Positron-Emission Tomography Chapter 17

Which of the following radionuclides is not a positron emitter?
(a) 82Rb
(b) 15O
(c) 18F
(d) 14C
(e) 68Ga

D. 14C
14C decays by beta emission. All of the other choices listed
decay by positron emission and some also by electron cap
ture

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

(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.

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.

(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.

Which of the following has the shortest half-life?
(a) 82Rb
(b) 13N
(c) 15O
(d) 18F

(a) 82Rb has a half-life of 1.3 min (75 s)
13N has a half-life of 10 min,
15O has a half-life of 122.2 s,
18F has a half-life of 109 min.

Normal 18F-FDG distribution would show the least activity in
the:
(a) Brain
(b) Bone
(c) Bladder
(d) Myocardium

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

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

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

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

(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. (See Chap. 10 ("Gastrointestinal Tract Scintigraphy"), Question 38.)

Which of the following is true regarding 18F-FDG?
(a) Its distribution in the brain is related only to blood fow.
(b) It is a potassium analog.
(c) It has a half-life of 60 min.
(d) It is taken up by disease-free myocardium

(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.

18F-FDG-PET images show a map of the _____ distribution
in the body.
(a) Oxygen
(b) Insulin
(c) Glucose
(d) Potassium

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

What length of time should a lactating female who undergoes
18F-FDG-PET scanning wait before resuming breastfeeding?
(a) 4 h
(b) 24 h
(c) 1 week
(d) Breastfeeding should be discontinued

(b) With a physical half-life of 109 min and a biological 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 14hr.

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

(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 measures attenuation through all lines of response.

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

(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.

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

(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 mis-positioned events decrease image contrast. The simultaneous detection of photons originating from different positrons is called a random coincidence, and the random 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.

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)

(c) Scintillation crystals that have been used in PET cameras
include Nal(Tl), LSO, BGO, and GSO. Lead sulfate, PbSO4,
is the white powder often seen on the electrodes of car batteries.

PET images may be reconstructed using:
(a) Coincidence detection
(b) Filtered back projection
(c) K-space flling
(d) Block detection

(b) Reconstruction of PET images can be performed using
filtered back projection 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 flling is a part of MRI reconstruction.
Block detection refers to the division of scintillators into separate channels using a material that will not transmit light.

Following a positron annihilation, two 511 keV photons are
emitted in opposite directions. If one of the photons is
defected in the body:
(a) Only the un-scattered photon will contribute to the image.
(b) The defected photon may be detected outside the correct
LOR.
(c) The LOR may contain an angle.
(d) The defected photon will have a higher energy than the
un-defected photon.

(b) The defected 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 defected photon transfers (loses) some
of its energy to an electron. Therefore, a defected photon's
energy is always lower than that of an un-defected photon

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.

(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.

Patient A and Patient B are injected with the same dose of 13N
ammonia for a myocardial PET scan. The patients are of 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

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

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

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

Why is it important to know if a patient has an infammatory
condition before performing an 18F-FDG-PET scan?
(a) 18F-FDG and certain anti-infammatory 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.

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

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

(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.

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.

(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.

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

(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-
740 MBq).

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

(a) See explanation for Question 4.

Relative to one another, 82Rb and 82Sr are:
(a) Isobars
(b) Isotopes
(c) Isotones
(d) None of the above

(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.

What determines the lower limit of spatial resolution in PET
scanners?
(a) Timing window
(b) Detector type
(c) Positron range
(d) Compton scatter

(c) The positron range is the distance that the 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.

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

(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.

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

(b) A random coincidence occurs when two annihilation pho
tons 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 have undergone Compton scatter, resulting in a change in direction.

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

(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.

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

(d) Muscle uptake may be the result of muscle contraction
during the uptake of 18F-FDG, so pre-medicating the patient
with diazepam (or other anxiolytic drugs), 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 difficulty to differentiate from lymph nodes

Dedicated PET scanners use:
(a) Flat-field collimation
(b) Electronic collimation
(c) Converging collimation
(d) No collimation

(b) Electronic collimation for PET means that pairs of pho
tons 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.

Standardized uptake values are useful for:
(a) Differential diagnosis of tumor type
(b) Assessing tumor size changes
(c) Assessing response to treatment

(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 (SUVmax), or from a volume of interest (VOI), as in the case of average SUV or peak SUV. Comparing SUVs in pre- and posttreatment images may show response to therapy.

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

(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.

