Clinical implication of down-scatter in attenuation-corrected myocardial SPECT

A study to improve the image quality in low-dose computed tomography (SPECT) using filtration

BACKGROUND

The output of the X-ray tube used in computed tomography (CT) provides a spectrum of photon energies. Low-energy photons are preferentially absorbed in tissue; the beam spectrum shifts toward the higher energy end as it passes through more tissue, thereby changing its effective attenuation coefficient and producing a variety of artifacts (beam-hardening effects) in images. Filtering of the beam may be used to remove low-energy photon component. The accuracy of attenuation coefficient calculation by bilinear model depends highly upon accuracy of Hounsfield units. Therefore, we have made an attempt to minimize the beam-hardening effects using additional copper filter in the X-ray beam. The quantitative evaluation were made to see the effect of additional filters on resulting CT images.

MATERIALS AND METHODS

This study was performed on dual-head SPECT (HAWKEYE 4, GE Healthcare) with low-dose CT which acquires images at peak voltages of 120/140 kV and a tube current of 2.5 mA. For the evaluation of image quality, we used CT QA Phantom (PHILIPS) having six different density pins of Water, Polyethylene, Nylon (Aculon), Lexan, Acrylic (Perspex) and Teflon. The axial images were acquired using copper filters of various thicknesses ranging from 1 to 5 mm in steps of 1 mm. The copper filter was designed in such a manner that it fits exactly on the collimator cover of CT X-ray tube. Appropriate fixation of the copper filter was ensured before starting the image acquisition. As our intention was only to see the effect of beam hardening on the attenuation map, no SPECT study was performed. First set of images was acquired without putting any filter into the beam. Then, successively, filters of different thicknesses were placed into the beam and calibration of the CT scanner was performed before acquiring the images. The X-ray tube parameters were kept the same as that of unfiltered X-ray beam. All the acquired image sets were displayed using Xeleris 2 (GE Healthcare) on a high-resolution monitor. Moreover, Jaszak's SPECT Phantom after removing the spheres was used to see the different contrast intensities by inserting the different contrast materials of iodine and bismuth in water as background media. Images were analyzed for visibility, spatial resolution and contrast.

RESULTS

Successive improvement in the image quality was noticed when we increased the filter thickness from 1 to 3 mm. The images acquired with 3-mm filter appeared almost with no artifacts and were visibly sharper. Lower energy photons from X-ray beam cause a number of artifacts, especially at bone-tissue interfaces. Additional filtrations removed lower energy photons and improved the image quality. Degradation in the image quality was noticed when we increased the filter thickness further to 4 and 5 mm. This degradation in image quality happened due to reduced photon flux of the resulting X-ray beam, causing high statistical noise. The spatial resolution for image matrix of 512 × 512 was found to be 1.29, 1.07, 0.64 and 0.54 mm for without filter, with 1, 2 and 3 mm filters, respectively. The image quality was further analyzed for signal-to-noise ratio (SNR). It was found to be 1.72, 1.78, 1.98 and 1.99 for open, with 1, 2 and 3 mm filters respectively. This shows that 3-mm filter results in an improvement of 15.7% in SNR.

CONCLUSION

On the basis of this study, we could conclude that use of 3-mm copper filter in the X-ray beam is optimal for removing the artifacts without causing any significant reduction in the photon flux of the resulting X-ray beam. We also propose that as artifacts have been removed from the images, the value of Hounsfield units will be more accurate and hence the value of attenuation coefficients lead to better contrast and visualization of SPECT images.

Disappearance of myocardial perfusion defects on prone SPECT imaging: comparison with cardiac magnetic resonance imaging in patients without established coronary artery disease

BACKGROUND

It is of great clinical importance to exclude myocardial infarction in patients with suspected coronary artery disease who do not have stress-induced ischemia. The diagnostic use of myocardial perfusion single-photon emission computed tomography (SPECT) in this situation is sometimes complicated by attenuation artifacts that mimic myocardial infarction. Imaging in the prone position has been suggested as a method to overcome this problem.

