Q.1 Which of the following is the primary source of noise in MRI images?
Patient motion
Thermal noise in RF coils
X-ray photon scattering
Ultrasound attenuation
Explanation - In MRI, thermal agitation of electrons in the receiver coils generates white Gaussian noise, which dominates image noise.
Correct answer is: Thermal noise in RF coils
Q.2 The resolution of a CT scan is mainly determined by which factor?
Scanner gantry rotation time
X‑ray beam collimation
Patient heart rate
Contrast agent dose
Explanation - Beam collimation defines the thickness of the x‑ray beam, directly affecting spatial resolution; narrower beams give better resolution.
Correct answer is: X‑ray beam collimation
Q.3 What does the term 'signal-to-noise ratio' (SNR) represent?
Ratio of pixel brightness to pixel darkness
Ratio of desired signal strength to background noise
Ratio of image width to height
Ratio of scanning time to reconstruction time
Explanation - SNR is a measure of image quality: higher SNR means clearer images with less visible noise.
Correct answer is: Ratio of desired signal strength to background noise
Q.4 Which imaging modality is most susceptible to photon starvation?
MRI
Ultrasound
X‑ray radiography
PET
Explanation - Photon starvation occurs when few x‑ray photons reach the detector, especially in dense tissues; this degrades image quality.
Correct answer is: X‑ray radiography
Q.5 In ultrasound imaging, increasing the frequency of the transducer primarily affects which aspect of the image?
Image depth
Spatial resolution
Noise level
Color mapping
Explanation - Higher frequency ultrasound yields finer spatial resolution but with reduced penetration depth.
Correct answer is: Spatial resolution
Q.6 Which factor does NOT improve the spatial resolution of a PET scanner?
Higher detector granularity
Increased positron emitter dose
Longer acquisition time
Advanced reconstruction algorithms
Explanation - While dose increases counts, it doesn't directly improve spatial resolution; detector size and reconstruction have bigger effects.
Correct answer is: Increased positron emitter dose
Q.7 In digital imaging, what is the main effect of increasing the bit depth from 8‑bit to 12‑bit?
Improved spatial resolution
Reduced noise
Greater dynamic range
Faster image acquisition
Explanation - Higher bit depth allows more gray levels, increasing dynamic range and reducing quantization noise.
Correct answer is: Greater dynamic range
Q.8 The term 'contrast-to-noise ratio' (CNR) is most important for detecting
Small lesions in dense tissue
Large tumors in low‑density tissue
Artifacts caused by motion
Color differentiation in images
Explanation - CNR measures how distinguishable a structure is from background noise; it is critical for small lesion detection.
Correct answer is: Small lesions in dense tissue
Q.9 Which of the following is a common method to reduce noise in MRI images?
Using higher magnetic field strength
Applying a Gaussian blur filter
Increasing repetition time (TR)
Employing parallel imaging techniques
Explanation - Parallel imaging accelerates acquisition and reduces noise by combining data from multiple coils.
Correct answer is: Employing parallel imaging techniques
Q.10 The spatial resolution limit imposed by diffraction in an optical system is described by
Rayleigh criterion
Abbe limit
Nyquist theorem
Fourier transform
Explanation - Abbe's criterion defines the smallest resolvable distance in diffraction-limited optics.
Correct answer is: Abbe limit
Q.11 Which parameter directly influences the depth of field in an x‑ray system?
Focal spot size
Tube voltage
Collimator setting
Exposure time
Explanation - Collimator size controls the beam's cone angle, which affects depth of field and image sharpness.
Correct answer is: Collimator setting
Q.12 The Nyquist sampling theorem is important for medical imaging because it ensures that
Images are displayed at the correct color scale
There is no aliasing due to undersampling
The scanner's battery lasts longer
Noise is minimized during acquisition
Explanation - Nyquist states sampling must be at least twice the highest spatial frequency to avoid aliasing artifacts.
Correct answer is: There is no aliasing due to undersampling
Q.13 In CT imaging, a 'kV' setting controls
Image contrast
X‑ray beam energy
Scanner rotation speed
Patient dose rate
Explanation - The kilovoltage determines photon energy, influencing penetration and contrast.
Correct answer is: X‑ray beam energy
Q.14 Which of these techniques is used to mitigate motion artifacts in MRI?
Increasing TR
Using breath‑hold techniques
Applying a stronger gradient
Reducing field of view
Explanation - Breath‑hold or gating methods synchronize imaging with patient motion to reduce artifacts.
