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NMTCB PET practice questions

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Question 1PET instrumentation and physics

A PET detector crystal is struck by a 511 keV photon and emits a burst of visible light that is converted to an electrical pulse. Which property of the scintillator primarily determines how accurately the system can reject scattered photons using the energy window?

  1. Crystal density
  2. Energy resolution at 511 keV Correct answer.
  3. Light output decay time
  4. Index of refraction

Explanation

The energy window accepts only pulses whose measured amplitude falls near the 511 keV photopeak, so the width of that peak (energy resolution) sets how well Compton-scattered events can be excluded. Better energy resolution narrows the accepted window and improves scatter rejection and image contrast.

Question 2PET instrumentation and physics

In a time-of-flight PET scanner, the timing resolution of the detector system most directly improves which aspect of image quality?

  1. Signal-to-noise ratio by localizing the annihilation along the line of response Correct answer.
  2. Accuracy of the attenuation correction map
  3. Spatial resolution of the reconstructed voxel grid
  4. Uniformity of the detector normalization

Explanation

TOF information constrains the annihilation point to a segment of the line of response, so fewer background events contribute to each voxel and noise propagates less. This raises the effective signal-to-noise ratio, especially in large patients, without changing the intrinsic spatial resolution.

Question 3PET instrumentation and physics

A PET detector uses photomultiplier tubes coupled to a scintillator array. Which statement best describes the role of the photocathode?

  1. It converts incident light photons into photoelectrons. Correct answer.
  2. It collects scintillation light and directs it to the anode.
  3. It multiplies electrons through secondary emission at each dynode.
  4. It provides the high-voltage gradient across the tube.

Explanation

The photocathode is the light-sensitive layer that absorbs scintillation photons and ejects photoelectrons through the photoelectric effect. Those electrons are then accelerated and multiplied by the dynode chain before reaching the anode.

Question 4PET instrumentation and physics

Two 511 keV photons from a single annihilation are detected within the coincidence timing window, but one of them has scattered in the patient before reaching the detector. What is the effect on the recorded line of response?

  1. The event is recorded as a random coincidence.
  2. The event is discarded by the energy window in all cases.
  3. The event is mispositioned and degrades image contrast. Correct answer.
  4. The event is correctly localized and improves contrast.

Explanation

A scattered photon changes direction, so the line drawn between the two detectors no longer passes through the true annihilation site. This scatter coincidence deposits counts in the wrong location and reduces contrast and quantitative accuracy.

Question 5PET instrumentation and physics

Which detector material property is most important for achieving high spatial resolution in a modern PET system?

  1. Large crystal volume with high light output
  2. Small crystal cross-section with minimal light sharing between elements Correct answer.
  3. Low atomic number with broad emission spectrum
  4. High density with a long scintillation decay time

Explanation

Spatial resolution depends on identifying which small crystal absorbed each photon, so narrow crystal elements with limited light spread between them sharpen localization. Density and light output affect efficiency and energy resolution but do not by themselves improve resolution.

Question 6PET instrumentation and physics

In PET, what is a random coincidence?

  1. Two photons from one annihilation detected by the same detector block
  2. Two photons from one annihilation detected after scattering in the patient
  3. Two photons from one annihilation detected outside the timing window
  4. Two photons from two separate annihilations detected within the timing window Correct answer.

Explanation

A random coincidence pairs photons from two unrelated annihilations that happen to arrive close enough in time to be accepted. Random rates rise with the square of activity and add uniform background counts across the field of view.

Question 7PET instrumentation and physics

A technologist notices that the random coincidence rate on a scanner increases substantially after a higher-activity injection. How does the random rate behave relative to the singles rate?

  1. It increases linearly with singles rate.
  2. It increases with the square of the singles rate. Correct answer.
  3. It decreases as activity increases.
  4. It remains constant regardless of activity.

Explanation

The chance that two independent photons arrive within the timing window is proportional to the product of the singles rates at the two detectors, so random rate scales as the square of activity. This is why randoms dominate at high count rates and are corrected using the timing window and singles rates.

