Q.1 Which of the following is the primary purpose of a scintillator in a medical X‑ray detector?
To convert incoming X‑rays into visible light
To amplify the electrical signal from a photodiode
To shield the patient from stray radiation
To filter out low‑energy photons
Explanation - Scintillators such as CsI:Tl absorb X‑ray photons and emit visible light, which is then detected by photodetectors.
Correct answer is: To convert incoming X‑rays into visible light
Q.2 Which detector material offers the highest intrinsic spatial resolution among the following?
Gadolinium oxysulfide (Gd2O2S)
Bismuth germanate (BGO)
Silicon photomultiplier (SiPM)
Cesium iodide (CsI)
Explanation - SiPMs consist of an array of micro‑cells providing high spatial granularity, leading to superior intrinsic resolution compared to scintillators.
Correct answer is: Silicon photomultiplier (SiPM)
Q.3 What is the main advantage of using a Time‑of‑Flight (TOF) PET scanner over a conventional PET scanner?
Higher energy resolution
Improved axial field‑of‑view
Reduced radiation dose to the patient
Better localization of the annihilation event along the line of response
Explanation - TOF PET measures the difference in arrival times of the two 511 keV photons, improving image contrast and allowing for lower doses.
Correct answer is: Better localization of the annihilation event along the line of response
Q.4 Which phenomenon limits the maximum count rate of a Geiger‑Müller tube?
Pulse pile‑up
Afterpulsing
Dead time
Quenching gas diffusion
Explanation - After a detection event, the tube requires a recovery period during which it cannot register another event, limiting count rate.
Correct answer is: Dead time
Q.5 In a flat‑panel detector, the term 'pixel pitch' refers to:
The distance between the center of adjacent pixels
The thickness of the photodiode layer
The width of the readout electronics
The width of the light guide
Explanation - Pixel pitch directly affects spatial resolution; smaller pitches yield higher resolution.
Correct answer is: The distance between the center of adjacent pixels
Q.6 Which of the following noise sources is primarily responsible for the low‑frequency flicker noise in semiconductor detectors?
Shot noise
Johnson–Nyquist noise
1/f (flicker) noise
Photon noise
Explanation - 1/f noise dominates at low frequencies and arises from traps and defects in semiconductor materials.
Correct answer is: 1/f (flicker) noise
Q.7 The term 'energy resolution' in a gamma‑ray detector is defined as:
The ability to differentiate between different photon energies
The ratio of full width at half maximum to the peak energy
The detector's sensitivity to low‑energy photons
The speed at which the detector can process events
Explanation - Energy resolution = (FWHM/E₀) × 100%, indicating how sharply the detector can resolve energies.
Correct answer is: The ratio of full width at half maximum to the peak energy
Q.8 Which component is essential for achieving high spatial resolution in a PET system using depth‑of‑interaction (DOI) readout?
Long scintillation crystals
Thick photomultiplier tubes
Thin scintillation crystals segmented into layers
High‑gain avalanche photodiodes
Explanation - DOI readout requires knowledge of the interaction depth, which is obtained by segmenting thin crystals.
Correct answer is: Thin scintillation crystals segmented into layers
Q.9 Which type of detector is most commonly used for high‑resolution SPECT imaging?
Solid‑state germanium detectors
Cadmium zinc telluride (CZT) detectors
NaI(Tl) scintillation crystals
Photodiode arrays
Explanation - NaI(Tl) crystals provide high sensitivity and sufficient resolution for clinical SPECT.
Correct answer is: NaI(Tl) scintillation crystals
Q.10 What is the main purpose of the anti‑scatter grid in X‑ray imaging?
To increase patient dose
To reduce scattered radiation reaching the detector
To focus the X‑ray beam
To improve detector quantum efficiency
Explanation - The grid attenuates scattered photons, improving image contrast but may increase dose.
Correct answer is: To reduce scattered radiation reaching the detector
Q.11 Which of the following parameters determines the modulation transfer function (MTF) of a digital X‑ray detector?
The detector’s noise floor
The size of the detector’s pixels
The energy of the incident photons
The thickness of the scintillator layer
Explanation - MTF describes spatial frequency response; pixel size limits highest resolvable frequency.
Correct answer is: The size of the detector’s pixels
Q.12 Which detector technology is preferred for portable, low‑dose mammography?
Boron carbide sensors
Digital flat‑panel detectors using amorphous silicon
Liquid metal ion conductors
Thermoluminescent dosimeters
Explanation - Amorphous silicon flat panels provide high efficiency, low noise and are compact for portable devices.
Correct answer is: Digital flat‑panel detectors using amorphous silicon
Q.13 The 'dead time' of a detector is best minimized by:
Increasing the quenching gas pressure
Using a faster electronics readout
Enlarging the detector area
Cooling the detector to cryogenic temperatures
Explanation - Fast electronics reduce the time between events, allowing higher count rates.
Correct answer is: Using a faster electronics readout
Q.14 Which of the following is NOT a common source of background noise in PET imaging?
Random coincidences
Scatter coincidences
Photon attenuation in the body
Electronic noise from the photodetector
Explanation - Attenuation reduces signal but does not constitute noise; it is corrected mathematically.
Correct answer is: Photon attenuation in the body
Q.15 In a SPECT system, the crystal thickness is increased to:
Improve energy resolution
Increase detection efficiency
Reduce photon scatter
Lower detector cost
Explanation - Thicker crystals absorb more photons, improving sensitivity at the expense of resolution.
Correct answer is: Increase detection efficiency
Q.16 What does the term 'Quantum Efficiency (QE)' refer to in the context of X‑ray detectors?
The fraction of incident X‑rays that produce a measurable signal
The efficiency of converting optical photons to electrons
The probability of photon absorption in the scintillator
The speed at which the detector processes events
Explanation - QE quantifies how effectively a detector converts incoming photons into useful electronic signals.
