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- unstable angina

- recent MI

- complex ventricular ectopy

- >2mm ST depression

- hypotension

- complex ventricular ectopy

- SVT

- severe hypertension

- hypertrophic cardio-myopathy

- arrhythmia

- WPW syndrome

- Î O2 demand

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Table 2. Comparison of 20rTl-chlonde and 99mTc-perfusion agents l1Tl-chloride

99mTc-perfusion agents

Physical properties decay mode physical half life emission

Effective dose

Blood flow Extraction efficiency Tracks high flow rates

Temporal distribution

Image quality

Accuracy electron capture 72 hours 63-80 keV

85% good isomeric transition 6 hours 140 keV

13 mSv (1.3 rems) for 750 MBq x 2 (stress & rest)

moderate washes out from heart no significant washout satisfactory good good good

201Tl is the radioactive form of thallium used for scanning. It has a half-life of 72 hours and decays by electron capture. Characteristic x-rays (range of 63-80 keV) produced during electron capture in addition to low abundance 135 and 167 keV gamma rays are imaged. 201Tl has relatively poor imaging characteristics. Its low photon energy makes it subject to attenuation by overlying soft tissues while its long half life limits the activity that can be administered rendering the images relatively count poor.

99mTc-sestamibi

This tracer is the most commonly used 99mTc-perfusion agent. It is a lipophilic monovalent cation that is passively taken up by myocytes along an electochemical gradient (and therefore requires cell membrane integrity). The extraction efficiency is approximately 65%. In the physiologic flow range, uptake is proportional to flow. However the deposition of the tracer does not increase linearly with flow but rather tends to level off at higher flow rates (Fig. 4). The distribution of 99mTc-sestamibi remains relatively fixed for several hours. This allows imaging to be delayed up to several hours after injection thereby facilitating the evaluation of patients presenting with acute chest pain (who can be injected during pain and imaged several hours later once stabilized).

Separate injections are required at stress and rest. These may be done on the same day (e.g., rest injection of 300 MBq and several hours later a stress injection of 1000 MBq) or as a two day protocol (e.g., 750 MBq injections at rest and stress on separate days). A third option is to administer 201Tl for the rest study and 99mTc-sestamibi for the stress study.

flow

Figure 4. Schematic representation of myocardial uptake versus flow for 201Tl and 99mTc-sestamibi. Uptake increases linearly with flow at low-flow rates for both tracers. At higher flow rates, there is a "roll-off" which is less marked for 201Tl (curve A) than for 99mTc-sestamibi. For two areas of the heart with flows of 'x' and 'y', there is a greater difference in uptake with 201Tl ('a') than with 99mTc-sestamibi ('b').

Hepatobiliary excretion of tracer may result in liver and/or gut activity obscuring the inferior wall of the left ventricle. To minimize adjacent infra-diaphragmatic activity imaging is delayed for at least 30 minutes after a stress injection and 60 minutes after a rest injection.

99mTc-tetrofosmin

99mTc-tetrofosmin is another commonly used 99mTc-perfusion agent with properties similar to 99mTc-sestamibi. While its extraction efficiency is somewhat lower than that of 99mTc-sestamibi there is less hepatobiliary extraction potentially decreasing interference with inferior wall assessment.

PET Tracers

While not available in many centers, PET radiopharmaceuticals can be used to assess myocardial flow and viability. 18F-fluorodeoxyglucose (FDG) is a glucose analogue. In the fasting state the myocardium normally uses fatty acids as its metabolic substrate. In ischemic myocardium, glucose is used preferentially. Increased uptake of FDG (in comparison to flow which is assessed with a separate radiopharmaceutical) in an ischemic segment indicates sustained metabolic activity and implies myocardial viability.

PET tracers to assess myocardial blood flow include 13N-ammonia and rubidium-82 (82Rb). 13N-ammonia has a high extraction efficiency of 90%. The energy of the positron and consequently its path range is low yielding improved resolution. Because of its short half-life of ten minutes, an on-site cyclotron is needed to produce 13N. 82Rb has properties similar to potassium and thallium. Since it is produced from a strontium-82 generator, an on-site cyclotron is not required. 82Rb has a short half life of 1.3 minutes with an extraction efficiency lower than that of 13N-ammonia. Its positron energy is greater than that of 13N-ammonia and hence resolution is poorer.

Figure 5. Normal planar 201Tl scan with images in the anterior, LAO and LAT projections. Panel A shows the nomenclature of the myocardial segments. There is uniform uptake of tracer in all segments of the left ventricle on the immediate poststress images (panel B) and on the delayed images 4 hours post injection (panel C). Typically, the counts in the heart decrease by approximately 50% over 4 hours. For the purpose of comparison, however, the intensity of uptake within the heart is normalized to the same brightness as the stress images and therefore the decrease in counts is not apparent. (Ant-anterior; AL-anterolateral; Ap-apex; Inf-inferior; IA-inferoapical; IS-inferoseptal; Post-posterior; PL-posterolateral; Sep-septum; RV-right ventricle)

Figure 5. Normal planar 201Tl scan with images in the anterior, LAO and LAT projections. Panel A shows the nomenclature of the myocardial segments. There is uniform uptake of tracer in all segments of the left ventricle on the immediate poststress images (panel B) and on the delayed images 4 hours post injection (panel C). Typically, the counts in the heart decrease by approximately 50% over 4 hours. For the purpose of comparison, however, the intensity of uptake within the heart is normalized to the same brightness as the stress images and therefore the decrease in counts is not apparent. (Ant-anterior; AL-anterolateral; Ap-apex; Inf-inferior; IA-inferoapical; IS-inferoseptal; Post-posterior; PL-posterolateral; Sep-septum; RV-right ventricle)

Planar, SPECT and Gated SPECT Imaging

Planar imaging usually consists of three views of the heart obtained in the anterior, LAO and steep LAO projections (Fig 5). While SPECT imaging (Figs. 6 and 7) is technically more demanding than planar imaging, advantages to its use include higher lesion contrast and an improved ability to localize defects. Conventionally a 180o acquisition (from the right anterior oblique to the left posterior oblique projection) is utilized for cardiac imaging rather than a full 360o acquisition since few cardiac photons are detected in the posterior projections.

SPECT was originally performed with single-head gamma cameras. These have largely been replaced with multi-head gamma cameras which increase counting efficiency. A common configuration used for cardiac imaging is a two-headed camera with the heads oriented at right angles. The gantry need only rotate 90o in order to obtain a 180o acquisition.

The principle of R-wave synchronized cardiac gating is discussed in Chapter 4. In the same way, cardiac SPECT data can be gated to obtain eight sets of tomographic

Figure 6. Schematic showing the three orthogonal planes used for SPECT myocardial imaging and segmental nomenclature. The short-axis (SA) slices can be considered as "cucumber" slices through the left ventricle. The horizontal long axis (HLA) slices are similar in orientation to the echocardiographic four chamber view. The vertical long axis (VLA) slices can be thought of as viewing the heart from a lateral projection. (ant-anterior; ap-apex; inf-inferior; lat-lateral; sep-septal)

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