Year 5, Number 18, October 2002

 

Cardiac SPECT.

Article N AJ18-13

 

 

3. Instrumentation and Acquisition Parameters


SPECT detectors are basically scintillation cameras mounted in a rotation gantry. The shape of the detector either circular or rectangular, although important for other organ studies, is not relevant for myocardial SPECT. Single-head cameras are the most popular ones for nuclear cardiology, although double-head systems with detectors in 90 are preferred in terms of time savings. However, these systems are more expensive and quality control requirements are more strict. High resolution, parallel-hole collimators and fan-beam collimators are suitable for myocardial perfusion SPECT. Thallium studies can be acquired using all-purpose collimators, specially for gated SPECT since low count density can otherwise compromise the study quality.

Traditionally, a 180 circular orbit is used starting in right anterior oblique (RAO) position and moving towards the left posterior oblique (LPO) in a step-and-shoot modality with 6 increments. However, other parameters are usually employed such as an elliptical orbit, continuous acquisition and 3 angular increment or less. The use of magnification factor (zoom) is also variable between 1 and 2. Parameters giving the best achievable resolution should be employed according to each manufacturer's recommendations and each department's experimental results. A non-controversial useful hint to always keep in mind is to minimize the distance between the detector and the patient in order to improve resolution. A 64x64 matrix is usually chosen for myocardial perfusion imaging, specially if a zoom factor of at least 1.5x is used.


GATED SPECT.

Gated myocardial perfusion SPECT should be routinely performed if hardware and software requirements are available, as recommended by the Society of Nuclear Medicine and the American Society of Nuclear Cardiology. A ECG-gated tomographic acquisition represents a variation of a conventional perfusion SPECT study in which temporal resolution is increased allowing the system to detect discrete phases of the cardiac cycle. Usually, 8 frames per cardiac cycle is considered a good compromise between temporal resolution, count density and acquisition time. If the SPECT study consists of 32 angular steps, then the final file will contain 32 x 8 = 256 frames. An adequate ECG signal with constant R-R interval is essential for study quality and reliability of quantitative results. Gated SPECT is usually performed using step-and-shoot acquisition modality.

Optimized protocols for gated SPECT should consider both gated and non-gated image quality. Enough count density must be guaranteed in each temporal frame at each projection angle, since each image of the cardiac cycle is individually reconstructed for tomographic dynamic display and quantitative calculation of ventricular function and then summed together to yield the conventional perfusion tomograms. Thus, it may be necessary to increase total acquisition time compared to the conventional non-gated acquisition, according to the achieved countrate. An acceptance window for bad-beat rejection must be specified. A narrow window will assure rejection of most ectopic beats and thus the value of ejection fraction will be more reliable. However, since some data is discarded, the conventional perfusion tomograms will suffer a decrease in count density with a magnitude dependant upon the number of rejected beats. On the other hand, a wide window will preserve perfusion data but can deteriorate functional gated information. However, since perfusion data is considered more relevant, current recommendations include the use of a 90%-100% acceptance window for gated SPECT.

Not every patient is suitable for gated SPECT acquisition. Complete arrhythmia secondary to atrial fibrillation, or frequent ectopic contractions either atrial of ventricular in origin usually produce poor gating performance with deterioration of the diastolic portion of the study. In general, a patient with regular beats occurring at least 80% of the time is considered adequate for gated acquisition.

With technetium-based agents, gating is recommended both for the stress and rest studies. Differences in ventricular function between the two situations may represent post-stress ventricular stunning (transient ischemic dysfunction) which can add to the clinical prognosis of the patient. Stunning can occur both after exercise or pharmachologic stress. If for logistic reasons only one study is to be gated, the rest one is suggested (which will represent true basal ventricular function), unless a single-day, rest-stress protocol is used in which a low dose is used for rest. With thallium, gated studies usually require longer acquisition times and image quality is suboptimal.

The official guidelines of the American Society of Nuclear Cardiology (ASNC) for cardiac SPECT imaging are listed in Table 1.

Table 1.- Imaging guidelines of the American Society of Nuclear Cardiology.

ONE-DAY PROTOCOL REST - STRESS WITH 99mTc-MIBI

-
Rest
Stress
Dose 8 - 9 mCi 22 - 25 mCi
Position Supine Same
Waiting time - -
Injection ===> Imaging 1 - 2 hs 15 min - 1 h
Rest ===> Stress 1 - 4 hs -
Acquisition - -
Energy window 20% symmetric Same

Collimator

LEHR Same
Orbit
180 (RAO-LPO) Same
Type of orbit Circular Same
- Elliptic Same
Pixel size 6.4 0.2 mm Same
Acquisition type Continuous (non-gated) Same
- Step-and-shoot Same
N of projections 64 Same
Matrix 64 x 64 Same
Zoom factor* 1x - 2x Same
Time / projection 25 seg 20 seg
Total time 30 min 25 min
ECG gating No Yes**
Frames / cycle N/A 8
R-R window N/A 100 %

* Optional - not included in the original guidelines.
** Post-stress ventricular function can be affected by ischemic myocardial stunning (author's note).