METHODS

In this study, 52 patients without known prior infarction and no stress-induced ischemia on SPECT imaging were examined in both supine and prone position. The results were compared with cardiac magnetic resonance imaging (CMR) with delayed-enhancement technique to confirm or exclude myocardial infarction.

RESULTS

There were 63 defects in supine-position images, 37 of which disappeared in the prone position. None of the 37 defects were associated with myocardial infarction by CMR, indicating that all of them represented attenuation artifacts. Of the remaining 26 defects that did not disappear on prone imaging, myocardial infarction was confirmed by CMR in 2; the remaining 24 had no sign of ischemic infarction but 2 had other kinds of myocardial injuries. In 3 patients, SPECT failed to detect small scars identified by CMR.

CONCLUSION

Perfusion defects in the supine position that disappeared in the prone position were caused by attenuation, not myocardial infarction. Hence, imaging in the prone position can help to rule out ischemic heart disease for some patients admitted for SPECT with suspected but not documented ischemic heart disease. This would indicate a better prognosis and prevent unnecessary further investigations and treatment.

Single-photon emission computed tomography/computed tomography: basic instrumentation and innovations

Correlation of the anatomical and functional information presented by single-photon emission computed tomography (SPECT) and computed tomography (CT) can aid in the decision-making process by enabling better localization and definition of organs and lesions and improving the precision of surgical biopsies. Technical developments over the past 20 years have led to the development of better software techniques for image fusion and, more recently, to the development of modern SPECT/CT systems. While image fusion techniques have been in clinical use for many years, the first commercial SPECT/CT system was only developed in 1999. Following the commercial success of PET/CT systems that employed multidetector CT (MDCT) scanners, there has been renewed interest in the development of comparable SPECT/CT systems. This has resulted in the development of a range of SPECT/CT devices varying from a simple CT add-on to a conventional SPECT system that can provide low-dose CT images to a full MDCT scanner integrated with a SPECT system. The advantages of combining SPECT with CT are numerous and are primarily due to the anatomic referencing and the attenuation correction capabilities of CT. Depending on system design, there are varying technical issues surrounding the different SPECT/CT devices, ranging from cost, radiation dose, planning, and siting requirements to system-specific issues such as table sag and CT artifacts due to patient motion. Motion artifacts should be less prevalent with the faster acquisition times of modern scanners, but are still problematic in the thorax and have not yet been fully resolved as they pertain to the use of CT data for cardiac attenuation correction. As this technology matures, we can expect to see a range of SPECT/CT devices available on the market that range from low-dose 1-4 slice inexpensive CT upgrades of conventional SPECT systems, to SPECT systems incorporating 64 or 128 slices CT scanners. The cost of the high-end CT scanners will exceed the cost of the SPECT scanner and hence the justification for such devices will be heavily dependent on clear demonstration of their value in clinical practice.

Clinical validation of SPECT attenuation correction using x-ray computed tomography-derived attenuation maps: multicenter clinical trial with angiographic correlation

BACKGROUND

Nonuniform attenuation artifacts cause suboptimal specificity of stress single photon emission computed tomography (SPECT) myocardial perfusion images. In phantoms, normal subjects, and patients suspected of having coronary artery disease (CAD), we evaluated a new hybrid attenuation correction (AC) system that combines x-ray computed tomography (CT) with conventional stress SPECT imaging.

METHODS AND RESULTS

The effect of CT-based AC was evaluated in phantoms by assessing homogeneity of normal cardiac inserts. AC improved homogeneity of normal cardiac phantoms from 11% +/- 2% to 5% +/- 1% (P < .001). Attenuation-corrected normal patient files were created from 37 normal subjects with a low likelihood (<3%) of CAD. The diagnostic performance of AC for detection of CAD was evaluated in 118 patients who had stress technetium 99m sestamibi or tetrofosmin stress SPECT imaging and coronary angiography. SPECT images with and without AC were interpreted by 4 blinded readers with different interpretative attitudes. Overall, AC improved the diagnostic performance of all readers, particularly the normalcy rate. The degree of improvement depended on interpretative attitude. Readers prone to high sensitivity or with less experience had the greatest gain in the normalcy rate, whereas a reader prone to higher specificity had improvements in sensitivity and specificity but not the normalcy rate. Importantly, improvement of one diagnostic variable was not associated with worsening of other variables.

