SP17:Positron Emission Tomography

Question 1

Which of the following radionuclides is not a positron emitter?
(a)82Rb

(b) 15O
(c) 18F
(d) 14C
(e) 68Ga

Answer 1

(d) 14C decays by beta emission. All of the other choices listed decay by positron emission and some also by electron capture.

Question 2

When a neutron-deficient nucleus emits a positron:
(a)Atomic mass number decreases by 1.

(b) Atomic mass number increases by 1.
(c) Atomic number increases by 1.
(d) Atomic mass number is unchanged.

Answer 2

(d) The atomic mass number is the number of particles in the nucleus. Because the proton is “converted” to a neutron, the number of protons decreases by one, and the number of neutrons is increased by one, so the net effect is no change to the atomic mass number. The atomic number, which is the number of protons in the nucleus, decreases by one.

Question 3

Which of the following has the shortest half-life?
(a)82Rb

(b) 13N
(c) 15O
(d) 18F

Answer 3

(a) 82Rb has a half-life of 1.3 min, 13N has a half-life of 10 min, 15O has a half-life of 122.2 s, and 18F has a half-life of 109 min.

Question 4

Which of the following will not affect the distribution of 18F-FDG on a PET image?
(a)Intense physical activity the day before imaging

(b) Serum insulin level
(c) Serum glucose level
(d) Bladder catheterization
(e) All of the above affect distribution

Answer 4

(e) Increased physical activity in the days prior to scanning will increase muscle uptake of 18F-FDG. Catheterization can decrease the activity in the pelvis.

Increased glucose level can decrease tumor uptake so many institutions reschedule a patient’s PET scan if their blood glucose level exceeds 200 mg/dL.

Question 5

Normal 18F-FDG distribution would show the least activity in the:
(a)Brain

(b) Bone
(c) Bladder
(d) Myocardium

Answer 5

(b) 18F-FDG will accumulate in the brain, urinary tract, and myocardium. Bone activity can be seen with 18F-NaF.

Question 6

All positron-emitting isotopes are produced in a cyclotron.
(a)True

(b)False

Answer 6

(b) Many positron emitters are cyclotron produced but not all. 82Rb is eluted from generators containing 82Sr.

Question 7

18F-FDG PET is not an important tool for:
(a)Restaging of colorectal cancer

(b) Monitoring response to treatment of non-Hodgkin’s lymphoma
(c) Detecting Helicobacter pylori
(d) Imaging metastases in breast cancer

Answer 7

(c) 18F-FDG PET is used for the indications in choices (a), (b), and (d). Detecting H. pylori is accomplished using a radioactive carbon breath test.

Question 8

Which of the following is true regarding 18F-FDG
(a)Its distribution in the brain is related only to blood flow

(b) It is a potassium analog
(c) It has a half-life of 60 min
(d) It is taken up by disease-free myocardium

Answer 8

(d) The normal distribution of 18F-FDG in the brain is related to both blood flow and metabolic rate, with gray matter showing the greatest uptake. It is a glucose analog, has a half-life of 109 min, and is taken up by normal myocardium.

Question 9

18F-FDG PET images show a map of the _____ distribution in the body.
(a)Oxygen

(b) Insulin
(c) Glucose
(d) Potassium

Answer 9

(c) Because FDG is a glucose analog, this study shows a map of glucose distribution.

Question 10

What length of time should a lactating female who undergoes 18F-FDG PET scanning wait before resuming breast-feeding?
(a)4 h

(b) 24 h
(c) 1 week
(d) Breast-feeding should be discontinued

Answer 10

(b) With a physical half-life of 109 min and a biologic half-life of 6 h, the effective half-life for 18F is approximately 1.4 h. Nonetheless, many imaging departments recommend that nursing mothers refrain from nursing for 24 h following a PET study with this isotope, although this varies greatly among labs. Some laboratories recommend as little as 6-h interruption after imaging, so it is completely eliminated from the body in 14 h.

Question 11

Because of the relatively high energy of the photons detected in PET imaging, attenuation correction does not need to be performed.
(a)True

(b)False

Answer 11

(b) Despite the higher energy of annihilation photons relative to other photons detected in nuclear medicine imaging, attenuation correction is even more important in PET because it requires detection of both annihilation photons from a single decay. Because these photons are oppositely directed, the pair of annihilation photons must pass through the entire width of the patient to reach the detector ring. Attenuation correction can be accomplished using a transmission scan from either a sealed source or a CT, which measure attenuation through all lines of response.

Question 12

What part of a PET scanner quality control regimen is necessary for the computation of attenuation factors?
(a)Coincidence timing calibration

(b) Normalization correction
(c) PMT gain adjustment
(d) Blank scan

Answer 12

(d) A blank scan is performed using a transmission source in an empty field of view. This is used to monitor system stability and is also needed along with the transmission scan to perform attenuation correction. In CT systems a blank scan is often referred to as an air cal.