Correct answer is: The fraction of incident X‑rays that produce a measurable signal
Q.17 Which of the following is a primary benefit of using a cadmium zinc telluride (CZT) detector over a traditional NaI(Tl) crystal for SPECT?
Higher temperature stability
Greater photon absorption efficiency at high energies
Lower cost
Simpler cooling requirements
Explanation - CZT can operate at room temperature with high performance, unlike NaI(Tl) which requires cooling.
Correct answer is: Higher temperature stability
Q.18 Which parameter directly affects the linearity of a scintillation detector’s response?
Photomultiplier tube gain
Crystal decay time
Photon detection efficiency
Scintillator light yield
Explanation - PMT gain must be linear with input light to maintain a linear detector response.
Correct answer is: Photomultiplier tube gain
Q.19 Which phenomenon primarily causes the 'Compton scatter' in CT imaging?
Elastic scattering of X‑rays
Inelastic scattering of X‑rays by electrons
Total internal reflection
Photoelectric absorption
Explanation - Compton scattering involves energy transfer from photons to electrons, altering photon direction and energy.
Correct answer is: Inelastic scattering of X‑rays by electrons
Q.20 In an X‑ray detector, the 'fill factor' of a pixel refers to:
The fraction of the pixel area occupied by the photosensitive region
The fraction of photons detected relative to those incident
The ratio of dark current to signal current
The portion of the pixel that can be read out simultaneously
Explanation - A higher fill factor improves sensitivity and reduces noise.
Correct answer is: The fraction of the pixel area occupied by the photosensitive region
Q.21 Which detector type is most suitable for detecting 511 keV annihilation photons with high efficiency?
Silicon photomultipliers (SiPM)
High‑purity germanium (HPGe)
Barium fluoride (BaF2) scintillators
CdTe detectors
Explanation - BaF2 offers fast response and high stopping power for 511 keV photons.
Correct answer is: Barium fluoride (BaF2) scintillators
Q.22 The term 'dead space' in a semiconductor detector refers to:
A region where charge carriers recombine before reaching the electrodes
A region not covered by the detector area
The time period during which the detector is inactive after an event
A zone of high electrical noise
Explanation - Dead space reduces charge collection efficiency and degrades energy resolution.
Correct answer is: A region where charge carriers recombine before reaching the electrodes
Q.23 Which of the following is a key advantage of using a digital photon counting detector in high‑energy imaging?
Increased dynamic range
Reduced electronic noise
Lower radiation dose
Higher spatial resolution
Explanation - Photon counting can handle a wide range of photon fluxes while maintaining linearity.
Correct answer is: Increased dynamic range
Q.24 Which factor is NOT directly related to the 'contrast‑noise trade‑off' in CT imaging?
X‑ray tube current
Detector quantum efficiency
Reconstruction kernel
Patient body mass index
Explanation - While QE affects signal strength, the contrast‑noise trade‑off is mainly governed by dose and reconstruction.
Correct answer is: Detector quantum efficiency
Q.25 Which of the following best describes the role of a 'collimator' in a gamma‑ray imaging system?
To accelerate photons
To limit the angles at which photons can reach the detector
To amplify the electronic signal
To filter out high‑energy photons
Explanation - Collimators restrict photon paths, improving image resolution at the cost of sensitivity.
Correct answer is: To limit the angles at which photons can reach the detector
Q.26 Which sensor type is commonly used in MRI to detect the weak magnetic resonance signals?
Photodiodes
Inductive radio‑frequency coils
Semiconductor detectors
Photomultiplier tubes
Explanation - RF coils convert tiny magnetic signals into measurable voltage signals in MRI.
Correct answer is: Inductive radio‑frequency coils
Q.27 What is the main advantage of using a 'photon‑counting' flat‑panel detector over a 'continuous‑output' detector?
Higher spatial resolution
Lower power consumption
Simpler calibration
Faster readout speed
Explanation - Photon‑counting detectors can achieve smaller effective pixel sizes, improving resolution.
Correct answer is: Higher spatial resolution
Q.28 The 'charge collection efficiency' in a semiconductor detector is primarily influenced by:
Photomultiplier tube gain
Crystal temperature
Electric field strength across the detector
Photodiode quantum efficiency
Explanation - A strong field reduces recombination and ensures efficient charge collection.
Correct answer is: Electric field strength across the detector
Q.29 Which type of detector is most commonly employed in ultrafast PET scanners?
Large‑area photomultiplier tubes
Silicon photomultipliers (SiPM)
High‑purity germanium crystals
Liquid scintillator detectors
Explanation - SiPMs offer fast timing response essential for ultrafast PET imaging.
Correct answer is: Silicon photomultipliers (SiPM)
Q.30 What is the primary reason for cooling semiconductor detectors like CZT?
To increase photon absorption
To reduce thermal noise
To accelerate charge transport
To enable operation in vacuum
Explanation - Lower temperatures suppress leakage current, improving energy resolution.
Correct answer is: To reduce thermal noise
Q.31 Which of the following is a key parameter for evaluating the 'noise figure' of a detector system?
Dark current
Quantum efficiency
Electronic bandwidth
Crystal thickness
Explanation - Dark current contributes to noise; a lower dark current yields a lower noise figure.
Correct answer is: Dark current
Q.32 In a PET scanner, the term 'coincidence window' refers to:
The time interval during which two photons must be detected to be considered coincident
The angular range over which photons are accepted
The physical width of the detector
The energy range accepted for photon detection
Explanation - A narrow coincidence window reduces random events but requires fast electronics.
Correct answer is: The time interval during which two photons must be detected to be considered coincident
Q.33 Which detector material is known for its excellent energy resolution at 662 keV?
Cesium iodide (CsI)
Barium fluoride (BaF2)
Germanium (Ge)
Silicon (Si)
Explanation - HPGe detectors offer the best energy resolution among semiconductor materials.
