SP4:Instrumentation and Quality Assurance

Question 1

How often should the COR check be performed?
(a)At least weekly

(b) Before each patient
(c) Every 6 months
(d) At least monthly

Answer 1

(a) COR stands for center of rotation. This should be checked at least weekly and preferably each day that tomography will be performed to ensure that the axis of rotation is in the center of the computer matrix for all projections and should be determined for each collimator used. While the computer can correct for misalignments, if the COR needs frequent adjustment, service may be needed.

Question 2

The abbreviation COR stands for:
(a)Circle of rotation

(b) Center of region
(c) Correction of rotation
(d) Center of rotation

Answer 2

(d) COR stands for center of rotation

Question 3

What is the minimum number of counts that should be obtained in a uniformity correction flood for a SPECT camera?
(a)10 K

(b) 10 million
(c) 30 million
(d) 60 million

Answer 3

(c) Field nonuniformity can cause artifacts in images, and this is a bigger issue in SPECT than in planar imaging because image reconstruction will amplify nonuniformities. While 3–5% nonuniformity is acceptable for planar imaging, SPECT requires 1% or less. Thirty million counts are needed for a flood image with a matrix of 64 × 64. If the matrix is 128 × 128, 120 million counts are required. Flood fields should be made according to the imaging parameters of the exam which will be performed using the system.

Question 4

Which of the following collimators will magnify an image?
(a)Flat field

(b) Diverging
(c) Converging
(d) High resolution

Answer 4

(c) The converging collimator magnifies the area imaged; the diverging collimator minifies it.

Question 5

Which of the following must be performed every 6 months at minimum?
(a)COR

(b) Sealed source leak test
(c) Linearity of the dose calibrator
(d) Area survey of hot lab

Answer 5

(b) Sealed sources must be leak checked during an inventory conducted every 6 months. Leak checks are made by wipe tests of the sealed sources.

Question 6

A dose calibrator must undergo repair or replacement if accuracy or constancy errors are:
(a)Present

(b) Greater than 5%
(c) Greater than 10%
(d) Greater than 20%

Answer 6

(c) According to the NRC, dose calibrators with accuracy or constancy errors >10% must be repaired or replaced.

Question 7

Accuracy of a dose calibrator must be checked upon installation and then:
(a)Monthly

(b) Quarterly
(c) Semiannually
(d) Annually

Answer 7

(d) Accuracy testing measures the ability of the dose calibrator to assay radionuclides of different energies and should be performed upon installation and annually at minimum. The NRC requires that records be kept for 3 years

Question 8

Linearity of a dose calibrator must be checked upon installation and then:
(a)Monthly

(b) Quarterly
(c) Semiannually
(d) Annually

Answer 8

(b) Linearity is the ability of the dose calibrator to detect a range of activities. This can be performed over 1 24-h period, using measurements and decay calculations for the isotope, or can be performed quickly using lead shields with a known attenuation. A linearity test should be performed at installation and assessed quarterly thereafter. The NRC requires that records be kept for 3 years.

Question 9

Constancy of a dose calibrator must be tested:
(a)Daily

(b) Quarterly
(c) Weekly
(d) Every 6 months

Answer 9

(a) Constancy is the ability of the dose calibrator to reproduce measurements of a source over time and should be checked daily. The NRC requires that records be kept for 3 years.

Question 10

A technologist wishes to evaluate the intrinsic uniformity of a camera used for planar imaging and is preparing a dose of 99mTc. How much activity is sufficient for the image?
(a)2 mCi

(b) 200 uCi
(c) 20 mCi
(d) 10 mCi

Answer 10

(b) Because the collimator is removed for intrinsic uniformity checking, only a small amount of 99mTc is needed. Field uniformity should be checked daily.

Question 11

The advantages of using a 57Co sheet source over a fluid-filled flood source with 99mTc added include:
(a)It does not have to be prepared each day

(b) Lower cost
(c) Shorter half-life
(d) Higher energy

Answer 11

(a) The cobalt sheet source is more expensive and has a longer half-life, but it does not require mixing as does the fluid-filled source. Insufficient mixing will result in suboptimal flood field images not related to uniformity, and there is also the possibility of contamination with this method.

Question 12

Geometric dependence of a dose calibrator must be checked:
(a)At installation

(b) Daily
(c) Quarterly
(d) Annually

Answer 12

(a) Geometry testing is performed by measuring the same amount of activity in varying volumes. This should be checked upon installation of the dose calibrator, and the records should be kept for 3 years.

