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Basic Science Investigations |
1 Division of Cardiology, Department of Medicine, University of Texas Medical School, Memorial Hermann Hospital, Houston, Texas
2 Weatherhead PET Center for Preventing and Reversing Atherosclerosis, University of Texas Medical School, Memorial Hermann Hospital, Houston, Texas
3 Department of Medicine, Division of Cardiology, University of Texas Medical School, Memorial Hermann Hospital, Houston, Texas
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Key Words: PET • image registration • heart • attenuation • artifact
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Therefore, the purpose of this study was to determine the frequency of attenuation–emission misregistration artifacts in cardiac PET perfusion imaging, to document the responsible mechanisms, to identify predictors of misregistration artifacts, and to validate their routine clinical identification, prevention, or correction.
Although there is a substantial literature on technical aspects of PET scanner performance regarding image reconstruction (12,13), registration (14–22), and 3-dimensional (3D) imaging (17,18,21,23), this report is, to our knowledge, the first to describe the dynamic changes in the attenuating structures of lungs, heart, and diaphragm from rest to pharmacologic stress that may cause significant artifactual defects due to misregistration of attenuation and emission images unless recognized and corrected for each patient. Consequently, our study is important for extending the limits to which cardiac PET perfusion imaging can be used for detecting mild differences in regional CFR or the base-to-apex longitudinal perfusion gradient indicating early CAD as the basis for lifelong lifestyle and medical treatment or for changes in perfusion as a guide to treatment (1,2,6–8).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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PET Imaging
Patients were instructed to fast for 4 h and abstain from caffeine, theophylline, and cigarettes for 24 h before study. As previously described (4–6,8), PET imaging was performed using the University of Texas–designed, Positron Posicam Auricle, bismuth germanate, 2-dimensional (2D) multislice tomograph with a reconstructed resolution of 10-mm full width at half maximum (FWHM). Based on a 5-min positioning transmission scan, patients were precisely positioned in the PET scanner with the first image plane 1 cm below the radiolucent bronchial carina on the positioning transmission scan. Laser guides aligned to external body markers were used to check correct position at least 3 times throughout data acquisition with adjustments before each image acquisition to maintain precise alignment according to the external body markers.
Using a rotating rod source containing 148–185 MBq (4–5 mCi) of 68Ge, transmission images to correct for photon attenuation contained 100–150 million counts; the transmission scan image is also called the attenuation image in this article. Emission images obtained after intravenous injection of 925 MBq to 2.22 GBq (25–60 mCi) of generator-produced 82Rb contained 40–60 million counts or after 666 MBq (18 mCi) of cyclotron-produced 13N contained 20–40 million counts.
Immediately after completing resting 82Rb or 30 min after administration of the first dose of ammonia, dipyridamole (0.142 mg/kg/min) was infused for 4 min. At 4 min after completion of the dipyridamole infusion, the same dose of the same radionuclide was given intravenously. For 82Rb, imaging data acquisition was started at 60 s (normally) to 90 s (for patients with heart failure or cardiac enlargement) after the beginning of 82Rb infusion. For 13N-ammonia, imaging data acquisition was started at 4 min after injection to allow blood-pool clearing needed for lung washout of 13N-ammonia. PET imaging was repeated by the same protocol as for the resting study. For angina, aminophylline (125 mg) was given intravenously.
Automated Quantitative Analysis of PET Images
Images were reconstructed using filtered backprojection with a Butterworth filter having a cutoff of 0.4 and roll off of 10. Completely automated analysis of severity and size of PET abnormalities was performed by previously described software (1–8). A 3D restructuring algorithm generates true short- and long-axis views from PET transaxial cardiac images, perpendicular to and parallel to the long axis of the left ventricle. From the tomographic data acquired in 2D mode to minimize scatter, circumferential profiles are used to reconstruct 3D topographic views of the left ventricle showing relative regional activity distribution divided into lateral, inferior, septal, and anterior quadrant views of the 3D topographic display corresponding to the coronary arteries as previously described (3–8) and illustrated in Figure 1A.
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Misregistration Artifacts
Misregistration artifacts were defined qualitatively and quantitatively as (a) abnormalities or defects on perfusion images that did not correspond to any of the coronary artery distribution territories in comparison with our previously reported precise myocardial perfusion map (4); (b) the area of the abnormality on the PET image that was associated with a corresponding area of misregistration of the transmission and emission scans when superimposed; and (c) the defects disappeared and the image normalized when the emission images were shifted to superimpose them precisely on the attenuation images and reconstructed again using the shifted emission data. Healthy control subjects were asymptomatic volunteers with normal rest and stress perfusion images and no risk factors for CAD.
Image Analysis Protocols
To determine the causes or mechanisms of misregistration producing image artifacts, 3 different attenuation-rest-stress imaging protocols were studied.
Protocol 1.
Protocol 1 consisted of 3 consecutive measured attenuation correction scans (MAC), with the first 2 scans obtained successively over 10 min each, followed by a third attenuation scan obtained over 20 min, followed by resting and dipyridamole emission scans.
Protocol 2.
Protocol 2 consisted of a measured attenuation correction scan obtained over 20 min at the beginning of the imaging sequence (early MAC), followed by resting emission and dipyridamole emission scans.
Protocol 3.