Correct answer is: Using breath‑hold techniques
Q.15 In ultrasound imaging, the 'apparent speed of sound' is affected by
Tissue density
Transducer frequency
Patient age
Room temperature
Explanation - Speed of sound varies with tissue composition; this influences depth calculations.
Correct answer is: Tissue density
Q.16 A higher dynamic range in an imaging detector allows
More precise color reproduction
Higher spatial resolution
Detection of subtle differences in signal intensity
Faster image transfer to the monitor
Explanation - Dynamic range defines how small intensity differences can be distinguished.
Correct answer is: Detection of subtle differences in signal intensity
Q.17 Which of the following is a type of noise commonly seen in PET images?
Ringing artifact
Random coincidences
Motion blur
Photon streaking
Explanation - Random coincidences arise from two unrelated photons detected within the coincidence window, producing noise.
Correct answer is: Random coincidences
Q.18 In digital radiography, a pixel size of 0.2 mm typically yields
Higher spatial resolution than 0.5 mm pixels
Greater patient dose
Longer exposure times
More image noise
Explanation - Smaller pixels sample the image more finely, improving spatial resolution.
Correct answer is: Higher spatial resolution than 0.5 mm pixels
Q.19 Which of these parameters is NOT directly related to image resolution in ultrasound?
Frequency of the transducer
Beam width
Depth of penetration
Pulse repetition frequency
Explanation - PRF affects imaging rate and aliasing, not spatial resolution.
Correct answer is: Pulse repetition frequency
Q.20 The term 'noise equivalent count' (NEC) is used in PET to assess
Maximum achievable spatial resolution
Signal-to-noise ratio of the reconstructed image
Patient radiation dose
Scanning time efficiency
Explanation - NEC quantifies the number of counts that would produce a comparable SNR if all were true coincidences.
Correct answer is: Signal-to-noise ratio of the reconstructed image
Q.21 Which factor primarily determines the depth of focus in a cone‑beam CT system?
Source‑to‑detector distance
Detector pixel size
X‑ray tube current
Reconstruction algorithm
Explanation - A larger source‑to‑detector distance increases depth of focus, reducing geometric blurring.
Correct answer is: Source‑to‑detector distance
Q.22 Increasing the number of detector elements in an MRI receiver coil array tends to
Decrease SNR
Improve parallel imaging acceleration
Reduce bandwidth
Increase RF noise
Explanation - More coil elements provide better spatial encoding, enabling faster scans.
Correct answer is: Improve parallel imaging acceleration
Q.23 The 'modulation transfer function' (MTF) describes
The detector's sensitivity to temperature changes
The relationship between input spatial frequency and output contrast
The timing accuracy of the scanner
The maximum dose deliverable by the system
Explanation - MTF quantifies how well an imaging system preserves contrast at different spatial frequencies.
Correct answer is: The relationship between input spatial frequency and output contrast
Q.24 Which of the following contributes most to scatter radiation in a CT scan?
High tube current
Small field of view
Long exposure time
Low tube voltage
Explanation - Higher current increases photon numbers, leading to more scatter events.
Correct answer is: High tube current
Q.25 In PET imaging, the term 'dead time' refers to
The time between patient arrival and scan start
The period during which the detector cannot record new events after an interaction
The delay in image reconstruction
The time required for the scanner to cool down
Explanation - Dead time causes loss of counts and reduces image quality if not corrected.
Correct answer is: The period during which the detector cannot record new events after an interaction
Q.26 In digital radiography, the term 'bit depth' refers to
The number of gray levels per pixel
The depth of the detector material
The number of bits in the scanner's firmware
The thickness of the film emulsion
Explanation - Bit depth determines how many distinct intensity values can be recorded.
Correct answer is: The number of gray levels per pixel
Q.27 Which imaging modality uses magnetic fields and radiofrequency pulses to generate images?
CT
PET
MRI
Ultrasound
Explanation - Magnetic Resonance Imaging relies on magnetic fields and RF pulses to excite nuclei.
Correct answer is: MRI
Q.28 In ultrasound, what is the main cause of 'speckle' noise?
Patient movement
Interference between multiple echo waves
Electronic amplifier noise
Incorrect probe placement
Explanation - Speckle arises from constructive and destructive interference of scattered waves from sub‑pixel scatterers.
Correct answer is: Interference between multiple echo waves
Q.29 What does the term 'geometric distortion' refer to in medical imaging?