Correct answer is: Germanium (Ge)
Q.34 The term 'gain variation' in a detector refers to:
Fluctuations in the amplification factor across different pixels
Change in gain with temperature
Variation in gain due to radiation damage
All of the above
Explanation - Gain can vary spatially, thermally, and with radiation exposure, affecting image quality.
Correct answer is: All of the above
Q.35 Which component in a scintillation detector chain is responsible for converting visible photons into electrons?
Scintillator
Photomultiplier tube
Photodiode
Amplifier
Explanation - Photodiodes (or PMTs) detect visible light and generate an electrical signal.
Correct answer is: Photodiode
Q.36 A 'dead zone' in a detector is:
A region where photons are absorbed with no signal generated
An area where the detector cannot record events due to readout limitations
A period after an event during which the detector is inactive
All of the above
Explanation - Dead zones can refer to spatial or temporal inactivity in detector systems.
Correct answer is: All of the above
Q.37 Which of the following best describes 'pulse height analysis' in gamma‑ray spectroscopy?
Measuring the duration of the signal pulse
Determining the energy of incoming photons from pulse amplitude
Counting the number of pulses per unit time
Filtering out low‑energy events
Explanation - Pulse height corresponds to the energy deposited, allowing spectroscopy.
Correct answer is: Determining the energy of incoming photons from pulse amplitude
Q.38 Which factor most directly limits the maximum photon flux that a photomultiplier tube can handle?
Dynode saturation
Photon energy
Anode resistance
Magnetic shielding
Explanation - Excess photons cause dynodes to saturate, reducing linearity.
Correct answer is: Dynode saturation
Q.39 In a CT scanner, the term 'pitch' refers to:
The ratio of table speed to beam width
The angular pitch of the X‑ray beam
The pitch of the detector array
The pitch of the collimator
Explanation - Pitch affects acquisition speed and image quality.
Correct answer is: The ratio of table speed to beam width
Q.40 Which type of detector provides the highest speed for detecting high‑energy photons in PET?
BGO scintillator
LSO scintillator
NaI(Tl)
LaBr3:Ce
Explanation - LaBr3:Ce has the fastest decay time among common scintillators.
Correct answer is: LaBr3:Ce
Q.41 The 'saturation' of a photodetector occurs when:
The photodetector is exposed to radiation beyond its tolerance
The number of incident photons exceeds the maximum count rate
The output signal reaches the maximum linear range of the amplifier
The detector is at full temperature
Explanation - Beyond this point, additional photons do not increase the output linearly.
Correct answer is: The output signal reaches the maximum linear range of the amplifier
Q.42 In medical imaging, 'dynamic range' refers to:
The range of photon energies a detector can measure
The ratio between the largest and smallest signal a detector can accurately measure
The speed at which a detector can process events
The spatial range over which the detector can image
Explanation - Dynamic range indicates the detector's ability to handle both weak and strong signals.
Correct answer is: The ratio between the largest and smallest signal a detector can accurately measure
Q.43 Which of the following is a major drawback of using NaI(Tl) crystals in PET?
Low light output
Long decay time
High cost
Low stopping power for 511 keV photons
Explanation - NaI(Tl) has a decay time of ~250 ns, limiting timing resolution in PET.
Correct answer is: Long decay time
Q.44 The 'optical coupling' between a scintillator and photodetector primarily influences:
Electrical noise
Charge collection efficiency
Light collection efficiency
Quantum efficiency
Explanation - Good coupling (e.g., optical grease) maximizes photon transfer to the detector.
Correct answer is: Light collection efficiency
Q.45 Which phenomenon causes the broadening of spectral lines in a detector?
Compton scattering
Photoelectric absorption
Statistical fluctuation in charge generation
Rayleigh scattering
Explanation - Random variations in the number of charge carriers cause energy resolution degradation.
Correct answer is: Statistical fluctuation in charge generation
Q.46 In PET imaging, the term 'random coincidences' refers to:
Coincidences that arise from two unrelated photons detected within the coincidence window
Coincidences that are deliberately introduced to calibrate the system
Coincidences from scattered photons only
Coincidences that are rejected by the software
Explanation - Randoms degrade image quality and are corrected through statistical methods.
Correct answer is: Coincidences that arise from two unrelated photons detected within the coincidence window
Q.47 Which detector geometry is most suitable for whole‑body PET scanners?
Ring of scintillator crystals
Linear array of detectors
Cylindrical array of SiPMs
Flat panel detectors
Explanation - A ring geometry provides full 3‑D coverage for whole‑body imaging.
Correct answer is: Ring of scintillator crystals
Q.48 What role does a 'shroud' play in a photomultiplier tube?
Provide mechanical support
Block stray photons
Reduce magnetic field effects
Guide electrons to the dynodes
Explanation - Shrouds prevent unwanted light from reaching the photocathode, reducing noise.
Correct answer is: Block stray photons
Q.49 Which of the following is NOT a characteristic of a 'single‑photon avalanche diode (SPAD)’?
Operates in Geiger mode
Produces a binary output per detection event
Requires high bias voltage
Has excellent energy resolution
Explanation - SPADs are optimized for timing and single‑photon detection, not energy resolution.
Correct answer is: Has excellent energy resolution
Q.50 In CT, increasing the tube voltage primarily:
Reduces photon attenuation
Reduces patient dose
Improves spatial resolution
Increases image noise
Explanation - Higher energy photons penetrate tissues more easily, lowering attenuation.
Correct answer is: Reduces photon attenuation
Q.51 The term 'dead layer' in a semiconductor detector refers to:
The region where incident photons cannot produce charge carriers
The time during which the detector cannot record events after a pulse
The surface layer where surface states trap carriers
A layer of dead time in the readout electronics
Explanation - Dead layers reduce active volume and charge collection, affecting efficiency.
Correct answer is: The surface layer where surface states trap carriers
Q.52 Which of the following is a key factor determining the temporal resolution of a PET scanner?
Scintillator decay time
Photon energy
Detector thickness
Tube current
Explanation - Short decay times allow faster signal rise and better timing.