Question 8PET instrumentation and physics

Which correction in PET reconstruction compensates for photons that are absorbed or scattered out of the patient before reaching the detectors?

  1. Decay correction
  2. Normalization correction
  3. Attenuation correction Correct answer.
  4. Dead-time correction

Explanation

Attenuation correction accounts for the probability that a photon pair is lost or scattered within the body, restoring quantitative accuracy and reducing artifacts. It is derived from transmission or CT data and applied along each line of response.

Question 9PET instrumentation and physics

Why does attenuation correction in PET require a transmission or CT-based map rather than a simple uniform correction factor?

  1. Attenuation is independent of photon energy and tissue type.
  2. Attenuation only affects the emission of the radionuclide.
  3. Attenuation depends on the path length and density of tissue along each line of response. Correct answer.
  4. Attenuation correction is only needed for scatter coincidences.

Explanation

The probability of photon loss depends on the total attenuating material along each line of response, which varies with body contour and tissue density. A measured attenuation map allows line-specific correction, whereas a single factor would misestimate activity in different regions.

Question 10PET instrumentation and physics

A PET scanner reports a spatial resolution of about 4 mm full width at half maximum. What does this value describe?

  1. The coincidence timing window
  2. The smallest distance at which two point sources can be distinguished Correct answer.
  3. The maximum count rate the scanner can handle
  4. The energy window width at 511 keV

Explanation

Spatial resolution expressed as FWHM describes the spread of a point source in the reconstructed image, which relates to the minimum separation needed to distinguish two sources. Smaller FWHM means sharper localization and better resolution.

Question 11PET instrumentation and physics

Which factor most limits the spatial resolution of a PET system that uses thick crystals?

  1. Depth of interaction uncertainty Correct answer.
  2. Energy window width
  3. Radionuclide half-life
  4. Photocathode quantum efficiency

Explanation

In a thick crystal, the photon may interact at different depths, and the assignment of the event to a particular crystal element becomes uncertain, especially away from the center of the field of view. Depth-of-interaction encoding reduces this parallax blur.

Question 12PET instrumentation and physics

What is the purpose of a normalization scan in PET?

  1. To set the energy window for scatter rejection
  2. To determine the radionuclide dose to be injected
  3. To measure and correct for variations in detector efficiency across the system Correct answer.
  4. To measure patient attenuation before emission imaging

Explanation

A normalization scan uses a uniform source to measure the relative sensitivity of each detector pair, and the resulting factors correct for efficiency differences that would otherwise create ring artifacts. It is a calibration step performed independently of the patient.

Question 13PET instrumentation and physics

Which radionuclide property makes fluorine-18 suitable for PET imaging?

  1. It emits a gamma photon directly at 140 keV.
  2. It emits only alpha particles.
  3. It emits a positron with a relatively short range and has a 110-minute half-life. Correct answer.
  4. It emits a positron with a very long range and a 13-second half-life.

Explanation

F-18 decays by positron emission with a half-life of about 110 minutes, long enough for transport and uptake, and its positron range in tissue is short enough to preserve resolution. The emitted positron annihilates to produce the 511 keV photon pair detected by PET.

Question 14PET instrumentation and physics

A PET system uses a coincidence timing window of 6 ns. What is the main consequence of widening this window to 12 ns?

  1. More random coincidences are accepted, degrading image quality. Correct answer.
  2. The spatial resolution of the system improves.
  3. The energy resolution of the detector improves.
  4. More true coincidences are rejected, reducing sensitivity.

Explanation

A wider timing window accepts more accidental pairings of unrelated photons, so the random coincidence rate increases and adds background noise. It may recover a few true events but at the cost of contrast and quantitative accuracy.

Question 15PET instrumentation and physics

Why is a light guide used between a scintillator array and the photomultiplier tubes?

  1. To spread scintillation light so that several tubes can share the signal for position calculation Correct answer.
  2. To absorb scattered photons before they reach the crystal
  3. To provide electrical shielding from external fields
  4. To convert 511 keV photons directly into electrons

Explanation

A light guide distributes scintillation light across multiple photomultiplier tubes so that the relative signals can be used in Anger-type positioning to locate the interaction crystal. This light sharing is essential for accurate event positioning in block detectors.

