Year 5, Number 18, October 2002

Cardiac SPECT.

Article N° AJ18-13

Margarita Nuñez.
Escuela Universitaria de Tecnología Médica, Centro de Medicina Nuclear, Hospital de Clínicas.


Escuela Universitaria de Tecnología Médica, Centro de Medicina Nuclear, Hospital de Clínicas. Av. Italia s/n Montevideo 11200 - Uruguay. Tel: (598 2) 487 1407 Fax: (598 2) 487 0230 Email:

Nuñez, Margarita. Cardiac SPECT. Alasbimn Journal 5(18): October 2002. Article N° AJ18-13.




1. Clinical Indications

Nuclear cardiology is a widely available, cost-effective, non-invasive methodology in use for more than two decades with which a large amount of experience has been accumulated. The technique is in constant development and its application has been growing to constitute nowadays about 30-40% of all nuclear medicine procedures in an average institution. Most indications are related to coronary artery disease (CAD) evaluation and are supported by large amounts of scientific evidence. The following are some of such indications:

a) Diagnosis of CAD.
This application was probably the first to demonstrate the efficacy of nuclear cardiology studies, specially the investigation of myocardial perfusion together with stress testing. According to Bayes' theorem, patients with intermediate pre-test probability of CAD benefit most from the study (i.e., patients with chest pain and negative exercise test, asymptomatic patients with ECG changes or patients with non-diagnostic ECG).

b) Prognosis and risk stratification in chronic CAD patients.
This is an application of increasing frequency regarding decision-making and rational patient management. Considering the findings of a myocardial perfusion study in terms of number, extension and severity of ischemic defects, the patient can be assigned a low, intermediate or high probability for future cardiac events and the most appropriate therapeutic choice can be selected. This is also useful for non-cardiac preoperative evaluation.

c) Unstable angina.
Most patients admitted with diagnosis of unstable angina are usually derived for immediate invasive evaluation. However, it has been demonstrated that if stabilization is achieved, these patients can be safely submitted for stress myocardial perfusion studies to determine the extent and severity of induced ischemia in order to choose between invasive and non-invasive strategies.

d) Myocardial infarction.
There is strong evidence that patients with the so-called "non-Q wave" myocardial infarction can benefit from non-invasive treatment unless a functional study result indicates high ischemic risk. In patients with transmural, non-complicated myocardial infarction, a non-invasive functional evaluation at discharge is suggested for risk-stratification, which can be done through a stress perfusion study preferably with gated SPECT, in order to determine the need for cardiac catheterization.
e) Evaluation of revascularization procedures.
Percutaneous transluminal coronary angioplasty (PTCA) is a minimal-invasive revascularization technique but has a relatively high rate of restenosis, even with stent implantation. In patients with chest pain and/or positive/indeterminate stress tests post-PTCA, myocardial perfusion studies can determine the presence of restenosis or detect a new affected vascular territory, or even rule out significant ischemia. In patients with previous by-pass surgery, perfusion studies are also useful for assessment of graft patency.

f) Chest pain evaluation in the emergency department.
Chest pain is one of the most frequent cause of emergency admissions worldwide. Many patients are really undergoing a cardiac ischemic episode, but many more have non-cardiac causes of pain that would need no hospitalization. Chest pain units have been installed in some institutions to rationally evaluate these patients, and acute rest myocardial perfusion studies have been demonstrated to represent a cost-effective procedure to be implemented within the diagnostic algorithm, specially in cases with non-interpretable ECGs.

g) Investigation of myocardial viability.
Patients with known CAD with previous myocardial infarction and poor ventricular function are potential candidates for revascularization procedures tending to improve survival and life quality. However, this can only be achieved if myocardial viability is present since non-viable tissue is not expected to improve function after restoration of blood flow. Hypokinetic but viable myocardium is known as "hibernating" myocardium and represents a condition believed to occur as a functional down-regulation in the presence of a diminished blood supply or as a consequence of repetitive acute ischemic episodes. Various nuclear medicine techniques have been developed to detect myocardial viability including PET, thallium perfusion with reinjection and late imaging, and nitrate-enhanced perfusion with technetium-based agents, with similar diagnostic efficacy. Gated SPECT with low-dose dobutamine infusion is also under clinical investigation.