Correct answer is: Scintillator decay time
Q.53 What does 'full width at tenth maximum (FWTM)' describe in the context of detector response?
Energy resolution
Timing resolution
Spatial resolution
Signal rise time
Explanation - FWTM measures the spread of the energy peak at 10% of its maximum, indicating resolution.
Correct answer is: Energy resolution
Q.54 In a photon‑counting detector, why is it important to set a low threshold energy?
To allow detection of low‑energy photons
To avoid saturation
To reduce dark current
To speed up readout
Explanation - A low threshold ensures that all relevant photons are counted, improving sensitivity.
Correct answer is: To allow detection of low‑energy photons
Q.55 Which detector material is known for its high light yield and fast decay time, making it suitable for high‑speed imaging?
BGO
LSO
LaBr3:Ce
NaI(Tl)
Explanation - LaBr3:Ce provides high light output (~63 kph/keV) and ~16 ns decay time.
Correct answer is: LaBr3:Ce
Q.56 Which of the following is a common method for reducing electronic noise in semiconductor detectors?
Increase bias voltage
Lower operating temperature
Increase detector thickness
Add magnetic shielding
Explanation - Cooling reduces leakage current, the main source of electronic noise.
Correct answer is: Lower operating temperature
Q.57 What is the primary purpose of the 'anticoincidence shield' in PET scanners?
To reject scattered photons
To detect photons from outside the field of view
To improve energy resolution
To provide mechanical support
Explanation - It identifies and rejects events originating outside the imaging volume, reducing background.
Correct answer is: To detect photons from outside the field of view
Q.58 Which of the following parameters directly influences the 'signal‑to‑noise ratio (SNR)' in digital imaging?
Quantum efficiency
Pixel size
Detector temperature
All of the above
Explanation - All these factors affect the magnitude of signal and noise contributions.
Correct answer is: All of the above
Q.59 In SPECT imaging, increasing the number of detector collimator steps results in:
Higher spatial resolution
Lower sensitivity
Both increased resolution and sensitivity
Neither resolution nor sensitivity changes
Explanation - More steps restrict photon paths, improving resolution at the cost of sensitivity.
Correct answer is: Higher spatial resolution
Q.60 Which of the following is NOT an advantage of using silicon photomultipliers (SiPM) over traditional photomultiplier tubes (PMTs) in PET?
Compact size
Lower operating voltage
Higher photodetection efficiency
Superior timing resolution
Explanation - While SiPMs are comparable, PMTs still have slightly better intrinsic timing for some configurations.
Correct answer is: Superior timing resolution
Q.61 What is the main function of a 'radiation‑hardening' process in detector fabrication?
Increase detector sensitivity
Enhance mechanical strength
Prevent degradation due to radiation exposure
Reduce manufacturing cost
Explanation - Hardening techniques mitigate damage to electronic components from ionizing radiation.
Correct answer is: Prevent degradation due to radiation exposure
Q.62 The 'spectral noise figure' of a detector system is defined as:
The ratio of noise power to signal power in a given bandwidth
The ratio of dark current to photo‑current
The amount of noise introduced by the detector electronics
The width of the energy peak at half maximum
Explanation - This figure quantifies how much noise the system adds relative to the desired signal.
Correct answer is: The ratio of noise power to signal power in a given bandwidth
Q.63 In medical imaging, the 'linear dynamic range' is primarily affected by:
Detector saturation
Photon statistics
Electronic noise
All of the above
Explanation - All these factors determine how well a detector can handle varying signal intensities.
Correct answer is: All of the above
Q.64 Which of the following is a key requirement for a detector used in high‑dose CT?
High energy resolution
High temporal resolution
High radiation hardness
Low cost
Explanation - Detectors in high‑dose environments must withstand significant radiation without performance loss.
Correct answer is: High radiation hardness
Q.65 The term 'gain linearity' in a photon‑counting detector refers to:
Uniform response across different pixels
Proportional increase of output with incident photon flux
Stability of gain over temperature changes
Constant dark current
Explanation - Linearity ensures accurate photon counting over the dynamic range.
Correct answer is: Proportional increase of output with incident photon flux
Q.66 Which phenomenon causes 'photon starvation' in low‑dose CT imaging?
Excessive scattering
Insufficient photon flux
High detector noise
Low spatial resolution
Explanation - With low dose, fewer photons reach the detector, increasing noise.
Correct answer is: Insufficient photon flux
Q.67 In a PET system, the 'time resolution' is crucial for:
Energy discrimination
Spatial resolution
Reducing random coincidences
Increasing detector thickness
Explanation - Better timing allows a narrower coincidence window, reducing randoms.
Correct answer is: Reducing random coincidences
Q.68 The 'dark count rate' of a SiPM is primarily influenced by:
Temperature
Bias voltage
Photodiode size
All of the above
Explanation - All factors contribute to thermally generated dark counts in SiPMs.
Correct answer is: All of the above
Q.69 Which detector architecture is used in digital mammography to achieve high spatial resolution?
Digital flat‑panel with amorphous silicon
Photomultiplier tube array
Semiconductor strip detector
NaI(Tl) crystal array
Explanation - Amorphous silicon allows fine pixelation and high resolution for breast imaging.
Correct answer is: Digital flat‑panel with amorphous silicon
Q.70 The 'noise equivalent dose' (NED) is used to:
Quantify the dose needed to achieve a specific image quality
Measure the total patient dose
Compare detector sensitivity
Determine the maximum permissible dose
Explanation - NED expresses the dose that would produce the same noise level as the detector system.
Correct answer is: Quantify the dose needed to achieve a specific image quality
Q.71 Which of the following best describes a 'photon‑number resolving detector'?
A detector that can count the exact number of incident photons in a single event
A detector that discriminates photon energy
A detector that only counts photons above a threshold
A detector that measures photon arrival time
Explanation - Such detectors provide detailed spectral information for each event.