TWO-DAY PROTOCOL WITH 99mTc-MIBI

Stress
Rest
Dose 20 - 30 mCi Same
Position
Supine Same
Waiting time - -
Injection ===> Imaging 15 min - 1 h 1 - 2 hs
Acquisition - -
Energy window 20% symmetric Same
Collimator LEHR Same
Orbit 180 (RAO-LPO) Same
Type of orbit Circular Same
- Elliptic Same
Pixel size 6.4 0.2 mm Same
Acquisition type Continuous (non-gated) Same
- Step-and-shoot Same
N of projections 64 Same
Matrix 64 x 64 Same
Zoom factor* 1x - 2x Same
Time / projection 20 seg 20 seg
Total time 25 min 25 min
ECG gating** Yes Yes
Frames / cycle 8 8
R-R window 100 % 100 %

* Optional - not included in the original guidelines.
** If only one study is to be gated, the rest one is recommended. However, gating of both studies is preferable since post-stress myocardial stunning can be detected (author's note).

DUAL-ISOTOPE PROTOCOL WITH 201Tl AND 99mTc-MIBI

--
Rest
Stress
Dose / radiopharmaceutical 2.5 mCi / 201Tl 22 - 25 mCi / 99mTc-MIBI
Position Supine Same
Waiting time - -
Injection ===> Imaging 15 min 15 min - 1 h
Rest ===> Stress
No waiting
-
Acquisition -- ---
Energy window

30% symmetric for 70 keV
20 % symmetric
for 167 keV

15% symmetric 140 keV
Collimator LEAP/LEHR Same
Orbit 180 (RAO-LPO) Same
Type of orbit Circular Same
Pixel size 6.4 0.2 mm Same
Acquisition type Continuous Same
N of projections 64 Same
Matrix 64 x 64 Same
Zoom factor* 1x - 2x Same
Time / projection 25 seg 20 seg
Total time 30 min 25 min
ECG gating No Yes**
Frames / cycle N/A 8
R-R window N/A 100 %


* Optional - not included in the original guidelines.
** Post-stress ventricular function could be affected by ischemic myocardial stunning (author's note).

CONVENTIONAL PROTOCOL WITH 201Tl

-
Stress
Rest (redistribution)
Dose 3 mCi N/A
Position Supine Same
Waiting time - -
Injection ===> Imaging 15 min N/A
Rest ===> Stress 4 hs -
Acquisition - -
Energy window 30% symmetric for 70 keV v -
Collimator LEAP/LEHR Same
Orbit 180 (RAO-LPO) Same

Type of orbit

Circular Same
Pixel size 6.4 0.2 mm Same
Acquisition type Continuous Same
N of projections 64 Same
Matrix 64 x 64 Same
Zoom factor* 1x - 2x Same
Time / projection 40 seg 40 seg
Total time 22 min 22 min
ECG gating** No No

* Optional - not included in the original guidelines.
** Physical characteristics of 201Tl are not ideal for gated SPECT, although it can be done at the expense of increasing acquisition time in order to improve count density and study quality (author's note).

REINJECTION PROTOCOL WITH 201Tl

-
Stress
Reinjection
Dose 3 mCi 1.5 mCi
Position Supine Same
Waiting time - -
Injection ===> Imaging 10 - 15 min -
Injection ===> Reinjection 2 - 3 hs -
Reinjection ===> Imaging - 10 - 30 min
Acquisition - -
Energy window 30 % symmetric for 70 keV
20 % symmetric for 167 keV
-
Collimator LEAP/LEHR Same
Orbit 180 (RAO-LPO) Same
Type of orbit Circular Same
Pixel size 6.4 0.2 mm Same
Acquisition type Continuous Same
- Step-and-shoot Same
N of projections 32 Same
Matrix 64 x 64 Same
Zoom factor* 1x - 2x Same
Time / projection 40 seg 40 seg
Total time 22 min 22 min
ECG gating** No No

* Optional - not included in the original guidelines.
** Physical characteristics of 201Tl are not ideal for gated SPECT, although it can be done at the expense of increasing acquisition time in order to improve count density and study quality (author's note).

24-HOUR IMAGING WITH 201Tl

At 24 hours post-injection, time per projection should be increased to 60 seconds so total study time will be prolonged to 32 minutes.

 


1. Clinical Indications | 2. Radiopharmaceuticals | 3. Instrumentation and Acquisition Parameters | 4. Processing and Display | 5. Quality Control | 6. Image Artifacts | In conclusion | References | Print

 

 

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