h) Evaluation of medical treatment.
Evolution of coronary atherosclerosis can be influenced by means of aggressive control of risk factors and medication. Coronary flow reserve and endothelial function can be effectively assessed in these patients to evaluate the efficacy of non-invasive therapy. This will probably constitute a main indication for perfusion studies in the near future as new pharmachologic products are released to the market raising the need for objective evaluation of cardiac effects.

i) Functional assessment of known anatomic lesions.
Patients with borderline (30-60%) coronary stenosis are frequently submitted for myocardial perfusion studies in order to evaluate the functional significance of the lesions and eventually identify the one(s) causing symptoms to the patient (culprit lesion). This helps the invasive cardiologist to make a decision on which vessel to attempt revascularization. Also, with the widespread use of electron-beam computerized tomography (EBCT) for detection of coronary calcifications, functional assessment will be indicated to determine the need for further invasive evaluation.




2. Radiopharmaceuticals

a) 201-Thallium chloride.
This is a potassium-analogue with active transport through the plasmatic membrane by means of the Na-K-ATPase pump. After intravenous administration, kinetics of 201Tl can be divided into two different phases: initial uptake and redistribution. Initial uptake depends on regional myocardial blood flow and extraction fraction, and the image at this stage represents myocardial perfusion. Myocardial concentration of 201Tl changes over time, which is known as redistribution, reaching an equilibrium with plasmatic concentration of the isotope. This phenomenon is evident when an initial perfusion defect is "filled in" or normalized in the late images hours after injection, which typically occurs under ischemic conditions. Late concentration of 201Tl is believed to represent integrity of cell membrane metabolic processes and thus, viability. Following this concept, four abnormal regional perfusion patterns can be defined: reversible perfusion defects, non-reversible defects, partially reversible defects and inverse or paradoxical reversibility. These patterns respectively represent ischemia, infarction and ischemia plus infarction, while inverse reversibility is still a controversial subject. Wash-out rate of 201Tl from the myocardium is also a useful functional parameter that can be measured with appropriate available software.

b) 99mTC- Isonitriles
Isonitriles are a group of cathionic liposoluble complexes designed as myocardial perfusion agents to be labelled with 99mTc, of which 6-methoxy-isobutyl-isonitrile (MIBI or sestamibi) is commercially available. As with 201 Tl, myocardial uptake of99mTc-MIBI is proportional to myocardial blood flow within a physiologic range. Extraction fraction is somewhat lower than that of 201Tl, underestimating high flows but being relatively higher at low flows. It enters the myocardial cell by passive diffusion and intracellular retention is prolonged, being trapped at the mithocondrial level with no significant redistribution. Experimental observations have demonstrated myocardial uptake even in the presence of severe metabolic alterations, however necrotic tissue does not exhibit retention of 99m Tc-MIBI so this should be considered not only a perfusion marker, but to some extent a viability agent too.

c) 99mTc-phosphines.
These are also cathionic lipophyllic agents, being tetrofosmin the only one commercially available. Several studies have demonstrated properties similar to those of  99mTc-MIBI regarding uptake and retention, although with more rapid hepatic clearance, allowing shorter acquisition waiting time after injection and slightly better dosimetry. Clinical efficacy in nuclear cardiology is totally similar to 99m Tc-MIBI, and technical protocols are almost identical.

d) Other radiopharmaceuticals.
Myocardial sympathetic innervation can be imaged with 123I-MIBG with applications in ischemic and non-ischemic heart disease. 99mTc-NOET is a perfusion agent with redistribution properties similar to those of thallium, not yet commercially available. Recently, nitroimidazoles labelled with 99mTc have been developed which accumulate in hypoxic tissues, giving a positive image of myocardial ischemia although still needing clinical validation. The process of apoptosis or programmed cell death has been successfully imaged experimentally with 99m Tc-annexin-V representing a promising and exciting new radiopharmaceutical with several potential clinical uses.