Correct answer is: A detector that can count the exact number of incident photons in a single event
Q.72 In SPECT, which factor most significantly impacts the ability to distinguish closely spaced radioactive sources?
Collimator geometry
Crystal thickness
Photon energy
Patient movement
Explanation - Collimators control the angular acceptance, directly affecting spatial resolution.
Correct answer is: Collimator geometry
Q.73 Which of the following is a key characteristic of 'hybrid photon counting detectors'?
They combine a scintillator with a solid‑state readout
They are used exclusively in PET
They rely on analog signal integration
They are insensitive to temperature
Explanation - Hybrid detectors merge scintillation light output with electronic photon counting.
Correct answer is: They combine a scintillator with a solid‑state readout
Q.74 In a digital X‑ray system, the term 'noise power spectrum' (NPS) characterizes:
The distribution of noise across spatial frequencies
The overall noise level
The detector's energy resolution
The photon flux
Explanation - NPS indicates how noise varies with image spatial frequency.
Correct answer is: The distribution of noise across spatial frequencies
Q.75 What is the primary function of the 'anti‑coinicidence shielding' in PET scanners?
Reduce the number of true coincidences
Reject random events originating outside the scanner bore
Increase photon flux to the detectors
Prevent detector overheating
Explanation - It identifies and discards events not relevant to the imaging volume.
Correct answer is: Reject random events originating outside the scanner bore
Q.76 Which detector property is directly linked to the ability to discriminate between photon energies?
Charge collection efficiency
Light yield
Energy resolution
Temporal resolution
Explanation - Better energy resolution allows for accurate discrimination of photon energies.
Correct answer is: Energy resolution
Q.77 Which of the following materials is commonly used as a scintillator for gamma cameras?
Bismuth germanate (BGO)
Silicon
Copper
Lead glass
Explanation - BGO provides high stopping power for 140 keV photons used in gamma cameras.
Correct answer is: Bismuth germanate (BGO)
Q.78 What is the significance of the 'rise time' of a detector signal?
Determines the energy resolution
Indicates the speed at which the detector can respond to a photon
Defines the maximum count rate
Measures the detector's quantum efficiency
Explanation - Fast rise time improves timing and reduces pile‑up.
Correct answer is: Indicates the speed at which the detector can respond to a photon
Q.79 Which of the following is a typical source of systematic error in PET image reconstruction?
Random coincidences
Non‑linear detector response
Photon attenuation
All of the above
Explanation - Each factor can bias reconstruction if not properly corrected.
Correct answer is: All of the above
Q.80 The 'photodetector efficiency' in a scintillation camera depends on:
Quantum efficiency of the photocathode
Size of the scintillator crystal
Magnetic shielding
Both a and b
Explanation - Efficiency is governed by light collection and the photocathode's quantum efficiency.
Correct answer is: Both a and b
Q.81 Which parameter is most critical for achieving high spatial resolution in a SPECT system?
Collimator slit width
Crystal thickness
Detector pixel size
Acquisition time
Explanation - Narrower slits improve resolution but reduce sensitivity.
Correct answer is: Collimator slit width
Q.82 In photon counting detectors, the term 'pile‑up' refers to:
When two photons arrive within a time interval shorter than the detector’s response time
When the detector is saturated by too many photons
When photons are absorbed in the wrong pixel
When the detector is blocked by stray light
Explanation - Pile‑up leads to miscounting and distorted energy spectra.
Correct answer is: When two photons arrive within a time interval shorter than the detector’s response time
Q.83 Which of the following is a primary challenge in designing a detector for high‑energy gamma imaging?
Low photon flux
High cost of scintillators
High attenuation by human tissue
Requirement for cryogenic cooling
Explanation - High‑energy photons are less attenuated, requiring thicker or more efficient detectors.
Correct answer is: High attenuation by human tissue
Q.84 Which of the following detector technologies offers the best temporal resolution for PET?
BGO scintillators
LSO scintillators
LSO combined with SiPM
NaI(Tl) crystals
Explanation - LSO’s fast decay and SiPM’s rapid response give superior timing.
Correct answer is: LSO combined with SiPM
Q.85 The term 'cross‑talk' in a pixelated detector refers to:
Electrical interference between pixels
Photon leakage from one pixel to another
Thermal interference
Mechanical vibration
Explanation - Cross‑talk reduces image fidelity by contaminating signals between adjacent pixels.
Correct answer is: Electrical interference between pixels
Q.86 Which factor does NOT affect the energy resolution of a scintillator detector?
Scintillator light yield
Photodetector quantum efficiency
Temperature fluctuations
Detector geometry
Explanation - Geometry influences spatial resolution, not energy resolution directly.
Correct answer is: Detector geometry
Q.87 What is the main benefit of using a 'dual‑energy' CT scanner?
Reduced patient dose
Improved image contrast
Faster acquisition time
Higher spatial resolution
Explanation - Dual‑energy allows material decomposition and better contrast.
Correct answer is: Improved image contrast
Q.88 Which of the following is a disadvantage of using high‑gain photomultiplier tubes in PET?
Limited spatial resolution
Susceptibility to magnetic fields
High dark current
High cost
Explanation - PMTs are sensitive to magnetic fields, complicating use in hybrid PET‑MRI.
Correct answer is: Susceptibility to magnetic fields
Q.89 In a digital X‑ray detector, 'fixed pattern noise' arises due to:
Pixel‑to‑pixel variations in response
Temporal fluctuations in photon flux
Statistical photon noise
Electronic cross‑talk
Explanation - Fixed pattern noise is inherent to the detector’s manufacturing variations.
Correct answer is: Pixel‑to‑pixel variations in response
Q.90 Which of the following describes a 'time‑of‑flight PET' system?
A system that measures the time difference between detected photons to improve image contrast
A system that uses a time‑varying X‑ray beam
A system that counts photons over a fixed time window
A system that performs imaging without timing information
Explanation - TOF PET uses precise timing to locate annihilation events along the line of response.