Question 16PET instrumentation and physics

Which statement correctly describes the annihilation radiation detected in PET?

  1. Two photons of 511 keV are emitted in opposite directions. Correct answer.
  2. Two photons of 140 keV are emitted at 90 degrees.
  3. A single photon of 511 keV is emitted toward the detector.
  4. Two photons of 511 keV are emitted in the same direction.

Explanation

When a positron annihilates with an electron, the mass energy is converted into two photons of about 511 keV emitted nearly back-to-back. The coincidence detection of this opposing pair is the basis of PET imaging.

Question 17PET instrumentation and physics

A detector's measured photopeak is broadened by poor light collection statistics. Which correction or hardware improvement would most directly address this?

  1. Increasing the coincidence timing window
  2. Reducing the number of detector rings
  3. Improving light collection efficiency and coupling between crystal and sensor Correct answer.
  4. Increasing the injected activity

Explanation

Energy resolution depends on the number of photoelectrons collected per event, so better light collection narrows the photopeak and sharpens scatter rejection. Improved coupling and reflective materials increase the collected light yield.

Question 18PET instrumentation and physics

Which of the following best describes a block detector design?

  1. A single large crystal with one photomultiplier tube
  2. A gas-filled chamber that detects ionization from photons
  3. A semiconductor pixel array with direct charge readout
  4. A crystal array coupled to a smaller number of photomultiplier tubes through light sharing Correct answer.

Explanation

A block detector uses a segmented crystal array read out by fewer photomultiplier tubes, relying on shared light and Anger logic to determine which crystal was struck. This reduces cost and complexity while preserving reasonable spatial resolution.

Question 19PET instrumentation and physics

In PET reconstruction, what is the role of the system matrix?

  1. It models the probability that an emission in a voxel is detected along a particular line of response. Correct answer.
  2. It sets the detector high voltage.
  3. It defines the radionuclide decay constant.
  4. It stores the patient's attenuation coefficients.

Explanation

The system matrix encodes the imaging physics, including detector geometry, resolution effects, and normalization, mapping voxel emissions to expected projection data. Reconstruction algorithms use it to recover the activity distribution from measured counts.

Question 20PET instrumentation and physics

Which of the following is a semiconductor detector material used in PET that converts 511 keV photons directly into electron-hole pairs?

  1. Lutetium oxyorthosilicate
  2. Sodium iodide
  3. Bismuth germanate
  4. Cadmium zinc telluride Correct answer.

Explanation

Cadmium zinc telluride is a semiconductor that produces electron-hole pairs when a photon interacts, providing direct conversion with good energy resolution. The other listed materials are scintillators that emit light requiring a separate photodetector.

Question 21PET instrumentation and physics

A patient with a large body habitus is imaged on a PET/CT system. Why does attenuation correction become more critical in this case?

  1. Larger patients have higher detector efficiency.
  2. Larger patients have faster radionuclide decay.
  3. Attenuation correction is only needed for small patients.
  4. More tissue increases the probability of photon loss and scatter, so uncorrected images show greater artifacts and underestimation. Correct answer.

Explanation

A larger body path length increases attenuation and scatter, so without correction the reconstructed activity is underestimated and nonuniform. Accurate attenuation maps preserve quantitative accuracy and contrast in these patients.

Question 22PET instrumentation and physics

Which effect does dead time have on PET image quality at high count rates?

  1. It increases the coincidence timing window.
  2. It causes count loss and can distort measured activity concentrations. Correct answer.
  3. It eliminates all random coincidences.
  4. It improves energy resolution by reducing noise.

Explanation

Dead time is the period after an event during which the detector or electronics cannot process another pulse, so high count rates lead to lost counts and nonlinear response. Correction algorithms compensate for this, but severe dead time can degrade quantitative accuracy.

Question 23PET instrumentation and physics

What is the function of a coincidence processor in a PET scanner?