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 90º 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 180º 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 6º increments. However, other parameters are usually employed such as an elliptical orbit, continuous acquisition and 3º 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 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.


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


180º (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
Nº 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).


Dose 20 - 30 mCi Same
Supine Same
Waiting time - -
Injection ===> Imaging 15 min - 1 h 1 - 2 hs
Acquisition - -
Energy window 20% symmetric Same
Collimator LEHR Same
Orbit 180º (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
Nº 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).


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 180º (RAO-LPO) Same
Type of orbit Circular Same
Pixel size 6.4 ± 0.2 mm Same
Acquisition type Continuous Same
Nº 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).


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 180º (RAO-LPO) Same

Type of orbit

Circular Same
Pixel size 6.4 ± 0.2 mm Same
Acquisition type Continuous Same
Nº 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).


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 180º (RAO-LPO) Same
Type of orbit Circular Same
Pixel size 6.4 ± 0.2 mm Same
Acquisition type Continuous Same
- Step-and-shoot Same
Nº 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).


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




4. Processing and Display

Transaxial reconstruction with ramp-filtered backprojection is most common. A Butterworth filter window is best suited for myocardial studies, with order number and cutoff frequency to be determined by the operator according to each system. However, other filters are equally acceptable as far as artifacts are not introduced. Iterative reconstruction methods are now available in modern systems and may represent a better alternative. Oblique-angle reorientation is necessary to obtain short-axis, horizontal and vertical long-axis of the left ventricle. Post-reconstruction magnification is recommended, specially for small hearts. Image display should include comparative slices 1-2 pixel width of stress and rest studies presented in the three planes. Short axis tomograms are displayed from apex to base, horizontal long axis with the apex pointing upwards and septal wall to the left, from anterior to inferior wall, and vertical long axis with the apex to the right and the anterior wall above, from septal to lateral wall.

Semi-quantitative evaluation of myocardial perfusion can be achieved through bull's eye display or polar maps, instrumented in most systems. This is a two-dimensional representation of a three-dimensional distribution of the radiotracer in the myocardium which allows visualization of perfusion defects in a compressed format, including their extension and degree of reversibility. Previous processing steps for the generation of polar maps are to be followed carefully as described by the manufacturer in order to avoid introduction of artifacts, and the results should always be compared with direct observation of conventional tomograms.

For gated SPECT, special software is commercially available from at least four different developers, although two of the program packages have gained more widely acceptance: QGSPECT (Cedars Sinai) and Emory Toolbox (Emory University). Processing usually requires minimal operator input and the results are automatically calculated and displayed in a fixed format containing qualitative and quantitative information on ventricular function including left ventricular ejection fraction and volumes. Observation of dynamic data in cine mode is essential to evaluate regional wall motion and thickening in the three orthogonal planes.

If available, attenuation correction should be applied as far as the method employed is reliable and has been validated. Attenuation correction offers the potential for improved diagnostic accuracy but require a modified approach to image interpretation because resultant images may have different "normal" patterns as compared to conventional, non-corrected images. The Society of Nuclear Medicine and the American Society of Nuclear Cardiology have published a joint position statement recommending that attenuation correction should be regarded as a method for which the weight of evidence and opinion is in favour of its usefulness.