Correct answer is: A system that measures the time difference between detected photons to improve image contrast
Q.91 In PET, the 'maximum count rate' is limited primarily by:
Photon flux
Detector dead time
Quantum efficiency
Energy resolution
Explanation - Dead time reduces the effective count rate at high activity concentrations.
Correct answer is: Detector dead time
Q.92 Which detector configuration is best suited for high‑resolution brain PET imaging?
Large crystal detectors with deep penetration
Thin, segmented crystal detectors with DOI capability
Flat‑panel detectors
Scintillators coupled with PMTs
Explanation - Thin crystals reduce parallax error, improving resolution for small structures.
Correct answer is: Thin, segmented crystal detectors with DOI capability
Q.93 The 'radiation hardness' of a detector refers to its ability to:
Operate at low temperature
Withstand high photon flux without performance degradation
Maintain a low dark current
Provide high spatial resolution
Explanation - Radiation hardness ensures detector reliability in high‑dose environments.
Correct answer is: Withstand high photon flux without performance degradation
Q.94 In a PET detector, the 'photodetector gain' is typically expressed in terms of:
Electrons per photon
Voltage per photon
Current per photon
Counts per second
Explanation - Gain is the number of electrons produced per incident photon in the photodetector.
Correct answer is: Electrons per photon
Q.95 Which parameter is directly influenced by the thickness of a scintillator crystal in a PET detector?
Energy resolution
Detection efficiency
Spatial resolution
Signal‑to‑noise ratio
Explanation - Thicker crystals absorb more photons, increasing detection efficiency.
Correct answer is: Detection efficiency
Q.96 In SPECT imaging, the term 'parallax error' arises because:
Photons are scattered before reaching the detector
Collimators are misaligned
Photons hit the detector at oblique angles, causing depth ambiguity
The detector is too thick
Explanation - Parallax error reduces resolution for off‑axis sources.
Correct answer is: Photons hit the detector at oblique angles, causing depth ambiguity
Q.97 Which of the following is a characteristic of a 'digital photon counting detector' compared to an analog detector?
Higher noise
Higher dynamic range
Lower spatial resolution
Slower readout time
Explanation - Photon counting can handle a wide range of photon fluxes while remaining linear.
Correct answer is: Higher dynamic range
Q.98 The 'energy window' setting in a PET scanner primarily affects:
Spatial resolution
Timing resolution
Count rate and image contrast
Patient comfort
Explanation - A narrower window rejects more scatter, improving contrast but reducing counts.
Correct answer is: Count rate and image contrast
Q.99 What does 'photon‑number resolving' capability enable in a medical imaging detector?
Measurement of the exact number of photons per event
Fast timing of photon arrivals
Increased sensitivity
Reduced power consumption
Explanation - It allows accurate energy determination for each photon, improving spectral imaging.
Correct answer is: Measurement of the exact number of photons per event
Q.100 Which of the following is a disadvantage of using 'lead glass' as a scintillator in gamma cameras?
Low light output
Slow decay time
High cost
Poor radiation hardness
Explanation - Lead glass typically emits less light compared to conventional scintillators.
Correct answer is: Low light output
Q.101 In medical imaging, the term 'signal‐to‐noise ratio (SNR)' is defined as:
Signal amplitude divided by noise amplitude
Signal amplitude times noise amplitude
Signal variance divided by noise variance
Noise amplitude divided by signal amplitude
Explanation - Higher SNR indicates clearer images with less noise relative to the signal.
Correct answer is: Signal amplitude divided by noise amplitude
Q.102 Which detector technology is most commonly used for flat‑panel mammography?
Amorphous silicon flat‑panel detectors
Photomultiplier tubes
BGO scintillators
NaI(Tl) crystals
Explanation - They provide high spatial resolution and low dose for breast imaging.
Correct answer is: Amorphous silicon flat‑panel detectors
Q.103 The 'dark current' of a photodiode is primarily caused by:
Thermally generated carriers
Photon absorption
Scintillator decay
Quantum tunneling
Explanation - Temperature increases carrier generation, raising dark current.
Correct answer is: Thermally generated carriers
Q.104 In a SPECT camera, the 'collimator–detector system' must be designed to:
Maximize sensitivity at all energies
Provide uniform response across the field of view
Minimize dead time
All of the above
Explanation - An optimal design balances sensitivity, uniformity, and timing constraints.
Correct answer is: All of the above
Q.105 Which of the following parameters is NOT directly related to the 'field of view (FOV)' in CT imaging?
Detector array size
X‑ray source diameter
Gantry diameter
Patient table speed
Explanation - Table speed affects scan time, not the spatial FOV.
Correct answer is: Patient table speed
Q.106 What is the purpose of the 'tuning capacitor' in a photomultiplier tube circuit?
To adjust the gain
To filter high‑frequency noise
To match impedance and shape the output pulse
To cool the PMT
Explanation - It helps in shaping the pulse for optimal timing and amplitude.
Correct answer is: To match impedance and shape the output pulse
Q.107 Which of the following is a key challenge when integrating PET detectors into an MRI system?
Magnetic field interference with PMTs
Increased radiation dose
Longer acquisition time
Higher cost of detectors
Explanation - PMTs are sensitive to magnetic fields; hybrid PET‑MRI requires magnetic‑resistant detectors.
Correct answer is: Magnetic field interference with PMTs
Q.108 In photon‑counting CT, the 'photon‑count threshold' is set to:
The lowest photon energy detectable
The minimum number of photons to register a hit
The energy that maximizes SNR
The maximum photon energy accepted
Explanation - A threshold ensures that only photons above a certain energy contribute to the image.
Correct answer is: The minimum number of photons to register a hit
Q.109 The 'photon flux' in a CT scan is determined by:
X‑ray tube current
Detector size
Patient size
All of the above
Explanation - Higher tube current increases the number of emitted photons.
Correct answer is: X‑ray tube current
Q.110 What is the primary advantage of using 'gadolinium oxysulfide (Gd2O2S)' as a scintillator in gamma cameras?