  1. To reconstruct the image from projection data
  2. To determine whether two detector signals occurred within the timing window and define a valid event Correct answer.
  3. To convert 511 keV photons into visible light
  4. To amplify the photomultiplier signal before digitization

Explanation

The coincidence processor compares the arrival times of signals from opposing detectors and accepts pairs that fall within the timing window. It is the stage that defines which events are recorded as lines of response.

Question 24PET instrumentation and physics

A PET scanner's energy window is set to 425 to 650 keV. What is the primary reason for the lower threshold?

  1. To reduce dead time by accepting more events
  2. To reject low-energy photons that have scattered and lost substantial energy Correct answer.
  3. To increase the timing resolution
  4. To compensate for radionuclide decay

Explanation

Scattered photons deposit less than 511 keV in the crystal, so a lower-level discriminator removes many of them before they create mispositioned coincidences. The window must still be wide enough to accept the photopeak and preserve sensitivity.

Question 25PET instrumentation and physics

Which statement about the positron range is correct?

  1. A longer positron range degrades spatial resolution because annihilation occurs away from the decay site. Correct answer.
  2. Positron range determines the energy window width.
  3. Positron range has no effect on PET resolution.
  4. Positron range improves resolution by spreading activity uniformly.

Explanation

The positron travels some distance before annihilating, so the detected annihilation point is displaced from the original decay location. For high-energy positron emitters, this displacement blurs the image and limits achievable resolution.

Question 26PET instrumentation and physics

In a PET scintillation detector, what is the primary role of the photomultiplier tube (PMT)?

  1. Convert 511 keV photons directly into electron-hole pairs
  2. Provide the high voltage used to accelerate the positron beam
  3. Convert the scintillation light into an electrical signal with gain Correct answer.
  4. Amplify the annihilation photon energy before it reaches the crystal

Explanation

The scintillator emits visible light when it absorbs a 511 keV photon, and the PMT photocathode converts that light into photoelectrons that are multiplied through dynode stages. The resulting electrical pulse carries the timing and energy information the scanner needs.

Question 27PET instrumentation and physics

Which statement describes the annihilation event that produces the two photons detected in PET?

  1. Two 511 keV photons collide inside the crystal and produce a positron-electron pair
  2. A positron decays into two gamma photons with energies that depend on the radionuclide used
  3. A positron and an electron combine and their mass is converted into two 511 keV photons emitted about 180 degrees apart Correct answer.
  4. An excited nucleus emits two gamma photons in cascade after beta decay

Explanation

When a positron slows and encounters an electron, the two particles annihilate and their combined rest mass (2 x 511 keV) appears as two photons. Conservation of momentum sends these photons in nearly opposite directions, which is the basis of coincidence detection.

Question 28PET instrumentation and physics

What does the term 'true coincidence' mean in PET data acquisition?

  1. A coincidence is recorded after scatter correction has been applied to the sinogram
  2. A single photon deposits its energy in two detector elements simultaneously
  3. Two photons from a single annihilation are detected within the timing window and define a valid line of response Correct answer.
  4. Two photons from two separate annihilations arrive within the timing window

Explanation

A true coincidence requires both annihilation photons from the same event to be detected within the coincidence timing window, which localizes the annihilation along the line joining the two detectors. Scattered and random events can also fall in the window but are not true coincidences.

Question 29PET instrumentation and physics

A technologist injects a patient with 18F-FDG and begins the PET acquisition immediately. What is the main problem with this protocol?

  1. The positron range increases with time after injection
  2. The 511 keV photons are emitted only after 2 hours
  3. Biodistribution is incomplete and the target-to-background ratio will be poor Correct answer.
  4. The scanner cannot detect 18F until 60 minutes after injection

Explanation

18F-FDG requires an uptake period, typically about 45 to 60 minutes, for cellular accumulation and for clearance of background activity from the blood pool. Imaging too early yields low target-to-background contrast and a nondiagnostic study.

Question 30PET instrumentation and physics

Which detector material used in PET scintillators is valued for its high light output and fast decay that support time-of-flight imaging?