CONCLUSION

CT-based AC of SPECT images consistently improved overall diagnostic performance of readers with different interpretive attitudes and experience. CT-based AC is well suited for routine use in clinical practice.

EANM/ESC procedural guidelines for myocardial perfusion imaging in nuclear cardiology

The European procedural guidelines for radionuclide imaging of myocardial perfusion and viability are presented in 13 sections covering patient information, radiopharmaceuticals, injected activities and dosimetry, stress tests, imaging protocols and acquisition, quality control and reconstruction methods, gated studies and attenuation-scatter compensation, data analysis, reports and image display, and positron emission tomography. If the specific recommendations given could not be based on evidence from original, scientific studies, we tried to express this state-of-art. The guidelines are designed to assist in the practice of performing, interpreting and reporting myocardial perfusion SPET. The guidelines do not discuss clinical indications, benefits or drawbacks of radionuclide myocardial imaging compared to non-nuclear techniques, nor do they cover cost benefit or cost effectiveness.

Contributions of subdiaphragmatic activity, attenuation, and diaphragmatic motion to inferior wall artifact in attenuation-corrected Tc-99m myocardial perfusion SPECT

BACKGROUND

Subdiaphragmatic activity and diaphragmatic motion both contribute to inferior wall artifacts in technetium 99m myocardial perfusion single photon emission computed tomography (SPECT).

METHODS AND RESULTS

We used an anthropomorphic phantom with ventricular wall activity, liver/spleen inserts containing variable Tc-99m activity, and variable vertical (diaphragmatic) motion amplitude. SPECT and transmission scans were obtained on a GE Optima NX camera. Data were processed by use of filtered backprojection or attenuation correction (AC). Resulting myocardial activity maps were analyzed with standardized inferior-anterior and anterior-lateral wall ratios. At a subdiaphragmatic-myocardial activity ratio of 0.5:1, inferior wall attenuation predominates, producing a cold artifact. AC corrects inferior wall activity to the level of the anterior wall irrespective of diaphragmatic motion. At a subdiaphragmatic-myocardial activity ratio of 1:1, inferior wall counts vary widely depending on the proximity of subdiaphragmatic activity to the ventricle. With increasing diaphragmatic amplitude, the overlap of subdiaphragmatic activity and inferior wall worsens, leading to a complex mixture of cold and hot artifacts, not corrected by AC.

CONCLUSIONS

Concentration and proximity of subdiaphragmatic Tc-99m activity relative to myocardium comprise a major factor in the nature and severity of inferior wall artifacts. If the subdiaphragmatic Tc-99m concentration is equivalent to that in the myocardium, complex, potentially uninterpretable hot and cold inferior wall artifacts are produced.

Attenuation correction single-photon emission computed tomography myocardial perfusion imaging

Clinicians now rely heavily on the results of single-photon emission computed tomography (SPECT) myocardial perfusion imaging for diagnosing coronary disease and for planning therapy. However, the technique is imperfect for these purposes, mainly because of technical limitations, the most prominent of which is the effect of soft-tissue attenuation on apparent tracer distribution. Providers have attempted to compensate for this by a number of indirect approaches. Recently, validated hardware and software solutions for directly correcting image data for soft-tissue attenuation have become widely available commercially. Optimal application requires an understanding of the technical details that differ somewhat from system to system, the quality control prerequisites, knowledge of the importance of the transmission map quality, and how dedicated SPECT and SPECT-computed tomography systems present different challenges. In addition, the clinical literature is expanding rapidly, including studies on diagnostic accuracy, image appearances, quantitative analysis, appropriate patients for attenuation correction, clinical utility, incremental value in relation to ECG-gating, and risk stratification.