High light yield
Fast decay time
High density and effective atomic number
Low cost
Explanation - Its high density improves detection efficiency for 140 keV photons.
Correct answer is: High density and effective atomic number
Q.111 In a PET detector, the term 'photon detection efficiency (PDE)' refers to:
The ratio of detected to incident photons
The speed of photon detection
The ratio of true to random coincidences
The efficiency of photon conversion to electrons
Explanation - PDE quantifies how many incident photons result in a measurable signal.
Correct answer is: The ratio of detected to incident photons
Q.112 Which of the following best describes a 'dual‑source' PET scanner?
A PET scanner that uses two different radionuclides simultaneously
A PET scanner equipped with two detector rings
A PET scanner that alternates between two different acquisition modes
A PET scanner that uses two different X‑ray tubes
Explanation - Dual‑source PET allows simultaneous imaging of two tracers.
Correct answer is: A PET scanner that uses two different radionuclides simultaneously
Q.113 In CT imaging, the term 'kV' refers to:
Kilovoltage peak of the X‑ray tube
Kilovolt current
Kilogram weight of the patient
Kilo‑electronvolt energy of photons
Explanation - kV controls the X‑ray photon energy spectrum.
Correct answer is: Kilovoltage peak of the X‑ray tube
Q.114 Which factor is NOT typically considered when selecting a scintillator for a PET system?
Light yield
Decay time
Density
Electrical conductivity
Explanation - Scintillators are insulating; conductivity is not relevant to PET performance.
Correct answer is: Electrical conductivity
Q.115 The 'field‑of‑view (FOV)' of a SPECT camera is primarily determined by:
The size of the detector array
The distance between the collimator and the detector
The energy of the gamma photons
All of the above
Explanation - A larger distance expands the FOV but reduces sensitivity.
Correct answer is: The distance between the collimator and the detector
Q.116 Which of the following is a direct consequence of high detector noise in digital imaging?
Improved contrast
Reduced image resolution
Lower dose requirement
Higher acquisition speed
Explanation - Noise obscures fine details, degrading spatial resolution.
Correct answer is: Reduced image resolution
Q.117 In PET imaging, the term 'parallax error' mainly arises due to:
Inaccurate timing
Detector geometry and depth of interaction
Photon scattering
Patient motion
Explanation - Misestimation of interaction depth leads to position errors.
Correct answer is: Detector geometry and depth of interaction
Q.118 What is the primary function of a 'pre‑amplifier' in a scintillation camera?
Amplify the photodetector signal
Filter noise
Convert light to electrons
Provide power to the detector
Explanation - It boosts the weak signal before further processing.
Correct answer is: Amplify the photodetector signal
Q.119 Which of the following is a major advantage of 'silicon photomultipliers (SiPM)' over traditional PMTs?
Higher magnetic field tolerance
Lower voltage operation
Higher light output
Both a and b
Explanation - SiPMs operate at low voltage and are insensitive to magnetic fields.
Correct answer is: Both a and b
Q.120 In a digital X‑ray detector, the 'noise equivalent dose (NED)' is used to:
Compare detector sensitivity to other systems
Measure the total dose to the patient
Determine the optimal exposure settings
Calculate the detector efficiency
Explanation - NED expresses the dose that produces the same noise level in a reference detector.
Correct answer is: Compare detector sensitivity to other systems
Q.121 Which detector material is known for its very fast decay time suitable for timing applications?
BGO
LaBr3:Ce
NaI(Tl)
Gd2O2S
Explanation - LaBr3:Ce decays in ~16 ns, ideal for precise timing.
Correct answer is: LaBr3:Ce
Q.122 In a PET system, the term 'scatter correction' refers to:
Removing scattered photons from the image
Compensating for photon attenuation
Adjusting for detector dead time
Correcting for patient motion
Explanation - Scatter degrades contrast; correction algorithms estimate and subtract it.
Correct answer is: Removing scattered photons from the image
Q.123 Which of the following is NOT a property of 'gated' PET imaging?
Reduces motion artifacts
Increases image noise
Enables cardiac imaging
Requires synchronization with the cardiac cycle
Explanation - Gating can reduce noise by selecting optimal timing windows.
Correct answer is: Increases image noise
Q.124 The 'sensitivity' of a gamma camera is defined as:
The proportion of emitted photons that are detected
The ratio of signal to noise
The photon detection efficiency
The field of view
Explanation - Sensitivity reflects how many photons are collected relative to the source emission.
Correct answer is: The proportion of emitted photons that are detected
Q.125 Which of the following detectors is most commonly used for 'digital breast tomosynthesis'?
Scintillating crystal array
Amorphous silicon flat‑panel detector
Photomultiplier tube array
Bismuth germanate (BGO) crystal
Explanation - Flat‑panel detectors provide high resolution and low dose for breast imaging.
Correct answer is: Amorphous silicon flat‑panel detector
Q.126 What is the main benefit of using a 'dual‑source' X‑ray CT system?
Higher spatial resolution
Reduced scan time
Lower radiation dose
Improved contrast
Explanation - Two X‑ray sources allow simultaneous data acquisition, shortening scans.
Correct answer is: Reduced scan time
Q.127 In SPECT imaging, the 'energy window' is chosen to:
Include only the photopeak photons
Reject scattered photons
Reduce the detector sensitivity
Both a and b
Explanation - An appropriately narrow window captures photopeak events and excludes scatter.
Correct answer is: Both a and b
Q.128 Which of the following factors most strongly influences the 'radiation dose' to a patient during CT?
Tube voltage (kV)
Tube current (mA)
Scan length
All of the above
Explanation - All parameters affect the total number of emitted photons and hence dose.
Correct answer is: All of the above
Q.129 In a PET scanner, 'dead time' is typically expressed as:
Seconds per event
Microseconds per event
Milliseconds per event
Nanoseconds per event
Explanation - PET detector dead times are on the order of a few microseconds.