Question 13

Which of the following describes a random coincidence?
(a)The simultaneous detection of annihilation photons originating from a single positron

(b) The simultaneous detection of annihilation photons originating from a single positron although one of the photons has been scattered before reaching the detector
(c) The simultaneous detection of photons originating from different positrons

Answer 13

(c) The simultaneous detection of annihilation photons originating from a single positron is called a true coincidence. The simultaneous detection of annihilation photons after one (or both) of the photons has undergone Compton scatter is called a scatter coincidence. Because Compton-scattered photons change direction, this type of coincidence will be assigned to an incorrect line of response (LOR). The resulting mispositioned events decrease image contrast. The simultaneous detection of photons originating from different positrons is called a random coincidence, and the randoms rate increases as the square of the amount of activity present in the FOV. This effect must be corrected for in order to obtain quantitative data.

Question 14

Which of the following is NOT a scintillation crystal found in PET cameras:
(a)Lutetium oxyorthosilicate (LSO)

(b) Bismuth germanate (BGO)
(c) Lead sulfate (PbSO4)
(d) Gadolinium oxyorthosilicate (GSO)

Answer 14

(c) Scintillation crystals that have been used in PET cameras include NaI(Tl), LSO, BGO, and GSO. Lead sulfate, PbSo4, is the white powder often seen on the electrodes of car batteries.

Question 15

PET images may be reconstructed using:
(a)Coincidence detection

(b) Filtered a backprojection
(c) K-space filling
(d) Block detection

Answer 15

(b) Reconstruction of PET images can be performed using filtered backprojection or iterative reconstruction schemes. Coincidence detection refers to the process by which photons are accepted or rejected as originating from a single annihilation event. K-space filling is a part of MRI reconstruction. Block detection refers to the division of scintillators into separate channels using material that will not transmit light.

Question 16

Following a positron annihilation, two 511 keV photons are emitted in opposite directions. If one of the photons is deflected in the body:
(a)Only the unscattered photon will contribute to the image

(b) The deflected photon may be detected outside the correct LOR
(c) The LOR may contain an angle
(d) The deflected photon will have a higher energy than the undeflected photon

Answer 16

(b) The deflected photon can be detected in a position that changes the assignment of the LOR. Choice (a) is not correct because an unpaired photon will not contribute to the image. Lines of response are lines and do not contain angles. In Compton scatter, the deflected photon transfers (loses) some of its energy to an electron. Therefore, a deflected photon’s energy is always lower than that of an undeflected photon.

Question 17

A patient has undergone myocardial PET scans using 13N ammonia and 18F-FDG. There is little inferoapical activity on the ammonia scan, but there is homogeneous uptake in the same area on the FDG images. This suggests that:
(a)An incorrect dose of 13N has been administered

(b) The inferoapical area may revascularize
(c) There is little viable tissue in the inferoapical wall
(d) The system requires a quality control check

Answer 17

(b) 13N ammonia can be used to image myocardial perfusion, and 18F-FDG can be used to assess myocardial metabolism. In ischemia, the myocardial metabolism of glucose increases, relative to fatty acids, so one would expect to see increased uptake of 18FDG in an ischemic area.

Question 18

Patient A and Patient B are injected with the same dose of 13N ammonia for a myocardial PET scan. The patients are the same height, but Patient A weighs 120 lbs while Patient B weighs 180 lbs. From which patient would more true coincidences be detected?
(a)Patient A

(b)Patient B

Answer 18

(a) Because there will be less attenuation, more true coincidences will be detected.

Question 19

Chewing gum between injection of 18F-FDG and PET scanning will have no effect on the resulting images.
(a)True

(b)False

Answer 19

(b) Any activity that utilizes a muscle has the potential to cause an increase in uptake of FDG in that muscle.

Question 20

Why is it important to know if a patient has an inflammatory condition before performing an 18F-FDG PET scan?
(a)18F-FDG and certain anti-inflammatory medications cause severe reactions when used simultaneously

(b) 18F-FDG can exacerbate inflammatory conditions
(c) Some inflammatory conditions cause areas of increased uptake of 18F-FDG

Answer 20

(c) Areas of inflammation often show increased activity on 18F-FDG images.

Question 21

Patient preparation for an 18FDG-PET scan of the whole body includes:
(a)NPO the day before imaging

(b) Fasting state of at least 4-6 h prior to injection
(c) Encouraging intense physical activity the day before imaging

Answer 21

(b) Dietary instructions prior to imaging vary greatly, but the minimum necessary fast is agreed by most to be 4 h prior to injection. Avoiding intense physical activity is also recommended for the days prior to imaging.

Question 22

18F-FDG is useful for imaging tumors because:
(a)It is a glucose analog

(b) There is an elevated rate of glycolysis in tumors
(c) It becomes trapped in tumor cells
(d) All of the above

Answer 22

(d) 18F-FDG is taken up by tumors according to their glycolytic rate, which is often higher than that of normal tissues, and once localized in tumor cells, it is phosphorylated and so remains there.