[Diagnostic accuracy of the SPECT of post-stress myocardial perfusion with attenuation and scatter correction]

OBJECTIVE

To evaluate the effect of attenuation and scatter correction (AC-SC) on the diagnostic accuracy of post-stress myocardial perfusion (MP) SPECT.

MATERIAL AND METHODS

The retrospective analysis included 121 patients who had a non-corrected (NC) and AC-SC 99mTc-Tetrofosmin MP SPECT after stress. The left ventricle was divided into 13 segments. Two observers performed a visual assessment of the MP on a scale from 0 (perfusion defect) to 3 (normal uptake). A consensus on concordances and discordances between the NC and AC-SC images was established. Final diagnosis of coronary artery disease (CAD) was established by coronary angiography (CANG) (stenosis > or = 70 %).

RESULTS

The combined analysis of NC and AC-SC images produced 93 concordances and 28 discordances. Of the 93 concordances, both studies were abnormal in 67 patients (abnormal CANG in 57) and normal in 26 patients (normal CANG in 20). Among the 28 discordances, 23 were abnormal NC/normal AC-SC (normal CANG in 18) and 5 normal NC/abnormal AC-SC. In these 5 patients AC-SC generated anterior perfusion defects but the CANG was normal. Overall, the appearance of NC and AC-SC images were in agreement with the CANG findings in the 72 % (87/121) and 78 % (95/121) of the patients, respectively. Sixty-seven of the 90 patients with abnormal NC had also abnormal AC-SC (abnormal CANG in 57) and the other 23 had normal AC-SC (normal CANG in 18). The appearance of AC-SC was in agreement with CANG finding in the 83 % (75/90) of patients with abnormal NC. MP abnormalities in NC normalized by AC-SC were more frequently located in inferior wall

CONCLUSION

AC-SC improves the diagnostic accuracy of post stress NC MP SPECT for the diagnosis of CAD. From these results we consider that AC-SC is of clinical value for the correction of attenuation artifacts, more frequently observed in the inferior wall. The presence of antero-apical perfusion defects in AC-SC with normal NC does not mean CAD. So it is necessary to adjust the normalcy pattern of MP SPECT when AC-SC is performed.

[Effect of attenuation correction and scatter compensation on the 99mTc-MIBI myocardial perfusion spect in patients without coronary artery disease]

OBJECTIVE

Attenuation correction (AC) and scatter compensation (SC) techniques are recent developments of myocardial perfusion SPECT. Our aim was to evaluate the effect of AC + SC on the myocardial distribution of 99mTc-MIBI in a population without coronary artery disease.

MATERIAL AND METHODS

A multiarray of Gd-153 linear sources was used for simultaneous transmission/emission 99mTc-MIBI myocardial perfusion SPECT in 27 patients without coronary artery disease. A visual analysis and polar map quantification was performed. Changes between non-corrected (NC) and corrected (AC + SC) studies were compared.

RESULTS

AC + SC produced an increase in liver activity and better visualization of the right ventricle. Intestinal activity increased in six patients. Myocardial homogeneity was increased by AC + SC. No differences by gender were observed after AC + SC. In females AC + SC led to a decrease of uptake in the anterior wall, apex and apical segments of the lateral wall and septum, and an increase in the inferior wall. In males AC + SC caused an increase of uptake in the inferior wall and in the basal segments of septum and a decrease of uptake in apex and apical segments of anterior and lateral walls. AC + SC generated false defects in the anterior wall of five patients.

CONCLUSIONS

Our results show the usefulness of AC + SC for compensating the interferences produced by attenuation on the myocardial distribution of 99mTc-MIBI. Because AC + SC may introduce false defects, it must not be applied to normal perfused myocardium.