  1. Cesium iodide doped with thallium (CsI(Tl))
  2. Lutetium oxyorthosilicate (LSO) Correct answer.
  3. Bismuth germanate (BGO)
  4. Sodium iodide doped with thallium (NaI(Tl))

Explanation

LSO has high light output, a short decay time of roughly 40 ns, and good stopping power for 511 keV photons, which supports both good energy resolution and time-of-flight timing. BGO has better stopping power but a much slower decay, and NaI(Tl) has high light output but lower density and slower timing than LSO.

Question 31PET instrumentation and physics

How does the coincidence timing window affect the count rate recorded by a PET scanner?

  1. The timing window determines the spatial resolution of the reconstructed image
  2. A wider window reduces random coincidences and lowers the recorded count rate
  3. The timing window changes the energy of the detected annihilation photons
  4. A wider window accepts more random coincidences and increases the recorded count rate Correct answer.

Explanation

The timing window sets the maximum time difference allowed for two detector events to be paired. Widening it increases the probability that unrelated single events are paired, raising the random coincidence rate and the total recorded count rate.

Question 32PET instrumentation and physics

What is the purpose of the energy window in a PET scanner's pulse processing?

  1. Control the reconstruction filter cutoff frequency
  2. Select the radionuclide to be imaged
  3. Set the coincidence timing tolerance between detectors
  4. Reject events whose deposited energy is inconsistent with 511 keV Correct answer.

Explanation

The energy window accepts pulses within a range around the 511 keV photopeak and rejects lower-energy events that are likely to be scattered photons or noise. This improves image contrast by removing events that would misplace the annihilation site.

Question 33PET instrumentation and physics

In PET, what does the line of response (LOR) represent?

  1. The straight line joining the two detectors that recorded a coincidence event Correct answer.
  2. The central axis of the scanner bore
  3. The path of the positron between emission and annihilation
  4. The trajectory of the CT X-ray tube during attenuation scanning

Explanation

The coincidence electronics define an LOR by the two detector elements that fired within the timing window, and the annihilation is assumed to lie somewhere along that line. All projection data in PET are organized as counts along these lines.

Question 34PET instrumentation and physics

A small lesion is imaged with a PET system that has a reconstructed resolution of about 5 mm FWHM. The lesion's true activity concentration is high but its measured peak value is lower than expected. What is this effect called?

  1. Normalization drift
  2. Dead time effect
  3. Noncollinearity effect
  4. Partial volume effect Correct answer.

Explanation

When a structure is comparable in size to the system resolution, its activity is spread over neighboring voxels and the measured peak underestimates the true concentration. This is the partial volume effect, and it becomes more pronounced as lesion size approaches the FWHM.

Question 35PET instrumentation and physics

Why does a PET scanner require a normalization scan?

  1. To correct for differences in detector efficiency across the ring Correct answer.
  2. To calibrate the injected activity in becquerels
  3. To determine the coincidence timing window
  4. To measure the patient's attenuation coefficient

Explanation

Detector elements and PMT channels have slightly different gains and sensitivities, so a uniform source would not produce uniform counts along all LORs. A normalization scan measures these variations and produces correction factors that equalize detector response.

Question 36PET instrumentation and physics

A PET/CT system uses CT images for attenuation correction. What must be done to the CT data before they can be used for 511 keV attenuation correction?

  1. Convert the CT Hounsfield units directly to activity concentrations
  2. Scale the CT attenuation values from the X-ray energies to 511 keV using a bilinear or similar conversion Correct answer.
  3. Reconstruct the CT images with a PET reconstruction algorithm
  4. Apply a random coincidence correction to the CT projections

Explanation

CT attenuation coefficients are measured at X-ray energies of roughly 70 to 140 keV, so they must be transformed to 511 keV values, typically with a bilinear scaling curve that treats soft tissue and bone differently. The resulting map is then used to correct the emission data.

Question 37PET instrumentation and physics

During a dynamic PET acquisition, the scanner's dead time increases as the bolus arrives. What is the expected effect on the measured activity concentration in the blood pool?