5. Quality Control

a) Center of rotation (COR).
An alignment error between the mechanical center of the rotational gantry and the center of the electronic matrix can produce reconstruction artifacts in the image. The effect is evident when the error is greater than 2 pixels in a 64x64 matrix. Smaller errors, however, can generate a loss in spatial resolution and image contrast. To a certain degree, COR errors can be electronically compensated but if a progressive deterioration is detected the camera should be serviced. COR checking should be performed frequently, usually on a weekly basis or depending on each manufacturer's recommendation and stability of the system.

b) Detector uniformity.
In SPECT, it is assumed that photon detection (sensitivity) is constant throughout the collimated surface of the detector when exposed to a flood source. Errors in flood field uniformity are produced with significant variations in detection efficiency, which can be due to physical or electronic factors. Uniformity is much more critical for SPECT as compared to planar imaging because more severe artifacts can be introduced. Most systems use previously acquired "correction maps" kept in memory to compensate for non-uniformities before reconstruction. Extrinsic uniformity (with collimator in place) should be daily checked for at least 3,000K counts in a 64x64 matrix using a refillable 99mTc phantom or a 57Co flat source. Larger matrices will require larger number of counts. Correction or sensitivity maps are to be acquired periodically for a total of 30,000-40,000K counts. Non-uniformity should be kept lower than 3% for SPECT.

c) Attenuation correction.
Although still expensive and not universally available, attenuation correction represents a rapidly evolving standard for myocardial SPECT. Accurate attenuation correction is mainly dependent upon high-quality transmission images. Therefore, QC of attenuation correction performance should include the following verifications: uniformity, variability and temporal drift of the reference transmission scan; consistency of hardware performance; pre-scanning methods to ensure adequate transmission scan counts; and algorithms that assist the technologist and physician in assessing the sufficiency of the data. Implementation of all these QC techniques, however, has not been incorporated in the current releases of all available attenuation correction protocols. Furthermore, QC of transmission data and attenuation-corrected reconstructed images should be performed for each patient and interpreted in comparison with non-corrected conventional tomograms.

d) Other verifications.
Energy resolution, spatial resolution, linearity, pixel size, tomographic resolution and tomographic uniformity should also be checked periodically in accordance to manufacturer's recommendations or established protocols. Detailed description of these procedures are beyond the scope of this presentation and can be found elsewhere.

e) Quality control of clinical studies.
After acquisition, raw images should be reviewed in cine mode to check for appropriate radiopharmaceutical biodistribution, total count density, data integrity, "upward creep" of the heart, attenuation, subdiaphragmatic activity and patient motion. Acquisition should be immediately repeated if necessary. Stress and rest reconstructed images should not contain artifacts (see below) and be properly aligned and count-normalized for comparison. ECG-gated images should be reviewed in cine display after reconstruction and checked for "flickering" effect produced by count loss in the last images of the cardiac cycle due to variable R-R interval or other ECG-gating related artifacts. QC of gated studies should also include the analysis of the volume curve which should be of expected shape (starting and ending at diastole), and checking for extracardiac activity that could be interpreted as belonging to the cardiac wall by the edge-tracking algorithm.




6. Image Artifacts

Artifacts and normal variants are a significant source of false-positive interpretations in myocardial SPECT. By anticipating and recognizing such findings, both the technologist and interpreting physician can increase test specificity in the diagnosis of myocardial ischemia or scar and avoid unnecessary invasive procedures in normal patients. There are several known sources of artifacts in myocardial perfusion SPECT:

a) Artifacts associated with instrumentation errors.