Question 23

Which dose and route of administration are the best choices when performing a whole body PET scan with 18F-FDG and a dedicated PET scanner?
(a)2-4 mCi IV

(b) 2-4 mCi IM
(c) 10-20 mCi IV
(d) 10-20 mCi IM
(e) 30-35 mCi IV

Answer 23

(c) The dose recommended by the Society of Nuclear Medicine Procedure Guideline for whole-body FDG PET scanning is 10-20 mCi injected intravenously (370 to 740 MBq).

Question 24

Fill in the blank. High blood glucose levels may _____ 18F-FDG uptake in tumors.
(a)Decrease

(b)Increase

Answer 24

(a) Increased glucose level can decrease tumor uptake so many institutions reschedule a patient’s PET scan if their blood glucose level exceeds 200 mg/dL.

Question 25

Relative to one another, 82Rb and 82Sr are:
(a)Isobars

(b) Isotopes
(c) Isotones
(d) None of the above

Answer 25

(a) Isobars are atoms of different elements; they have different numbers of protons but have the same mass number (number of protons plus neutrons). Isotopes are members of the same element, so they have the same number of protons but have differing numbers of neutrons. Isotones have the same number of neutrons.

Question 26

What determines the lower limit of spatial resolution in PET scanners?
(a)Timing window

(b) Detector type
(c) Positron range
(d) Compton scatter

Answer 26

(c) The positron range is the distance that he positron travels before undergoing annihilation with an electron. The distance is radionuclide dependent and is, on average, 0.6 mm in water for 18F and 5.9 mm in water for 82Rb. Because PET scanners detect the products of positron annihilation rather than the positron itself the line of response will be assigned a slight distance away from the actual site of positron emission.

Question 27

Parenteral nutrition should be discontinued for several hours prior to 18FDG imaging.
(a)True

(b)False

Answer 27

(a) Regardless of how a patient receives his or her nutrition, increased glucose level may decrease tumor uptake because glucose will compete with FDG for cellular uptake. Of course, the decision to remove nutrition is up to the clinician.

Question 28

Two unrelated annihilation photons are detected within the timing window and judged to be in coincidence. This is called a:
(a)True coincidence

(b) Random coincidence
(c) Scatter coincidence

Answer 28

(b) A random coincidence occurs when two annihilation photons from different annihilations are detected within the coincidence timing window, and treated as a true coincidence. A true coincidence refers to the detection of two annihilation photons from a single annihilation event. A scatter coincidence is the term for detection of two annihilation photons from a single annihilation event, where one or both photons has undergone Compton scatter, resulting in a change in direction.

Question 29

Noncolinearity refers to:
(a)Annihilation photons that are not emitted at exactly 180 to one another due to particle momentum

(b) The detection of two annihilation photons, at least one of which has been redirected by Compton scatter
(c) The distance a positron travels before undergoing annihilation

Answer 29

(a) The angle difference from 180 is typically +- 0.25% and negatively affects spatial resolution. The effect is increased with increasing diameter of the scanner.

Question 30

On an 18F-FDG-PET scan of the whole body, muscle uptake can be decreased by:
(a)Administering diazepam before injection of 18F-FDG

(b) Maintaining the patient in the supine position during uptake
(c) Minimizing chewing, talking, and swallowing during uptake
(d) All of the above
(e) (a) and (b) only

Answer 30

(d) Muscle uptake may be the result of muscle contraction during the uptake of 18F-FDG, so premedicating the patient with diazepam (or other anxiolytic drug), keeping the patient supine and relaxed, and minimizing the use of facial and neck muscles can decrease this uptake. Muscular uptake in the neck can be difficult to differentiate from lymph nodes.

Question 31

Dedicated PET scanners use:
(a)Flat-field collimation

(b) Electronic collimation
(c) Converging collimation
(d) No collimation

Answer 31

(b) Electronic collimation for PET means that pairs of photons striking the detector ring within a short timing window are assumed to come from the same positron decay, so the event is positioned by drawing a straight line (line of response) between the two detectors.

Question 32

Standardized uptake values are useful for:
(a)Differential diagnosis of tumor type

(b) Assessing tumor size changes
(c) Assessing response to treatment

Answer 32

(c) Standardized Uptake Value (SUV) is the measured activity concentration divided by the average activity concentration in the patient’s body, commonly calculated as the administered activity divided by body weight. The measured activity concentration can be obtained from a single voxel, as in the case of Maximum SUV (SUV max), or from a volume of interest (VOI), as in the case of Average SUV or Peak SUV. Comparing SUVs in pre- and post- treatment images may show response to therapy.

Question 33

A patient presents for PET scanning with 18FDG for suspected tumor of the left axilla. Which injection site should be avoided?
(a)Right antecubital vein

(b) Left antecubital vein
(c) Right foot
(d) Left foot

Answer 33

(b) If any of the dose is extravasated, it will be cleared by the lymph nodes. This may cause axillary uptake to complicate scan interpretation. Hence, the injection site should be contralateral to the area in question or in the foot.