Pixel intensity variation
Temporal blurring of moving tissues
Incorrect representation of object shapes
Increased noise levels
Explanation - Geometric distortion causes the image to misrepresent real spatial relationships.
Correct answer is: Incorrect representation of object shapes
Q.30 Which of the following best improves the signal-to-noise ratio (SNR) in a digital X‑ray image?
Using a smaller detector
Reducing the exposure time
Increasing the pixel size
Employing a higher tube current
Explanation - Higher current yields more photons, increasing the signal relative to noise.
Correct answer is: Employing a higher tube current
Q.31 In CT, 'pitch' is defined as the
Ratio of table speed to gantry rotation time
Ratio of x‑ray beam width to detector size
Ratio of tube voltage to current
Ratio of reconstruction kernel sharpness to blur
Explanation - Pitch affects image overlap and scan speed in helical CT.
Correct answer is: Ratio of table speed to gantry rotation time
Q.32 What is the primary function of a collimator in an X‑ray system?
To focus the X‑ray beam
To reduce radiation dose
To limit the beam to the area of interest
To cool the X‑ray tube
Explanation - Collimators narrow the beam, limiting exposure to the target region.
Correct answer is: To limit the beam to the area of interest
Q.33 Which parameter directly affects the 'depth resolution' in a PET scanner?
Positron range
Time-of-flight resolution
Magnetic field strength
Scanner rotation speed
Explanation - TOF PET uses timing information to improve localization along the line of response.
Correct answer is: Time-of-flight resolution
Q.34 The term 'contrast agent' in MRI is used to
Increase the speed of the scan
Enhance the relaxation rates of tissues
Reduce patient motion
Calibrate the scanner's gradient coils
Explanation - Contrast agents like gadolinium alter T1 or T2 relaxation, improving tissue differentiation.
Correct answer is: Enhance the relaxation rates of tissues
Q.35 In CT imaging, the 'kernel' refers to
The algorithm used to reconstruct images
The physical shape of the X‑ray source
The type of detector used
The software license key
Explanation - The reconstruction kernel (e.g., sharp or soft) influences spatial resolution and noise.
Correct answer is: The algorithm used to reconstruct images
Q.36 Which of the following is a consequence of undersampling in MRI?
Motion artifacts
Aliasing artifacts
Increased contrast
Reduced patient dose
Explanation - Undersampling leads to aliasing where high-frequency components fold into lower frequencies.
Correct answer is: Aliasing artifacts
Q.37 In ultrasound, the term 'beamforming' refers to
The process of creating a focused acoustic beam
The measurement of tissue elasticity
The conversion of sound to electrical signals
The calibration of the transducer array
Explanation - Beamforming controls the timing of individual elements to focus sound energy.
Correct answer is: The process of creating a focused acoustic beam
Q.38 Which type of noise is typically reduced by increasing the integration time in a detector?
Thermal noise
Shot noise
Electronic noise
Ambient light noise
Explanation - Shot noise scales with the square root of the signal; longer integration averages it out.
Correct answer is: Shot noise
Q.39 In PET, a 'random coincidence' occurs when two photons from
the same decay event are detected at different times
different decay events are detected within the coincidence window
a single photon is scattered twice
a photon interacts with the detector twice
Explanation - Randoms are unrelated photons that happen to fall into the coincidence time window.
Correct answer is: different decay events are detected within the coincidence window
Q.40 What is the effect of increasing the f/ratio of a digital radiography system?
Greater depth of field
Higher spatial resolution
Lower patient dose
Increased detector noise
Explanation - A higher f/ratio reduces the cone angle, increasing depth of field.
Correct answer is: Greater depth of field
Q.41 The 'effective dose' in radiology refers to
The actual amount of radiation delivered to the patient
The biological risk of the radiation exposure
The dose measured by a dosimeter
The amount of dose needed to achieve diagnostic quality
Explanation - Effective dose incorporates tissue weighting factors to reflect risk.
Correct answer is: The biological risk of the radiation exposure
Q.42 Which factor is most responsible for the 'halo' artifact in CT images?
Beam hardening
Motion blur
Scatter radiation
Insufficient reconstruction kernel
Explanation - Beam hardening causes bright halos around dense structures.
Correct answer is: Beam hardening
Q.43 In digital imaging, a 12-bit image can represent how many distinct intensity levels?