  • Flood field non-uniformity: This may be secondary to defective photomultiplier tubes, damaged collimators, problems with camera electronics, or defects in the sodium-iodine crystal. To detect such abnormalities, intrinsic and extrinsic flood fields should be obtained as described in protocols. Flood field non-uniformities can be detected either visually or quantitatively by appropriate QC software or manual calculations. When a SPECT study is acquired using a non-uniform camera, each flood field defect will be propagated into a circular arch generating ring artifacts that can potentially cause irregularities in myocardial count density. These artifacts are more evident towards the center of the reconstruction matrix and can be either "hot" or "cold". With dual-head cameras, minor differences in detector sensitivity can also cause image artifacts even if absolute values are acceptable separately.
  • Center-of-rotation (COR) error. The COR allows SPECT data to be back-projected onto a central point in the volumetric matrix. Exact centering of data reconstruction is critical to accurately reproduce anatomic structures and count density distribution. Error in the COR will result in misregistration of tomographic data, image blurring and artifactual SPECT myocardial perfusion defects. The myocardial image may appear to be misaligned or skewed specially towards the apex, which is more evident in the horizontal long axis plane. With dual-head, variable angle cameras COR alignment can potentially represent a more complex problem.
  • Camera head tilt. Similar to the problem with COR, if one detector is tilted while using a variable angle multihead camera, the detector heads will not record exactly the same image. Back projection and reconstruction of these different images into a single cardiac image may also produce regional myocardial perfusion artifacts.
  • Detector-to-patient distance. Although not technically an artifact per se, minimizing detector-to-patient distance is a very important technical consideration to assure optimal image quality and diagnostic accuracy of the procedure. Increased distance results in suboptimal defect contrast and spatial resolution. When defining the camera orbit, radius of rotation should be kept as smaller as possible. Also, with non-circular orbits, careful must be taken to avoid significant variations in radius of rotation at different angles since backprojection of images of different resolution can also produce artifacts.

b) Patient related artifacts.

  • Patient motion. This is one of the most common sources of SPECT myocardial perfusion artifacts. During SPECT acquisition, patients may move in the vertical (axial), lateral or rotational direction. The best means to detect patient motion is to observe the planar raw images of the SPECT acquisition in cine mode, but condensed images such as sinogram and linogram are also useful. Whether patient movement is abrupt or gradual, myocardial perfusion image artifacts may be created. The cause of the reconstruction artifact is similar to that due to an erroneous COR, but with patient motion the heart itself is at a different location during different portions of the SPECT acquisition. During reconstruction, data are backprojected onto different points of the volumetric matrix, and thereby a misregistration error occurs.

Respiratory motion is very prominent in some patients but the images are often still of diagnostic value, because there is a periodical movement of the heart in the axial direction causing some blurring of the backprojected image but usually not simulating a perfusion defect. A change in body position at the middle of the acquisition is much worse in consequence. Characteristically, with patient motion reconstructed images demonstrate opposed defects in contralateral walls. Frequently the image contains "tails" streaming from the edges of the defect. The overall visual effect is known as the "hurricane sign" in the short axis views, while in the horizontal long axis misalignment of the lateral and septal wall towards the apex can be apparent as described for COR errors. Motion correction can be attempted although most software approaches are imperfect and sometimes represent additional sources of error by themselves.

The technologist can be of great help in minimizing patient motion. SPECT acquisition should be fully explained to the patient and he/she should be encouraged not to talk, yawn, sigh or fall sleep (and snore). The lumbar curvature of the back should be supported, the knees slightly elevated and supported in order to minimize back strain, and the left arm and shoulder supported and restrained. Although SPECT can be performed with both arms at the side, in order to minimize detector distance and avoid the attenuation caused by the arms, elevation of at least the left arm above the head is preferable. Commercially available arm holders are useful for this purpose. Multiheaded detector systems have been beneficial to decrease SPECT image acquisition time, improve patient tolerance, and thereby decrease the possibility of patient motion. On the other hand, a single motion episode will be duplicated with a dual-head camera since it will be "seen" simultaneously by both detectors at different angles.

If significant motion is detected and reconstruction artifacts are present, a new acquisition is recommended. However, if repeating a stress 201Tl study significant redistribution could have occurred and scheduling the patient for a different day may be preferable. This is not a problem with 99mTc-labelled agents.

  • Soft tissue attenuation. The effect of soft tissue attenuation may be generalized or localized. Generalized attenuation, encountered in obese patients and in individuals with a large chest circumference, results in decreased counts density and poor image quality. More frequent are fixed, localized soft tissue attenuation artifacts secondary to the left hemidiaphragm, large abdomen, the left breast, and lateral chest wall fat, usually mimic myocardial scarring. Breast attenuation generates apparent hipoperfusion of the anterior wall and can be seen not only in women with large breasts but also in those with relatively small but dense breasts. Shifting breast attenuation can be caused by different position of the breast in the stress and rest acquisitions. Such differences will result in attenuation artifacts that affect different portions of the left ventricle in the stress and rest images and thereby simulate stress-induced ischemia and/or reverse distribution. Localized diaphragmatic attenuation generally creates a fixed inferior defect. Unlike the breast, the position of the left hemidiaphragm is relatively stable and therefore, shifting attenuation artifacts are relatively rare.