The recommended dose of gallium-68 DOTATATE (Netspot®)
is:
(a) 0.054 mCi/kg, up to 5.4 mCi
(b) 10 mCi
(c) 1 mCi
(d) 25 mCi

(a) The recommended dose is 0.054 mCi/kg of body weight,
with a maximum dose of 5.4 mCi.

F-18 PSMA is a PET imaging agent for patients with
________.
(a) Breast cancer
(b) Lung cancer
(c) Prostate cancer

(c) F-18 PSMA is used to image patients with prostate cancer.

Indications for performing Netspot® PET imaging are:
(a) Localization of somatostatin receptor-positive neuroen
docrine tumors
(b) Prostate cancer
(c) Breast cancer
(d) Lung cancer

(a) Netspot® imaging is used for localization of somatostatin
receptor-positive neuroendocrine tumors.

Axumin (fuciclovine F-18) is a PET imaging tracer used to
image:
(a) Breast cancer
(b) Prostate cancer
(c) Lung cancer
(d) Infection

(b) Prostate cancer is imaged with Axumin (fuciclovine
F-18).

What is the administration method for F-18 PSMA?
(a) Intravenous
(b) Intramuscular
(c) Oral
(d) Inhalation

(a) F-18 PSMA is administered via intravenous injection.

Imaging with fluciclovine F-18 (Axumin) should be initiated:
(a) 3-5 min postinjection
(b) 30 min postinjection
(c) 60 min postinjection
(d) 90-120 min postinjection

(a) Imaging with fuciclovine F-18 should begin 3-5 min
after administration.

Netspot® can be used to image both pediatric and adult
patients.
(a) True
(b) False

(a) True, both adult and pediatric patients can be imaged with Netspot®.

Typical dose of F-18 PSMA is:
(a) 15 mCi
(b) 9 mCi
(c) 12 mCi
(d) 5 mCi

(b) 9 mCi is the typical dose administered for F-18 PSMA
PET imaging.

What is the delay time between injection and scanning with
F-18 PSMA?
(a) Immediate postinjection
(b) 15 min postinjection
(c) 60 min postinjection
(d) 2 h postinjection

(c) The delay between injection and scanning with F-18
PSMA is 60 min

Ga-68 DOTATATE is generator produced.
(a) True
(b) False

(a) True, Ga-68 DOTATATE is generator produced.

The typical dose of fluciclovine F-18 is:
(a) 10 mCi
(b) 15 mCi
(c) 20 mCi
(d) 25 mCi

(a) The typical dose of fuciclovine F-18 for PET imaging is
10 mCi.

There are no contraindications to using Axumin (fuciclovine
F-18).
(a) True
(b) False

(a) True, there are no contraindications to using Axumin.

Patient preparation includes being NPO prior to the F-18
PSMA injection.
(a) True
(b) False

(b) False, patients do not have to be NPO prior to F-18 PSMA
PET imaging

Ga-68 DOTATATE PET imaging, from skull to mid-thigh,
can be acquired:
(a) Immediately postinjection
(b) 15 min postinjection
(c) 40-90 min postinjection
(d) 2-4 h postinjection

(c) The delay between injection and scanning of Ga-68
DOTATATE is 40-90 min.

Tumors that do not bear somatostatin receptors will not be
visualized.
(a) True
(b) False

(a) True, tumors that do not bear somatostatin receptors will
not visualize on the PET imaging

F-18 PSMA is used to scan the female patient population.
(a) True
(b) False

(b) False, F-18 PSMA is not for imaging female patients as it
is utilized to image patients with prostate cancer

The half-life of Ga-68 DOTATATE is:
(a) 68 min
(b) 110 min
(c) 6 h
(d) 78.3 h

(a) The half-life of Ga-68 DOTATATE is 68 min.

Patient should be encouraged to increase hydration prior to
the study and to increase frequency of voiding after the
administration of F-18 PSMA.
(a) True
(b) False

(a) True, patients receiving F-18 PSMA for PET imaging are
encouraged to increase hydration and frequency of urination.

Ga-68 DOTATATE PET should be performed prior to admin
istering long-acting somatostatin analogs. When can short
acting analogs be used?
(a) Up to 24 h before
(b) 2 weeks before
(c) Never
(d) Time does not matter

(a) Short-acting analogs can be used up to 24 h before the
tracer injection for the Ga-68 DOTATATE PET scan.

How is Ga-68 DOTATATE administered?
(a) Oral
(b) Intravenous
(c) Intramuscular
(d) Inhalation

(b) Ga-68 DOTATATE is administered via intravenous
injection