Question 13

What happens to image resolution as the distance between a patient and a parallel-hole collimator decreases?
(a)Improves

(b) Worsens
(c) Does not change

Answer 13

(a) When the distance between the patient and collimator is decreased, the system resolution improves, not only for parallel-hole collimators but also for diverging, converging, and pinhole collimators.

Question 14

What happens to the image size as the distance between a patient and a parallel-hole collimator decreases?
(a)Increases

(b) Decrease
(c) Does not change

Answer 14

(c) The resulting image will not change in size with changing patient to collimator distance, but resolution will improve.

Question 15

What part of an imaging system emits light when it had absorbed a photon?
(a)Photomultiplier tube

(b) Pulse height analyzer
(c) Scintillation crystal
(d) Collimator

Answer 15

(c) When a gamma ray interacts with a scintillation crystal such as a thallium-activated sodium iodide, thousands of electrons are promoted to higher orbits. As they relax and fall back to their original positions, their excess energy is released as light.

Question 16

Most of the photons emitted from the radiopharmaceutical which has been administered to a patient:
(a)Do not contribute to the final image

(b) Are received by the sodium iodide crystal
(c) Are converted to a voltage signal
(d) Are in the visible spectrum

Answer 16

(a) Many emitted photons will be attenuated in the patient, deflected away from detector, or absorbed in the collimator, so most photons emitted do not contribute to the final image.

Question 17

To obtain high-resolution images of a small organ, gland, or joint, which collimator will be most useful?
(a)Low-energy parallel hole

(b) Flat field
(c) Diverging
(d) Pinhole

Answer 17

(d) A pinhole collimator produces high-resolution images but suffers form low sensitivity because a very low percentage of the photons incident on the collimator will pass through the pinhole.

Question 18

Which instrument should be used to determine the location of a 99mTc spill?
(a)Geiger-Muller (GM) survey meter

(b) Portable ionization chamber
(c) NaI well counter
(d) Pocket dosimeter

Answer 18

(a) Geiger-Muller (GM) survey meters are more sensitive than ionization chamber survey meters by a factor of ~10. This allows them to detect spills with very low levels of contamination.

Question 19

A technologist covers the collimator (which is facing the ceiling) with absorbent paper and obtains a uniformity image using a liquid flood source with added 99mTc. A round cold spot is seen on the image. Subsequently the technologist removes the collimator, turns the camera face down, and obtains another image using a point source placed on the floor. This image appears uniform. What is the most likely cause of the cold spot on the first image?
(a)Collimator defect

(b) Decoupled photomultiplier tube
(c) Subtle crystal crack
(d) Improper energy peaking

Answer 19

(b) The PM tube may have slipped out of position when the detector faced upward and dropped back into place when the detector head was face down, causing a defect that was only observable in the first image.

Question 20

If two photons strike a sodium iodide crystal at the same time, what will occur?
(a)Neither event will trigger production of light.

(b) The system will perceive only one event which will contain the summed energy of both photons.
(c) Both events will be correctly perceived.
(d) Only the photon closer to the center of the crystal will be perceived.

Answer 20

(b) When photons interact with the crystal simultaneously, or nearly simultaneously, they may be perceived as a single event with the energies summed.

Question 21

The time after a crystal has absorbed a photon before it is able to respond to another event is called:
(a)Count rate

(b) Distortion
(c) Dead time
(d) Sensitivity

Answer 21

(c) Dead time is the time required after the absorption of a photon before the next photon can be detected.

Question 22

The purpose of the photomultiplier tube is:
(a)To keep any of the electrical signal which resulted from scatter or background form contributing to the final image

(b) To convert light into an electrical signal and to magnify that signal
(c) To convert radioactivity into light
(d) To filter out photons which strike the crystal from oblique angles

Answer 22

(b) The amount of light produced in the crystal is small and is converted into an electrical signal and multiplied in the photomultiplier tube.

Question 23

The purpose of the pulse height analyzer is:
(a)To minimize the amount of scattered radiation in the final image

(b) To convert light into an electrical single and to magnify that signal
(c) To convert radioactivity into light
(d) To filter out photons which strike the crystal from oblique angles

Answer 23

(a) In a scintillation camera, the pulse height analyzer determines the energy that each gamma ray has deposited in the crystal by measuring the summed output of all photomultiplier tubes (known as the Z signal). This information is used to limit the range of photon energies accepted into the final image, so that fewer Compton-scattered photons will be included.

Question 24

A camera with three-pulse height analyzers will be most useful for imaging:
(a)99mTc

(b) 67Ga
(c) 111In
(d) 133 Xe

Answer 24

(b) Multiple PHAs will be useful for imaging radionuclides with multiple peaks. 67Ga has four photopeaks, 99mTc and 133Xe each has one photopeak, and 1111In has two.