Repeating the acquisition with the left breast elevated and restrained with tape, or in the prone position, can diminish the attenuation effect of breast tissue and the left hemidiaphragm, respectively. Attenuation correction, now commercially available on many scintillation camera systems, is of benefit in minimizing or eliminating many of these artifacts. However, the interpreting physician must be cautions of unique artifacts produced by current attenuation correction algorithms, including truncation, defects produced by inadequate transmission source count density and uniformity, exaggeration of apical thinning, and scatter of subdiaphragmatic activity into the inferior wall of the left ventricle. Gated perfusion SPECT is an efficient alternative to minimize false positive studies because portions of the heart with attenuation will have preserved wall motion and thickening, while true fybrotic tissue will not.

c) Image processing and display artifacts.

  • Filtering. Commercial manufacturers have incorporated empirically determined optimal filters for SPECT myocardial perfusion protocols. Technologists and interpreting physicians should be aware of the consequences of altering filter parameters and they should also be able to recognize the effects of applying standard filters to images of unusually low or high count density.
    Decreasing the filter cutoff (critical frequency) will render very smooth images with loss of detail and contrast resolution, and hence with decreased sensitivity in detecting perfusion abnormalities. On the other hand, increasing the cutoff value will accentuate high frequency data and exaggerate noise, giving the tomogram an unpleasant, difficult to interpret appearance. Likewise, lowering the critical frequency will decrease the apparent extent and severity of a true perfusion abnormality, whereas increasing the critical frequency may appear to accentuate the defect. The result is similar if the filter cutoff is held constant and the count density of an image decreases: true perfusion abnormalities as well as attenuation artifacts may appear more evident and severe. It is possible to adjust a filter to match the count density of an image (adaptive filtering). However, unless the technologist is exceptionally knowledgeable about SPECT filters, it is probably advisable not to adjust prescribed filters but to instead anticipate filtering effects in particular patients.
  • Adjacent subdiaphragmatic activity. Abdominal tracer concentration adjacent to the inferior wall of the left ventricle may create significant image artifacts. 201Tl localizes in the liver, and MIBI and tetrofosmin have initial liver uptake and are then excreted via the hepatobiliary tract to the duodenum. The tracer moves distally in the small bowel, which may be positioned in the left upper quadrant of the abdomen, or it may sometimes reflux into the stomach. Subdiaphragmatic activity may artifactually increase inferior wall count density due to scatter, or decrease count density secondary to the application of the ramp filter during backprojection process.

Scatter: photons emanating from subdiaphragmatic activity may artifactually increase inferior wall count density. This phenomenon may obscure true inferior wall perfusion defects or, in normal patients, the inferior wall may become relatively hot. By subsequent image normalization an artifactual defect may be created in the contralateral anterior wall. Such artifacts are more common with 201Tl than with 99mTc because of the larger Compton angle, and when an all purpose collimator is used instead of a high resolution collimator.

Ramp filter artifact: with filtered backprojection, the ramp filter is used to eliminate the star artifact associated with reconstruction by applying negative values adjacent to the projection profiles. When there is intense, localized tracer concentration next to the heart, present in the liver, stomach, or small bowel, the negative values of the corresponding projection profiles may cause an artifactual decrease in count density in the inferior wall of the left ventricle. One way to check if the finding is an artifact caused by this effect is to raise the cutoff frequency of the filter, which will result in a noisier image but the "cold" area will be reduced. This effect is not evident with iterative reconstruction algorithms.