Efficient simulation of SPECT down-scatter including photon interactions with crystal and lead

A major image degrading factor in simultaneous Dual Isotope (DI) SPECT or simultaneous Emission-Transmission (ECT-TCT) imaging, is the detection of photons emitted by the higher energy isotope in the energy window used for imaging the lower energy isotope. In Tc-99m/Tl-201 DI-SPECT typically tens of percents of the total detected down-scatter is caused by lead x rays. In Tc-99m/Gd-153 ECT-TCT, a comparable fraction of the down-scatter originates from Tc-99m photons which only partly deposit their energy in the detector crystal (i.e., due to crystal interactions). Efficient simulation methods which model down-scatter can be used to optimize DI-SPECT or ECT-TCT imaging acquisition or reconstruction protocols. In this paper we adapt a previously proposed efficient down-scatter simulation method, to include the interactions of photons with the detector crystal and collimator lead. To this end, point spread function tables including crystal and lead interactions are precalculated. Subsequently, photons are traced through the patient body until their last scatter position, and the precalculated responses are used to project the photons onto the detector plane, while photon attenuation in the patient is taken into account. The approach is evaluated by comparing simulated Tc-99m down-scatter projections with measured projections. Incorporation of photon interaction with crystal and lead leads to significantly improved accuracy of the shape of down-scatter responses, while differences in total counts between simulated and measured projections typically decrease from tens of percents to a couple of percents. Calculating 60 down-scatter projections of an extended distribution on a 64 x 64 x 64 grid takes about three minutes on a PC with two 1.2 GHz processors. We conclude that accurate and efficient simulation of down-scatter is now possible including the major effects of the nonuniform mass density of the patient as well as photon interactions with the crystal and collimator lead.

Should SPET attenuation correction be more widely employed in routine clinical practice? Against

The development of reliable and accurate devices for the correction of nonuniform soft tissue attenuation is essential for the future clinical use of SPET myocardial perfusion imaging. In addition to abolishing false-positive defects, which is the chief goal, such corrected SPET images may allow for improved detection of coronary artery disease and perhaps ultimately for true quantification of regional myocardial blood flow. Although progress has been made, most existing attenuation correction devices are not yet ready for prime time. To date the literature shows as many positive results as negative results. There is considerable uncertainty, confusion, and skepticism about the true reliability and value of currently available attenuation correction packages. Although commonly referred to as "attenuation correction devices," these packages are in fact much more complex systems and contain novel mechanical designs, novel image acquisition and image reconstruction algorithms, scatter correction, and depth-dependent resolution compensation, in addition to attenuation correction. Each of these variables needs to be better understood and tested prior to clinical implementation. Although the general concepts are shared, there are as may different approaches to attenuation correction as there are vendors. In order to minimize the confusion of potential buyers about such complex systems, it is desirable that, before attenuation correction is implemented in routine clinical practice, each attenuation correction device is rigorously tested using a standardized testing protocol. Potential buyers of equipment should be able to compare the results of testing with various devices against predefined criteria in order to make an educated decision. Such standards have as yet not been developed. At the present time it is unclear whether attenuation correction of cardiac SPET will remain the emperor's new clothes or will develop into a fashionable Armani suit. Until further progress has been made, one cannot recommend attenuation correction devices for routine clinical practice.

Performance of simultaneous emission-transmission systems for attenuation-corrected SPEct: a method for validation applied to two camera systems

Several commercially available systems for attenuation correction in single photon emission computed tomography (SPECT) based on a transmission scan have been introduced that vary in performance. A test procedure for attenuation correction in SPECT is described and applied to two principally different gamma camera systems (the Siemens Multispect 3 triple-headed system [3HS] and the ADAC Genesys Vertex double-headed system [2HS]). The test procedure was based on geometrically well-defined phantoms. A torso phantom was used to illustrate the attenuation correction methods. The test procedure can be used without detailed knowledge of or access to the algorithms used for attenuation correction. The influence on the transmission measurement of radioactivity in a phantom was higher for the 2HS than for the 3HS. The 3HS produced satisfactory attenuation maps and corrected emission count rates to a constant value independent of phantom density and size. With the 2HS, there was a progressive decrease in the correction of emission count rates with increasing phantom density, and about 30% lower corrected count rates in the large compared with the small phantom. A decrease in measured attenuation coefficients in the vicinity of an emission source was demonstrated in large but not small phantoms. A likely explanation is erroneous correction of downscatter into the transmission energy window. This study demonstrates the need for independent evaluation of systems for attenuation correction in SPECT.