positron emission tomography (PET) Thyroid scan (I-123) SPECT vs PET

1. PET CT & THYROID SCAN Dr Muhammad Qasim
Khan

2. SPECT VS PET
Single photon emission computed tomography (SPECT) and positron
emission tomography (PET) are nuclear medicine imaging techniques
which provide metabolic and functional information unlike CT and
MRI.
They have been combined with CT and MRI to provide detailed
anatomical and metabolic information.

3. Positron emission tomography (PET):
is very expensive
uses positron emitting radioisotope (tracer)
fluorine-18
gives better contrast and spatial resolution (cf.
SPECT)

4. Single-photon emission computed tomography (SPECT):
is lower cost
uses gamma emitting radioisotope (tracer):
technetium-99m
iodine-123
iodine-131
gives poorer contrast and spatial resolution (cf. PET)

5. POSITRON EMISSION
TOMOGRAPHY

6. Positron emission tomography (PET) is a modern non-invasive
imaging technique for quantification of radioactivity in vivo. It
involves the intravenous injection of a positron-emitting
radiopharmaceutical, waiting to allow for systemic distribution, and
then scanning for detection and quantification of patterns of
radiopharmaceutical accumulation in the body.
As with SPECT imaging, PET scan data can be reconstructed and
displayed as a three dimensional image. This is in contrast
to scintigraphy, which yields planar data which can only be used to
create a two dimensional image.

7. Although the physiologic information afforded by PET and SPECT
imaging is invaluable, the quality of obtained data is poor/noisy and
limits imaging spatial resolution. For this reason, PET and SPECT
scans are often combined with CT imaging, allowing correlation
between functional and anatomical imaging ("hybrid imaging"). More
recently PET-MRI scanners have become available although their use
remains limited and they are generally only found in the larger
academic centers, often in a research setting.

8. Physics
A radiolabelled biological compound such as F-18
fluorodeoxyglucose (FDG) is injected intravenously.
Uptake of this compound followed by further breakdown occurs in
the cells. Tumor cells have a high metabolic rate, and hence this
compound is also metabolized by tumor cells.
FDG is metabolized to FDG-6-phosphate which cannot be further
metabolized by tumor cells, and hence it accumulates and
concentrates in tumor cells. This accumulation is detected and
quantified.

9. RADIOPHARMACEUTICAL
DETECTION
The positron emitting isotope administered to the patient
undergoes β+ decay in the body, with a proton being converted to a
neutron, a positron (the antiparticle of the electron, sometimes
referred to as a β+ particle) and a neutrino. The positron travels a
short distance and annihilates with an electron.
The annihilation reaction results in the formation of two high energy
photons which travel in diametrically opposite directions.
Each photon has an energy of 511 keV. Two detectors at opposite
ends facing each other detect these two photons traveling in opposite
directions, and the radioactivity is localized somewhere along a line
between the two detectors. This is referred to as the line of response.

10. PROCEDURE
fasting for 4-6 hours
blood glucose level <150 mg/dL
avoid strenuous activity 24 hours prior to imaging
avoid speech 20 minutes prior to imaging
the scan is carried out 60 minutes post-injection of FDG
In cases of fusion imaging such as PET-CT, the whole body CT scan is
conducted first, followed by the whole-body PET scan and subsequently the
two sets of images are co-registered.
A standardized uptake value (SUV) is calculated at the end of the study i.e.
ratio of activity per unit mass tissue to injected dose per unit body mass.

11. LIMITATIONS
Motion artifacts result in an inaccurate anatomical coregistration of
the CT and PET studies. The distance (2-3 mm) the positron travels
before annihilation and the detector element size both contribute to
relatively poor spatial resolution.
Physiological muscle uptake usually appears symmetrically and
diffusely on PET imaging.

12. ATTENUATION CORRECTION
When combined with CT, the CT imaging can be used to generate an
attenuation map which is used to correct the PET imaging for attenuation.
This attenuation correction can add a number of further artifacts.
breathing
 artifacts related to respiratory motion causes the 'mushroom effect' where an artifact is
sometimes seen in the lung bases because of the different phases of respiratory motion
implants and prostheses
 metallic implants such as joint prostheses can create significant artifact on PET images
as the attenuation correction cannot deal with/correct for markedly high densities
truncation
 CT field of view is limited whereas PET field of view is usually larger; if patients are
scanned with arms by their side this can lead to abnormal reconstruction of the images

13. NORMAL PHYSIOLOGICAL UPTAKE
brain
Waldeyer ring, e.g. palatine tonsils symmetrically especially when younger 7
skeletal muscle, especially after strenuous activity and laryngeal muscles following speech
myocardium
gastrointestinal tract, e.g. intestinal wall
genitourinary tract: FDG is excreted via the renal system and passes into the collecting
systems
brown fat
thymus 4
bone marrow 5
lactating breasts

14. FALSE-POSITIVE FDG UPTAKE
This may occur due to the following conditions:
granulomatous disease
abscess
surgical changes
foreign body reaction e.g. talc pleurodesis
excessive bowel uptake with metformin therapy
inflammation (although at times e.g. evaluating for vasculitis, this
may be the finding of interest)
fat necrosis

15. APPLICATIONS
oncologic
 detection, staging, response to treatment
 differentiation between radiation necrosis and recurrence
neurologic
 early diagnosis of Alzheimer disease
 localization of seizure focus in interictal phase
 localizing eloquent areas (e.g. speech, motor function)
cardiac
 identification of hibernating myocardium
infection/inflammation
 pyrexia of unknown origin (PUO)
 vasculitis

16. THYROID SCAN (I-123)

17. Thyroid scan (thyroid scintigraphy) is a nuclear medicine examination
used to evaluate thyroid tissue.
Clinical indications
functional status of a thyroid nodule
thyrotoxicosis: differential diagnosis
thyroid cancer
 whole body scan for distant metastases
 estimation of local residual thyroid post thyroidectomy
 follow-up for tumor recurrence

18. PATIENT PREPARATION
Medications that interfere with thyroid uptake of radioiodine should
be discontinued. Review of the history should be carried out to
ensure the patient has not received iodine-containing contrast (e.g.
for CT or angiography). Patients should be fasted for 4 hours prior to
study.

19. DOSE, ROUTE OF ADMINISTRATION
AND TIMING
Iodine-123 is the most commonly used radioisotope. It is
administered orally in capsule form (3.7-14.8 MBq (100-400 μCi)).
Scanning is performed either at 4-6 or 24 hours.
An alternative radioisotope is Tc-99m pertechnetate. Administration
is intravenous, and imaging must be done early (maximum uptake at
about 20 minutes).The pertechnetate anion, captured by the follicular
cells of the thyroid - unlike radio-iodine - is not organificated.

20. EQUIPMENT
camera: gamma camera
collimator: 3-6 mm aperture pinhole collimator
window: 20% energy window centered at 159 keV

21. PROCEDURE
The patient is positioned supine with the chin up and the neck
extended. The collimator is then positioned so that the thyroid fills
about two-thirds of the diameter of the field of view. Mark the chin
and suprasternal notch. Note the position and mark palpable nodules
and surgical scars. Place marker sources lateral to the thyroid to
calibrate size.
Three views are typically obtained: anterior; 45-degree LAO; and 45
degree RAO (move the collimator, if possible, rather than the patient).
Each view should have 100-250,000 counts.

22. THANK YOU