  • Improper selection of the apex and base for polar map reconstruction. Accurate and reproducible selection of the apex and base of the left ventricular myocardium is necessary in stress and rest images. Positioning the limits for slice selection too far basally will result in an apparent basal myocardial perfusion defect, while positioning the limits distal to the apex will result in an apparent apical defect. In contrast, positioning slice limits too tightly, so that they do no encompass the entire heart, may result in underestimation of the size and extent of a defect.
  • Errors in axis reorientation. If the long axis of the ventricle is defined incorrectly for reorientation on either transaxial or midventricular vertical long axis slices, the geometry of the heart in subsequently reconstructed orthogonal tomographic slices can be distorted. Consequently, the apparent regional count density can be altered resulting in artifactual perfusion defects.
  • Inadequate image display. Stress and perfusion tomograms should each be individually normalized to maximal myocardial count density so that stress and rest perfusion defects can be compared and assessed for reversibility. If images are incorrectly normalized to non-cardiac activity, particularly subdiaphragmatic tracer concentration, accurate comparison of stress and rest images is not possible.

d) Artifacts related to non-coronary disease.

  • Left bundle-branch block (LBBB). This conduction abnormality produces apparent septal hypoperfusion that can be either reversible or fixed, lowering the specificity of the study for detection of ischemic heart disease. In gated studies, however, the septal wall usually exhibits paradoxical motion but with preserved thickening, aiding in the differential diagnosis.
  • Left ventricular hypertrophy. It can cause localized hot spots with relative hypoactivity of remote areas, and other myocardial non-homogeneities.
  • Long membranous /short muscular septum. It can produce false impression of hypoperfused basal septum.
  • Apical thinning. This can be either a normal physiological variant or an artifact produced by varying spatial resolution of the acquired images (higher at the apex because of detector proximity). It usually simulates a fixed defect and can be differentiated from scar by gating since myocardial thickening and wall motion will be preserved.
  • The eleven / seven o'clock defect. More frequent at anterior-septal region (11 o'clock) in short-axis tomograms, this pseudo-defect probably represents attenuation by the right ventricle since it usually lies, together with the 7 o'clock defect, close to the insertion points of the right ventricular wall. Again, gated SPECT can aid in diagnosis.

e) Gating artifacts.

  • With severe arrhythmia or heart rate changes during SPECT acquisition, short or long cardiac cycles will be rejected according to the tolerance window selected as mentioned in the section on instrumentation and acquisition parameters. If the proportion of irregular beat rejection varies during acquisition, the total number of cardiac cycles acquired for each projection will be different, assuming that each projection image is acquired for the same length of time as is the standard protocol for gated perfusion SPECT. Therefore, projection images will differ in count density. When viewed in endless loop cine mode, the projection images will appear to flash. Acquisition of a variable number of counts in the projection images may potentially result in errors in filtered backprojection and consequently perfusion artifacts. However, it has been reported that clinically significant perfusion artifacts are produced only with severe arrhythmias, such as associated with atrial fibrillation.
  • Variation in cycle length also can produce low count density images specially those from the final portion of the cardiac cycle (usually frames 6-8). As a result of the normalization process, these images are assigned an average count rate after multiplying by a certain factor, with the effect of increasing noise value which is already present in excess. During backprojection, this noise significantly alters the quality of reconstructed images, potentially introducing hot and cold artifacts of variable shape and location.
  • Gating with waves other than R wave can result in false interpretation of end diastole, with a volume curve starting elsewhere in the cardiac cycle. This fact, however, rarely causes a problem other than a volume curve of unexpected shape.
  • The ECG signal can be lost during acquisition because of patient motion or electronic problems, thus causing severe deterioration of functional data. The technologist must constantly supervise the gated acquisition process, preferably with the aid of a ECG monitor or an acoustic signal.




In conclusion

Cardiac perfusion SPECT is a powerful diagnostic tool of increasing utilization worldwide. However, several technical aspects are critical to obtain good quality, reliable studies. Technological improvements such as multi-head cameras, attenuation correction devices and gating procedures represent new challenges for the technologist who must keep high educational and training standards to encompass such evolution.






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