Question 25

A technologist has set a 15% symmetric window for a 140 keV photopeak, What will happen to a 158 keV signal?
(a)It will be accepted by the pulse height analyzer.

(b)It will be rejected by the pulse height analyzer.

Answer 25

(b) A symmetric window will be divided equally above and below the photopeak. Fifteen percent of 140 keV means that 21 keV around the photopeak will be accepted, 10.5 keV above and 10.5 keV below. The 158 keV photon is 18 keV above the photopeak and will be rejected.

Question 26

A typical energy spectrum of 99mTc from a patient contains a broad peak around 90-140 keV that is not present on the energy spectrum from a point source of the same isotope in air. This represents:
(a)The iodine escape peak

(b) The signal from 99mTc
(c) Lead x-ray peak
(d) Compton scatter

Answer 26

(d) Compton scatter occurs when a photon interacts with a loosely bound outer-shell electron. The photon transfers some of its energy to the electron and is deflected in a new direction with lower energy. The more electrons that a photon encounters, the more likely it is that this process will occur. Therefore, photons traveling through air rarely experience Compton scattering, while photons traveling through patients do.

Question 27

Which of the following can be measured in millimeters?
(a)Energy resolution

(b) Spatial resolution
(c) Field uniformity
(d) Temporal resolution

Answer 27

(b) Spatial resolution refers to the ability of a system to discriminate between two structures located close to one another and is typically measured in millimeters. Phantoms containing either lead bars that may vary in thickness and distance from one another or multiple holes in lead sheets are used to assess spatial resolution.

Question 28

The energy resolution of an image is better with a _____ energy isotope than with a _____ energy isotope
(a)Higher, lower

(b)lower, higher

Answer 28

(a) Energy resolution, like intrinsic spatial resolution, is primarily dependent on statistical fluctuations in the distribution of photons among photomultiplier tubes from one scintillation event to the next. Higher-energy gamma rays deposit more energy in the scintillation crystal, producing more secondary photons. With more photons, there is lower statistical fluctuation in their distribution (per Poisson statistics), so energy resolution is improved.

Question 29

Of the following types of transmission phantoms, which one requires the fewest images to test the spatial resolution over the entire detector face?
(a)Hine-Duley phantom

(b) Four-quadrant bar phantom
(c) Parallel-line equal-space phantom
(d) Orthogonal-hole phantom

Answer 29

(d) The orthogonal-hole phantom contains a lead sheet with holes in it and requires only one image to assess resolution. The other phantoms contain lead bars and spaces and require multiple images to assess resolution across the entire detector.

Question 30

According to the curve for 137Cs shown in Fig. 1, what is the percent energy resolution of the system? (Maximum count 11,000 at 662 keV)(FWHM 5500 count at 701-623 keV, 78 keV)
(a)6.9%

(b) 8.2%
(c) 10.5%
(d) 11.8%

Answer 30

(d) The maximum count rate is 11,000, so full width at half maximum is the width at 5500 counts or 78 keV (701-623 keV).
% energy resolution = (FWHM/radionuclide photopeak) x 100. Applying the formula, energy resolution is found to be [(701 keV - 623 keV)/662 keV] x 100 = 11.8%.

Question 31

An orthogonal-hole phantom cannot be used to test spatial linearity since it does not contain lead bars.
(a)True

(b)False

Answer 31

(b) The orthogonal-hole phantom, although it does not contain lead bars, can still be used to assess linearity because holes are arranged in parallel lines.

Question 32

When using a 150 uCi source of 99mTc to check uniformity, a technologist measures the counts in 1 min as 53,125. If the background in the room is 405 cpm, what is the sensitivity of the system?
(a)351 cpm/uCi

(b) 354 cpm/uCi
(c) 356 cpm/uCi
(d) 350 cpm/uCi

Answer 32

(a) Sensitivity is expressed as counts per time per mCi. The background count rate must be subtracted from the measured count rate before dividing by the number of uCi.

Question 33

On Monday the extrinsic uniformity flood for a gamma camera has a uniformity of 3%. On Tuesday the uniformity is 8%. The technologist then verifies that the photopeak is still centered in the energy window. What should be the next step?
(a)Scan a bar phantom instead.

(b) Call for service.
(c) Remove the collimator and obtain an intrinsic uniformity flood.

Answer 33

(c) If removing the collimator and obtaining an intrinsic flood results in a significantly lower uniformity %, the problem could be either a damaged collimator or a poorly mixed tank source.