256
512
1024
4096
Explanation - Each bit adds a factor of 2; 2^12 = 4096 levels.
Correct answer is: 4096
Q.44 The 'z‑resolution' in PET imaging is improved by
Increasing the field of view
Using a thicker detector
Implementing time-of-flight techniques
Reducing the number of detector layers
Explanation - TOF PET improves the localization along the line of response, enhancing z‑resolution.
Correct answer is: Implementing time-of-flight techniques
Q.45 Which of the following is NOT a type of detector used in PET scanners?
Scintillation crystals
Silicon photomultipliers
Semiconductor detectors
Photographic film
Explanation - Modern PET uses electronic detectors; photographic film is obsolete.
Correct answer is: Photographic film
Q.46 In ultrasound imaging, increasing the pulse repetition frequency (PRF) primarily
Reduces axial resolution
Improves temporal resolution
Decreases lateral resolution
Increases beam penetration depth
Explanation - Higher PRF allows more frames per second, better capturing motion.
Correct answer is: Improves temporal resolution
Q.47 The 'half‑value layer' (HVL) in X‑ray physics refers to
The thickness of material that reduces intensity by half
The energy level where absorption is maximal
The time required for half the photons to be absorbed
The depth at which half the signal remains
Explanation - HVL is a measure of beam quality and filtration.
Correct answer is: The thickness of material that reduces intensity by half
Q.48 Which of the following best describes 'quantization noise'?
Random noise due to thermal effects
Error introduced when converting analog signals to digital values
Noise caused by motion artifacts
Noise from photon scattering
Explanation - Quantization noise arises because continuous signals are approximated by discrete levels.
Correct answer is: Error introduced when converting analog signals to digital values
Q.49 In MRI, the 'TR' parameter influences
The strength of the magnetic field
The repetition time between successive pulse sequences
The bandwidth of the receiver coil
The spatial resolution of the image
Explanation - TR determines T1 weighting and affects image contrast and acquisition time.
Correct answer is: The repetition time between successive pulse sequences
Q.50 What is the main advantage of using a 'soft' reconstruction kernel in CT?
Higher spatial resolution
Lower noise
Faster reconstruction time
Greater dose reduction
Explanation - Soft kernels produce smoother images with reduced noise but at the cost of resolution.
Correct answer is: Lower noise
Q.51 In digital radiography, an exposure that is too high will cause
Under‑exposed images
Over‑exposed images
Improved contrast
Reduced detector noise
Explanation - Too many photons saturate the detector, losing detail.
Correct answer is: Over‑exposed images
Q.52 The 'point spread function' (PSF) in imaging systems describes
The response of the system to a point source
The distribution of pixel intensities
The detector's sensitivity to magnetic fields
The noise floor of the system
Explanation - PSF shows how a point is blurred by the system, influencing resolution.
Correct answer is: The response of the system to a point source
Q.53 Which of the following improves the SNR of a PET image without increasing radiation dose?
Increasing the number of detector layers
Using a longer acquisition time
Increasing the injected dose
Employing a higher magnetic field
Explanation - Longer scans collect more counts, improving SNR without changing dose per unit time.
Correct answer is: Using a longer acquisition time
Q.54 The 'detector fill factor' refers to
The proportion of detector area that is active
The ratio of detector size to field of view
The sensitivity of the detector to temperature changes
The amount of electronic noise generated
Explanation - A higher fill factor means more of the detector contributes to signal, improving efficiency.
Correct answer is: The proportion of detector area that is active
Q.55 In ultrasound, why is the image depth expressed in centimeters?
Because sound travels 1 cm per microsecond
Because the human body is measured in centimeters
Because the transducer operates in centimeter units
Because the speed of sound in tissue is about 1 cm/microsecond
Explanation - Depth calculation uses time delay and the ~1540 m/s speed, roughly 1 cm per μs.
Correct answer is: Because the speed of sound in tissue is about 1 cm/microsecond
Q.56 Which of the following is a primary source of image blur in X‑ray CT?
Patient motion
Detector pixel size
Beam collimation
Pulse repetition frequency
Explanation - Motion during acquisition causes spatial blurring of structures.
Correct answer is: Patient motion
Q.57 The 'contrast resolution' of an imaging system refers to
The ability to detect differences in pixel intensity
The maximum spatial frequency that can be resolved
The resolution of the display monitor
The dynamic range of the detector
Explanation - Contrast resolution quantifies how small intensity differences can be distinguished.