  1. It is overestimated because of pulse pile-up
  2. It is underestimated unless dead time correction is applied Correct answer.
  3. It is unaffected because the blood pool is a large structure
  4. It remains accurate because dead time affects only random coincidences

Explanation

At high count rates, detector and electronic processing time causes events to be lost, so the recorded count rate is lower than the true rate. Dead time correction models this loss and rescales the data; without it, high-activity regions such as the blood pool during the bolus are underestimated.

Question 38PET instrumentation and physics

Which acquisition mode in a modern PET scanner allows the system to record the arrival time of each photon so that coincidences can be paired in software after the scan?

  1. List mode Correct answer.
  2. Step-and-shoot mode
  3. Cine CT mode
  4. Frame mode

Explanation

In list mode, each detected event is stored with its detector position and timestamp, and coincidences are formed later by matching events within the timing window. This permits flexible re-binning into frames and retrospective manipulation of timing and energy criteria.

Question 39PET instrumentation and physics

A technologist observes that the randoms rate is high on a scan acquired with a short coincidence timing window and high injected activity. Which relationship best explains this observation?

  1. The randoms rate increases only when the patient moves during the scan
  2. The randoms rate is independent of the timing window and depends only on the energy window
  3. The randoms rate increases with the product of the singles rates in the two detectors and with the timing window Correct answer.
  4. The randoms rate decreases as the square of the singles rate increases

Explanation

Random coincidences arise when two unrelated single events fall within the timing window, so the rate scales with the singles rates in the two detectors multiplied by the window width. Higher activity raises singles rates, and a longer window increases the chance of accidental pairing.

Question 40PET instrumentation and physics

What is the function of a septal collimator in a two-dimensional (2D) PET acquisition?

  1. Focus the annihilation photons toward the detector crystals
  2. Provide the timing signal for coincidence detection
  3. Convert scattered photons into 511 keV photons
  4. Reduce the detection of photons that originate outside the imaging plane Correct answer.

Explanation

In 2D mode, septa made of tungsten or lead extend between detector rings to restrict the axial acceptance angle, reducing scatter and random events from activity outside the imaged slice. Removing the septa gives 3D mode with higher sensitivity but more scatter.

Question 41PET instrumentation and physics

Which correction addresses the effect of accidental coincidences that are counted during acquisition?

  1. Normalization correction
  2. Attenuation correction
  3. Dead time correction
  4. Delayed-window or model-based randoms correction Correct answer.

Explanation

Randoms correction estimates the accidental coincidence contribution, commonly by using a delayed coincidence window or a model based on singles rates, and subtracts it from the prompt data. Attenuation, dead time, and normalization corrections address different physical effects.

Question 42PET instrumentation and physics

A PET detector block uses a 4x4 array of small crystals coupled to four PMTs, and the position of an event is determined from the relative light distribution among the PMTs. What is this technique called?

  1. Pulse-height discrimination
  2. Anger logic (position-sensitive light sharing) Correct answer.
  3. Coincidence gating
  4. Time-of-flight localization

Explanation

In an Anger-type block detector, the scintillation light is shared among several PMTs, and the centroid of the light distribution computed from the PMT signals identifies which crystal was hit. This allows small crystals to be decoded with fewer photodetectors.

Question 43PET instrumentation and physics

Which statement about the positron range and its effect on PET image quality is correct?

  1. A longer positron range degrades spatial resolution because annihilation occurs away from the emission site Correct answer.
  2. Positron range affects only the timing resolution of the scanner
  3. Positron range is independent of the emitted positron energy
  4. A longer positron range improves spatial resolution by spreading the signal

Explanation

The positron travels some distance in tissue before annihilating, so the detected LOR does not point exactly to the decay site. Higher positron energy means a longer range and greater blurring, which degrades resolution, especially for radionuclides such as 82Rb.

Question 44PET instrumentation and physics

Why does a PET scanner with thicker scintillation crystals have worse spatial resolution than one with thinner crystals?

  1. Thicker crystals have lower stopping power for 511 keV photons
  2. Thicker crystals require a longer coincidence timing window
  3. Thicker crystals emit less scintillation light per absorbed photon
  4. Thicker crystals allow photons to interact at greater depth, causing depth-of-interaction uncertainty along the LOR Correct answer.