Question 34

Why might a technologist use an asymmetric energy window around a photopeak?
(a)To include other emissions

(b) To exclude edge packing
(c) To exclude Compton scatter
(d) To increase the count rate

Answer 34

(c) The asymmetric window can be used to exclude photons that have undergone Compton scatter.

Question 35

What energies are included if a 20% symmetric window is used for the 364 keV photopeak of 131I?
(a)191-437

(b) 328-400
(c) 337-391
(d) 344-384

Answer 35

(b) A symmetric window will be divided equally above and below the photopeak.

Question 36

The image acquired as a SPECT camera rotates around a patient are called:
(a)Reconstructions

(b) Arcs
(c) Projections
(d) Azimuth stops

Answer 36

(c) Images obtained are called projections; these can be processes to create reconstructions. The detector head rotates in an arc, and the motion may be interrupted by azimuth stops during which imaging is performed.

Question 37

With a matrix size of 64 x 64 and two sources placed 30 cm apart, there are 44 pixels between the activity peaks on the activity profiled that is generated. What is the pixel size?
(a)6.8 cm/pixel

(b) 6.8 mm/pixel
(c) 1.4 mm/pixel
(d) 0.14 cm/pixel

Answer 37

(b) The distance between the sources is divided by the number of pixels between the activity peaks to find 0.681 cm or 6.8 mm.

Question 38

What steps should be taken to reduce a star artifact from reconstructed SPECT images?
(a)Decrease the matrix size

(b) Increase the time per projection
(c) Increase the radius of projection
(d) Increase the number of projections

Answer 38

(d) Increasing the number of projections will reduce the star artifact. This problem was largely solved by systems that use filtered as opposed to simple back projection.

Question 39

The _____ method of SPECT acquisition requires slightly more time but has less blurring than the _____ method.
(a)Step and shoot, continuous rotation

(b)Continuous rotation, step and shoot

Answer 39

(a) In step and shoot mode data acquisition stops while the camera head(s) move to the next azimuthal angle. In continuous mode the camera(s) rotate continuously while data is acquired continuously, so data acquisition takes less time. However, the continuously-acquired data must be interpolated back into azimuthal steps, and that results in slightly worse spatial resolution. Or, to phrase it another way, step and shoot mode has better spatial resolution because there is no blurring caused by camera rotation during acquisition.

Question 40

In SPECT imaging, if the image matrix size is increased from 64 x 64 to 128 x 128, which one of the following is true?
(a)Total imaging time may need to be increased.

(b) Spatial resolution will decrease.
(c) Image reconstruction will be faster
(d) Image file size will decrease.
(e) Count rate will decrease

Answer 40

(a) Increasing the matrix size divides the same physical space into smaller “chunks” or picture elements (pixels). Therefore, spatial resolution should increase. However, because a 128 x 128 matrix has 4x more pixels than a 64 x 64 matrix, for the same imaging time each of its pixels will contain only 1/4 of the counts, resulting in noisier images due to Poisson statistics. Increasing the imaging time helps counteract this effect. The object that is emitting photons (the patient) is unchanged, so the count rate will not change.

Question 41

If the distance between two point sources placed on the camera surface is 35 cm in both the x and y directions and the number of pixels between their activity profile peaks is 52 in the x-axis and 55 in the y-axis, are the pixel dimensions in the x and y direction within 0.5 mm of one another?
(a)Yes

(b)No

Answer 41

(a) The pixel size for x is 6.73 mm/pixel (35 cm/52 pixels), and the size in the y direction is 6.36 mm/pixel, so they are within 0.5 mm of one another; this is critical for volume calculations to be considered accurate.

Question 42

A technologist measures a standard of known activity in the dose calibrator three times. She subtracts background form each measurement and compares these measurements to the decay-corrected activity of the standard. This was a test of:
(a)Constancy

(b) Linearity
(c) Accuracy
(d) Geometric dependence

Answer 42

(c) This is a partial test of accuracy. Accuracy is the ability of the system to measure samples with different energies, so at least two different isotopes must be measured.

Question 43

Records of dose calibrator checks must be kept for:
(a)3 years

(b) 5 years
(c) 10 years
(d) As long as the license is valid

Answer 43

(a) Quality control records for dose calibrators should be kept for 3 years

Question 44

In PET imaging, what is the outcome of imaging positrons with a long range of travel before undergoing annihilation
(a)Increased rate of random detection

(b) Increase in streaking artifacts
(c) Mispositioning where the radioactivity is located
(d) Improved image resolution

Answer 44

(c) Mispositioning where the radioactivity is located. A positron with a long range can travel millimeters away from the site of the radioactive decay that produced it. A PET camera can only detect where a positron annihilates, not where it originated.