Correct answer is: The ability to detect differences in pixel intensity
Q.58 Which of the following parameters is NOT directly controlled by the user in a PET scanner?
Coincidence timing window
Scanner geometry
Reconstruction algorithm
Magnetic field strength
Explanation - PET scanners do not use magnetic fields; that is a property of MRI.
Correct answer is: Magnetic field strength
Q.59 The 'slice thickness' in MRI affects
The number of slices per volume
The spatial resolution in the slice direction
The field of view
The T1 relaxation time
Explanation - Thicker slices average signal over a larger volume, reducing z‑resolution.
Correct answer is: The spatial resolution in the slice direction
Q.60 Which of the following best describes 'noise figure' in imaging electronics?
The ratio of signal to noise at the detector output
The amount of electronic noise added by a device relative to an ideal device
The noise power in a 1 Hz bandwidth
The number of noise pixels per image
Explanation - Noise figure compares the degradation of SNR by a component.
Correct answer is: The amount of electronic noise added by a device relative to an ideal device
Q.61 In a CT scan, a higher 'kVp' setting increases
Image contrast
Photon penetration depth
Patient dose
Detector noise
Explanation - Higher kVp generates higher energy photons that penetrate tissues better, reducing contrast but increasing depth.
Correct answer is: Photon penetration depth
Q.62 Which of the following best describes the 'modulation transfer function' (MTF) in CT?
The rate at which the scanner updates images
The relationship between spatial frequency and image contrast
The time required to reconstruct a volume
The proportion of scatter radiation absorbed
Explanation - MTF quantifies how contrast is preserved across spatial frequencies.
Correct answer is: The relationship between spatial frequency and image contrast
Q.63 The 'beam current' in an X‑ray tube is measured in
Volts
Amperes
Hertz
Joules
Explanation - Beam current (mA) indicates the electron flow, controlling photon output.
Correct answer is: Amperes
Q.64 In PET, the term 'coincidence loss' refers to
Loss of true coincidences due to high count rates
The loss of patient motion data
The loss of image contrast over time
The loss of detector efficiency with age
Explanation - High count rates cause multiple events to occur within the coincidence window, reducing true counts.
Correct answer is: Loss of true coincidences due to high count rates
Q.65 Which factor most directly affects the 'penumbra' in a radiation field?
Field size at the source
Distance from source to target
Beam hardening
Detector type
Explanation - Penumbra widens with increasing distance due to geometric spread.
Correct answer is: Distance from source to target
Q.66 In ultrasound, the 'beam width' is inversely proportional to
Frequency of the transducer
Speed of sound
Pulse repetition frequency
Transducer size
Explanation - Higher frequency produces narrower beams, improving resolution.
Correct answer is: Frequency of the transducer
Q.67 Which of the following is NOT a typical type of image artifact in CT?
Beam hardening
Metal streaks
Motion blur
Color inversion
Explanation - CT images are grayscale; color inversion is not a known artifact.
Correct answer is: Color inversion
Q.68 The 'contrast' in an MRI image depends on
The patient's blood pressure
The magnetic field strength and pulse sequence parameters
The size of the scanner gantry
The room temperature
Explanation - Contrast arises from differences in T1/T2 relaxation influenced by field strength and sequence.
Correct answer is: The magnetic field strength and pulse sequence parameters
Q.69 In a CT system, the 'tube voltage' (kVp) is primarily used to control
The angular speed of the gantry
The energy of the emitted X‑ray photons
The detector gain
The patient’s motion
Explanation - kVp sets the peak voltage, determining photon energy distribution.
Correct answer is: The energy of the emitted X‑ray photons
Q.70 Which of the following is a direct consequence of increasing the detector pixel size?
Higher spatial resolution
Lower detector noise
Reduced dynamic range
Higher patient dose
Explanation - Larger pixels collect more photons, reducing noise but at the cost of resolution.
Correct answer is: Lower detector noise
Q.71 The term 'image grade' in diagnostic imaging refers to
The subjective assessment of image quality by the radiologist
The objective measurement of signal-to-noise ratio
The number of slices per volume
The dose index of the scan
Explanation - Image grade is a qualitative score given by the clinician.
Correct answer is: The subjective assessment of image quality by the radiologist
Q.72 Which of the following best improves the axial resolution in an ultrasound image?
Increase the transducer diameter
Use a lower frequency
Reduce the depth of focus
Use a narrow aperture
Explanation - A narrower aperture improves axial resolution by limiting the beam width.