Correct answer is: Microseconds per event
Q.130 Which of the following is a characteristic of a 'photon counting CT' detector?
It can discriminate photon energy
It uses analog integration
It requires a high dose
It operates at cryogenic temperatures
Explanation - Photon counting CT measures individual photon energies for material decomposition.
Correct answer is: It can discriminate photon energy
Q.131 In a scintillation camera, the 'photocathode material' is chosen based on:
Its sensitivity to visible light
Its magnetic susceptibility
Its high voltage tolerance
All of the above
Explanation - The photocathode must efficiently convert light photons into electrons.
Correct answer is: Its sensitivity to visible light
Q.132 Which detector type offers the highest energy resolution for gamma spectroscopy?
NaI(Tl) scintillator
BGO crystal
High‑purity germanium (HPGe)
Cadmium zinc telluride (CZT)
Explanation - HPGe provides sub‑% energy resolution due to charge‑carrier collection.
Correct answer is: High‑purity germanium (HPGe)
Q.133 What does the term 'count rate linearity' describe in PET detector performance?
The linear increase of count rate with activity concentration
The linear relation between count rate and energy
The linear relation between count rate and scan time
The linearity of photon arrival times
Explanation - Linearity ensures accurate quantification across activity ranges.
Correct answer is: The linear increase of count rate with activity concentration
Q.134 Which of the following is a typical use of a 'photodiode array' in medical imaging?
X‑ray flat‑panel detection
Gamma camera light collection
PET photon detection
All of the above
Explanation - Photodiode arrays convert scintillator light to electronic signals in flat panels.
Correct answer is: X‑ray flat‑panel detection
Q.135 Which factor limits the 'maximum photon energy' that can be detected by a scintillator?
Scintillator light yield
Crystal density
Decay time
Detector size
Explanation - Higher density improves absorption of high‑energy photons.
Correct answer is: Crystal density
Q.136 In PET, the 'system matrix' used in image reconstruction primarily accounts for:
Detector geometry and sensitivity
Photon energy distribution
Patient motion
Radiation dose
Explanation - The matrix maps activity to measured data based on system characteristics.
Correct answer is: Detector geometry and sensitivity
Q.137 Which of the following is a main advantage of 'digital photon‑counting CT' over conventional CT?
Lower radiation dose
Improved contrast resolution
Both a and b
Faster acquisition speed
Explanation - Photon‑counting detects individual photons, reducing noise and dose.
Correct answer is: Both a and b
Q.138 In SPECT imaging, 'collimator efficiency' is defined as:
The fraction of photons passing through the collimator holes
The fraction of photons absorbed by the collimator
The ratio of true to scatter coincidences
The detector sensitivity
Explanation - Collimator efficiency depends on hole geometry and material.
Correct answer is: The fraction of photons passing through the collimator holes
Q.139 Which detector technology is best suited for high‑speed, high‑resolution PET imaging?
NaI(Tl) crystals
BGO scintillators
LSO crystals coupled with SiPM
Photomultiplier tubes
Explanation - Fast decay and rapid photodetector response enable high‑speed imaging.
Correct answer is: LSO crystals coupled with SiPM
Q.140 The 'effective atomic number' of a scintillator affects its:
Stopping power for X‑rays
Decay time
Light output
Quantum efficiency
Explanation - A higher Z improves absorption of high‑energy photons.
Correct answer is: Stopping power for X‑rays
Q.141 In CT imaging, the term 'pitch' is defined as the:
Ratio of table speed to rotation time
Ratio of table speed to beam width
Ratio of tube voltage to current
Ratio of detector thickness to pixel size
Explanation - Pitch determines overlap of X‑ray beams and affects image quality.
Correct answer is: Ratio of table speed to beam width
Q.142 Which of the following best describes the 'signal‑to‑noise ratio (SNR)' in a PET scan?
The ratio of true to random events
The ratio of detected signal to noise in the image
The ratio of energy resolution to noise
The ratio of count rate to noise
Explanation - High SNR indicates clearer images with fewer artifacts.
Correct answer is: The ratio of detected signal to noise in the image
Q.143 The 'dead time' in a detector is most effectively reduced by:
Increasing the quenching gas pressure
Using a faster readout electronics
Enlarging the detector area
Cooling the detector to cryogenic temperatures
Explanation - Fast electronics shorten the time the detector is unable to register events.
Correct answer is: Using a faster readout electronics
Q.144 Which of the following is a characteristic of 'hybrid photon‑counting detectors' in CT?
They combine a scintillator with a digital sensor
They are purely analog devices
They require high magnetic fields
They cannot be used for dual‑energy imaging
Explanation - Hybrid systems enable both fast timing and energy discrimination.
Correct answer is: They combine a scintillator with a digital sensor
Q.145 Which of the following is a typical source of 'scatter' in a PET scanner?
Photons interacting within the detector material
Photons scattered in the patient body
Photons absorbed in the collimator
None of the above
Explanation - Scattered photons degrade image quality and must be corrected.
Correct answer is: Photons scattered in the patient body
Q.146 In SPECT imaging, the 'energy window' is chosen to:
Include the photopeak and exclude scatter
Maximize sensitivity
Minimize patient dose
Both a and b
Explanation - A carefully chosen window balances sensitivity and contrast.
Correct answer is: Both a and b
Q.147 Which of the following best describes a 'depth‑of‑interaction (DOI)' detector?
Detects the exact depth where a photon interacts within a crystal
Measures the energy of the incident photon
Measures the arrival time of the photon
Detects photons only at the surface of the crystal
Explanation - DOI reduces parallax error by providing depth information.
Correct answer is: Detects the exact depth where a photon interacts within a crystal
Q.148 The 'light yield' of a scintillator influences which performance metric?
Energy resolution
Spatial resolution
Timing resolution
All of the above
Explanation - Light yield affects signal amplitude, affecting energy, spatial, and timing resolution.
Correct answer is: All of the above