Question 45

Survey meters must be checked for proper operation against a sealed source:
(a)Before each day of use

(b) Weekly
(c) Monthly
(d) Every 6 months

Answer 45

(a) Before using a GM meter each day the user should perform: (1) battery test - check that needle goes up into region marked BAT TEST, (2) constancy test - hold the detector up against a check source and compare the measurement (minus background) to the expected value.

Question 46

Which of the following would be used to perform a survey of an area in which a small amount of radioactivity is expected to be present?
(a)Ionization chamber

(b) Geiger-Muller counter
(c) Well counter
(d) Single probe counting system

Answer 46

(b) Geiger-Muller meters are designed to detect small amounts of contamination. Ionization chambers are not sensitive enough to use for contamination monitoring.

Question 47

An ionization chamber with a relatively low applied voltage from the anode to the cathode has a low efficiency and is therefore best for measuring:
(a)Low exposure rates

(b) High exposure rates
(c) Background radiation
(d) Absorbed dose

Answer 47

(b) Because of the low voltage, there is no avalanche effect, and therefore no dead time.

Question 48

Most well counters measure radioactivity by the use of:
(a)An applied voltage from anode to cathode

(b) A sodium iodide crystal
(c) A silver halide layer
(d) Thermoluminescent crystals

Answer 48

(b) Thermoluminescent crystals are used for monitoring radiation exposure to occupational personnel. The Geiger-Muller counter uses an applied voltage from anode to cathode. Silver halide coats the film often used in nuclear medicine departments.

Question 49

When measuring samples in a well counter, which one of the following is true?
(a)Sample volumes should be large.

(b) Samples should have high radioactivity.
(c) Sample geometry should be consistent.
(d) Sample containers don’t affect the measurement.

Answer 49

(c) Well counters best measure samples containing small amounts of radioactivity and are useful for counting in vitro studies, wipe tests, etc. Samples should have identical geometry, i.e., the volumes, and containers used should be identical for all samples.

Question 50

A dose calibrator is received in the nuclear medicine department following repair. A linearity test using 99mTc is performed on the dose calibrator, and the results are as follows:
33.9 mCi - 17Jan 0800
30.3 mCi - 17Jan 0900
9.2 mCi - 17Jan 1900
2.3 mCi - 18Jan 0700
0.5 mCi - 18Jan 2000
0.2 mCi - 19Jan 0300
What is the next step the technologist should take?
(a)Send the instrument for further repair.

(b) Accept the instrument for use.
(c) Reject the instrument for use.
(d) Change the battery.

Answer 50

(b) One must calculate the expected activity for each time point and compare that with the measured activity. The activity measured over time, or using shielding techniques, should agree with expected activity (calculated using decay factors) to within 10%. Decay factors for 99mTc for the time points in the table are 0.891 (at 1 h), 0.282 (at 11 h), 0.071, 0.016, and 0.009.

Question 51

A check source is calibrated for June 13, 2010, at noon and contains 145 uCi of 137Cs (T1/2 = 30 years). If a dose calibrator is used to measure that dose on June 13, 2016, what is the range of acceptable activities to prove that the accuracy is within 10% (6-year decay factor is 0.871)?
(a)113-139 uCi

(b) 119-125 uCi
(c) 120-133 uCi
(d) 125-140 uCi

Answer 51

(a) Multiplying the original activity by the decay factor, one finds that expected activity is 126 uCi; 10% of the expected activity must be added to and subtracted from the expected activity to find the range.

Question 52

1 ml of a solution is assayed and contains 206 mCi. If the solution is diluted to 9 ml and the activity is now 170, what is the geometric correction factor?
(a)0.65

(b) 1.2
(c) 10.9
(d) 18

Answer 52

(b) The true activity (measured in the smallest volume) is divided by the measured activity to find the correction factor.

Question 53

If the energy range for 137Cs at half maximum ranges from 623 keV to 701 keV, what is the percent energy resolution
(a)7.5%

(b) 10.8%
(c) 11.8%
(d) 13.5%

Answer 53

(c) Energy resolution is the ability of the system to reliably convert light to an electrical impulse, and it should be assessed annually. It requires a graph of the counts vs. the energy spectrum and knowledge of the full width of the surve at half maximum. It can be calculated using the following formula: % energy resolution = (FWHM/radionuclide photopeak) x 100. Applying the formula, energy resolution is found to be [(701 keV - 623 keV)/662 keV] x 100 = 11.8%.