Correct answer is: Use a narrow aperture
Q.73 The 'beam hardening' effect in CT imaging results in
A reduction in patient dose
Increased contrast in bone structures
Artifacts such as streaks and halos
Improved spatial resolution
Explanation - Beam hardening causes non-uniform attenuation leading to artifacts.
Correct answer is: Artifacts such as streaks and halos
Q.74 Which of the following is a common method to reduce noise in ultrasound images?
Increasing the gain setting
Using a lower transducer frequency
Applying a speckle reduction filter
Increasing the pulse repetition frequency
Explanation - Speckle filters smooth the image while preserving edges.
Correct answer is: Applying a speckle reduction filter
Q.75 The 'time‑of‑flight' (TOF) capability in PET imaging improves
Spatial resolution by localizing events along the line of response
Patient comfort
Contrast between tissues
Scanning speed
Explanation - TOF uses timing differences to pinpoint annihilation events, enhancing resolution.
Correct answer is: Spatial resolution by localizing events along the line of response
Q.76 In a digital detector, the 'full well capacity' refers to
The maximum number of photons that can be recorded per pixel
The maximum dose the detector can tolerate
The size of the detector chip
The maximum dynamic range
Explanation - Full well capacity limits the pixel’s charge storage, influencing saturation.
Correct answer is: The maximum number of photons that can be recorded per pixel
Q.77 Which of the following is the most significant factor in determining the 'contrast-to-noise ratio' (CNR) in an MRI image?
Magnetic field homogeneity
Pulse sequence timing parameters
The patient's heart rate
The room temperature
Explanation - Sequence timing (TR/TE) influences contrast and noise characteristics.
Correct answer is: Pulse sequence timing parameters
Q.78 In CT, the 'half‑width at half maximum' (HWHM) of the MTF indicates
The spatial frequency at which the system’s contrast is reduced by half
The width of the X‑ray beam
The time required to acquire a slice
The amount of scatter radiation
Explanation - HWHM is a metric of resolution derived from the MTF curve.
Correct answer is: The spatial frequency at which the system’s contrast is reduced by half
Q.79 The 'dynamic range' of an imaging system is best described as
The range of pixel intensities the system can record
The range of spatial frequencies the system can resolve
The range of patient sizes the scanner can accommodate
The range of acceptable noise levels
Explanation - Dynamic range is the ratio between maximum and minimum detectable signals.
Correct answer is: The range of pixel intensities the system can record
Q.80 In MRI, increasing the field of view (FOV) while keeping pixel size constant results in
Higher spatial resolution
Lower spatial resolution
No change in resolution
Increased signal-to-noise ratio
Explanation - Increasing FOV with the same number of pixels spreads the same pixel area over a larger area, reducing resolution.
Correct answer is: Lower spatial resolution
Q.81 Which of the following describes the main advantage of using a 'high‑resolution' reconstruction kernel in CT?
It reduces the radiation dose required
It improves the visibility of small structures at the expense of increased noise
It shortens the scan time
It eliminates motion artifacts
Explanation - High‑resolution kernels sharpen edges but amplify noise.
Correct answer is: It improves the visibility of small structures at the expense of increased noise
Q.82 What is the effect of using a higher 'kV' setting in CT imaging?
It increases the patient dose but improves image contrast
It reduces beam hardening artifacts but decreases contrast
It shortens acquisition time
It increases the number of slices per second
Explanation - Higher kV yields higher energy photons that are less attenuated, reducing artifacts but also lowering contrast.
Correct answer is: It reduces beam hardening artifacts but decreases contrast
Q.83 In PET, the 'system sensitivity' is defined as
The number of detected true coincidences per unit of injected activity
The maximum radiation dose the scanner can deliver
The speed of image reconstruction
The contrast resolution of the system
Explanation - Sensitivity quantifies how effectively the system detects annihilation events.
Correct answer is: The number of detected true coincidences per unit of injected activity
Q.84 Which factor is most responsible for the 'penumbra' effect observed in CT imaging?
Scatter radiation
Finite focal spot size
Patient motion
Detector pixel size
Explanation - A finite focal spot creates a blurring region at the edges of structures.
Correct answer is: Finite focal spot size
Q.85 Which of the following is a characteristic of a 'soft' PET reconstruction algorithm?
Higher spatial resolution
Lower image noise
Higher radiation dose
Faster acquisition
Explanation - Soft reconstructions emphasize noise suppression over sharpness.