Explanation

A 511 keV photon may interact anywhere along the crystal depth, and the detector electronics cannot tell where. For off-center LORs, this depth uncertainty projects into a positional error that widens the line spread function, so thicker crystals improve sensitivity at the cost of resolution.

Question 45PET instrumentation and physics

A PET scanner is operated in 3D mode without septa. Compared with 2D mode, what is the expected trade-off?

  1. Higher sensitivity but increased scatter and random coincidences Correct answer.
  2. Lower sensitivity but improved energy resolution
  3. Lower sensitivity and lower count rate
  4. Higher sensitivity with no change in scatter fraction

Explanation

Removing the septa allows coincidences from a much larger axial acceptance angle, which increases sensitivity but also admits more scattered and random events. The scatter fraction rises, so accurate scatter correction becomes more important.

Question 46PET instrumentation and physics

A technologist performs a daily quality control scan and finds that the photopeak position for 511 keV events has shifted downward. Which component is the most direct cause to investigate?

  1. The reconstruction filter cutoff
  2. The CT tube current
  3. The patient's blood glucose level
  4. The detector gain and the high-voltage or amplifier settings Correct answer.

Explanation

The photopeak position in the energy spectrum depends on the total signal gain from the crystal through the PMT and amplifier electronics. A downward shift suggests reduced gain, such as a drifting PMT high voltage or a failing amplifier, and the energy window may then reject valid events.

Question 47PET instrumentation and physics

Which statement best explains why time-of-flight PET improves image quality in a large patient?

  1. It removes the partial volume effect for small lesions
  2. It eliminates the need for attenuation correction
  3. It increases the number of detected true coincidences by widening the energy window
  4. The arrival time difference localizes the annihilation along the LOR, reducing the effect of noise from the rest of the line Correct answer.

Explanation

In TOF PET, the small difference in photon arrival times estimates where along the LOR the annihilation occurred, so the reconstruction can weight the data to a segment of the line instead of the whole line. This reduces propagation of noise from unrelated activity and improves contrast, especially in large patients.

Question 48PET instrumentation and physics

A small hot sphere is imaged in a water-filled phantom, and the measured activity concentration in the sphere's central voxel is only 60 percent of the true value. Which combination best explains this underestimation?

  1. An energy window that is too narrow
  2. Complete failure of the coincidence processor
  3. Finite spatial resolution causing spill-out combined with attenuation and scatter effects Correct answer.
  4. A timing window that is too wide

Explanation

When a source is small relative to the reconstructed resolution, activity spills into surrounding voxels and the peak value is reduced, which is the partial volume effect. Attenuation and scatter further reduce and redistribute counts, so accurate quantification requires correction of all these effects.

Question 49PET instrumentation and physics

A PET/CT study shows a focal area of increased uptake on the PET images that aligns with a dense metallic implant on CT. Which artifact mechanism is most likely responsible?

  1. Random coincidence pile-up in the implant
  2. Increased radiotracer uptake by the metal itself
  3. Overcorrection of attenuation from the high-density implant in the CT-derived attenuation map Correct answer.
  4. Positron range enhancement inside metal

Explanation

Metal causes beam hardening and streak artifacts on CT, and converting these very high CT values to 511 keV attenuation coefficients can overestimate attenuation along affected LORs. The reconstruction then adds excess activity, producing a false hot spot near the implant.

Question 50PET instrumentation and physics

A scanner's coincidence timing resolution is 500 ps FWHM. What is the approximate uncertainty along the line of response from TOF information alone, given that light travels about 30 cm per nanosecond?

  1. About 7.5 cm Correct answer.
  2. About 15 cm
  3. About 30 cm
  4. About 1.5 cm

Explanation

The time difference corresponding to a position offset d along the LOR is 2d/c, so the position uncertainty is about (c x timing FWHM)/2. With 500 ps and c of roughly 30 cm/ns, the calculation gives 0.5 ns x 30 cm/ns / 2, or about 7.5 cm.