Correct answer is: Lower image noise
Q.86 In medical imaging, the term 'contrast agent' refers to
A device that amplifies the magnetic field
A substance introduced into the body to enhance image contrast
The software that reconstructs images
The protective shield used during scanning
Explanation - Contrast agents, such as iodine or gadolinium, alter tissue attenuation or relaxation times.
Correct answer is: A substance introduced into the body to enhance image contrast
Q.87 The 'pixel pitch' in a digital detector is
The distance between adjacent pixels
The depth of the detector material
The thickness of the X‑ray tube
The gain of the detector electronics
Explanation - Pixel pitch determines the physical spacing between pixels.
Correct answer is: The distance between adjacent pixels
Q.88 Which of the following is a common source of 'photon starvation' in CT?
High patient BMI
Low kVp setting
Large field of view
All of the above
Explanation - Any factor that reduces photon flux can lead to photon starvation.
Correct answer is: All of the above
Q.89 In ultrasound imaging, the 'aperture size' influences
The depth of field
The speed of sound
The magnetic field strength
The patient dose
Explanation - A larger aperture increases the depth of field and lateral resolution.
Correct answer is: The depth of field
Q.90 The 'modulation transfer function' (MTF) is used to evaluate
The system's ability to preserve contrast across spatial frequencies
The patient’s radiation exposure
The speed of image reconstruction
The detector's dynamic range
Explanation - MTF measures how contrast is maintained at different spatial frequencies.
Correct answer is: The system's ability to preserve contrast across spatial frequencies
Q.91 In CT, increasing the 'tube current' (mA) primarily
Reduces image noise
Reduces patient dose
Improves spatial resolution
Decreases scanning time
Explanation - Higher mA generates more photons, lowering noise.
Correct answer is: Reduces image noise
Q.92 Which of the following best describes the 'noise power spectrum' (NPS) in imaging?
A measure of noise distribution over spatial frequencies
The total amount of noise in an image
The maximum noise level
The ratio of signal to noise
Explanation - NPS quantifies how noise power varies with spatial frequency.
Correct answer is: A measure of noise distribution over spatial frequencies
Q.93 The term 'penetration' in X‑ray physics refers to
How deeply X‑rays can travel into tissue before being absorbed
The ability to penetrate the scanner’s shielding
The depth of the detector array
The rate at which images are reconstructed
Explanation - Penetration depends on photon energy and tissue density.
Correct answer is: How deeply X‑rays can travel into tissue before being absorbed
Q.94 Which of the following is NOT a typical cause of motion artifacts in MRI?
Patient breathing
Heartbeats
Transducer misalignment
Scanner gradient instability
Explanation - Transducers are not used in MRI; motion artifacts come from patient or hardware movement.
Correct answer is: Transducer misalignment
Q.95 In a PET scanner, a 'dead time' correction is necessary because
The detector electronics have a finite processing time after each event
The scanner requires time to cool down between scans
The patient needs rest time between injections
The reconstruction algorithm needs time to converge
Explanation - During dead time, the detector cannot record new events, leading to loss of counts.
Correct answer is: The detector electronics have a finite processing time after each event
Q.96 What is the primary reason for using a 'high‑dose' protocol in CT imaging?
To increase image resolution at the cost of higher dose
To reduce scan time for critically ill patients
To improve contrast in soft tissues for better diagnosis
All of the above
Explanation - Higher dose protocols provide higher SNR, improved resolution, and faster acquisition.
Correct answer is: All of the above
Q.97 Which of the following is a common metric used to assess the spatial resolution of a CT system?
Point spread function
Modulation transfer function
Signal-to-noise ratio
Dynamic range
Explanation - MTF provides a quantitative measure of how contrast is preserved across spatial frequencies.
Correct answer is: Modulation transfer function
Q.98 In ultrasound imaging, 'depth of focus' refers to
The range over which the beam remains in focus
The maximum depth the transducer can image
The time between pulses
The number of pixels per image
Explanation - Depth of focus is the axial distance over which image sharpness is maintained.
Correct answer is: The range over which the beam remains in focus
Q.99 Which of the following is a key determinant of the 'signal' in a PET image?
Injected radiotracer dose
Patient movement
Scanner geometry
Ambient temperature
Explanation - Higher dose increases the number of decay events and thus the signal.
Correct answer is: Injected radiotracer dose