Protocol 3 consisted of a MAC attenuation scan obtained over 20 min at the end of the imaging sequence (late MAC) after resting and dipyridamole emission scans. In this later protocol, in addition to the late MAC attenuation image, 5-min segmented attenuation scans (SAC) were obtained before the resting emission scan (rest SAC) and immediately after the dipyridamole emission scan (stress SAC) just before the late MAC attenuation image.
Quantitative Measurements of Misregistration on PET Scans
The following objective quantitative measurements were made: (i) heart size; (ii) vertical and lateral heart displacement at rest compared with dipyridamole stress; (iii) diaphragm displacement between the initial attenuation scans and between rest and dipyridamole; (iv) size of misregistration of the attenuation and emission images expressed as the percentage of the emission heart area outside the heart contour superimposed on the attenuation image in the horizontal plane; and (v) amount of shifting of the emission image in centimeters that was required to correctly superimpose the attenuation and emission images sufficient to make the artifactual defect disappear (normalize) in the area of prior misregistration on repeated reconstruction using shifted emission data.
These endpoints are listed in a numbered sequence for conceptual clarity. However, for efficiency in image processing and making the measurements, these objective quantitative measurements were mechanically made in detail in a different lettered sequence as follows:
250 pixels. Misregistration areas of <250 pixels were not considered significant. The heart and misregistration areas were expressed in square centimeters calculated from the reconstructed image pixel resolution of 1.7 x 1.7 mm.
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Combined size and severity of perfusion defects were defined as the percentage of cardiac image with relative activity of <60% of maximum activity that is 3 SDs below the mean maximum activity of healthy control subjects. The apical quadrant was not included. This measure of perfusion defect severity has been a highly reproducible endpoint with a narrow SD that sensitively quantifies perfusion defects and changes among sequential PET scans with high statistical significance (2–8).
To determine the mechanisms underlying the attenuation and emission misregistration artifacts, a detailed analysis was performed in a subset of 54 patients from the early MAC protocol and in a subset of 55 patients from the late MAC protocol, all with characteristic, definite, and comparable misregistration artifacts. A group of 18 healthy volunteer control subjects imaged using the late MAC protocol was also analyzed. Since protocol 3 (late MAC) had rest SAC, stress SAC, and late MAC attenuation images, a more complete analysis of the mechanisms underlying misregistration of emission images and attenuation was obtained in this group. Additionally, precise measurement of diaphragm motion was obtained using protocol 1 in a group consisting of 45 randomly selected patients, divided in 3 equal groups based on their body mass index (BMI).
Statistical Analysis
All statistical analyses were performed using SPSS version 11.5 (SPSS Inc.). Data are reported as mean ± 1 SD or SEM. Differences among the means of continuous variables for patients and the healthy control group were analyzed with an independent samples, 2-tailed t test using the Levene test for equality of variances. ANOVA was performed for significance of variance with the Games–Howell post hoc test for unequal variances. Pearson bivariate correlation coefficients were calculated. Single stepwise multivariate linear regression analysis was performed to identify predictors for the quantitative degree of misregistration. Multivariate logistic regression analysis was performed to identify predictors for the presence or absence of misregistration artifacts in nonshifted images. A 2-tailed P value of <0.05 was considered statistically significant.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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To obtain additional mechanistic insights, we also examined the frequency of misregistration artifacts on resting compared with the dipyridamole images in each of these protocols. Artifactual defects on resting emission images were significantly more frequent using the postdipyridamole attenuation of the late MAC protocol (44/55 [80%]) than that using the resting attenuation of the early MAC protocol (25/54 [46%]), a significant difference with P = 0.02. Artifactual defects on stress emission images using the resting attenuation of the early MAC protocol (29/54 [54%]) were significantly more frequent than that using the postdipyridamole attenuation of the late MAC protocol (11/55 [20%]), a significant difference with P = 0.003.
Figure 1A illustrates a severe defect on the resting perfusion scan that normalized after shifting the emission image to precisely superimpose it on the attenuation scan and reconstructing the image using the shifted data. Figure 1B left upper panels show the outline of the heart from the emission image superimposed on the attenuation image with a large area outside the heart border on the attenuation image in vertical and horizontal views. Figure 1B right upper panels show the same views and outline of the heart after shifting the emission image to obtain correct coregistration. The bottom panel shows the scanner in-plane acquisition shift view of the heart emission image superimposed on the attenuation image before (left) and after (right) shifting the emission image to achieve correct coregistration in routine clinical operations. Figures 2A and 2B illustrate a similar severe artifactual defect on a dipyridamole PET scan. Figures 3 and 4 illustrate milder artifactual defects on rest and dipyridamole images in a similar format to show the range of artifactual defects caused by attenuation–emission misregistration.
The baseline data for the late MAC subgroup are given in Table 1. The control group consisted of younger, leaner patients, with significantly less diaphragmatic motion and smaller hearts, differences that are important for analyzing the mechanisms of misregistration of the attenuation and emission images. Small degrees of misregistration were found in the healthy control group but significantly greater misregistration was measured in the patients (P < 0.001). Interestingly, downward heart displacement rest to dipyridamole was not significantly different between patients and control subjects (P = 0.210).
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In Figures 1 through Figure 4, the rotated transmission images before shifting emission data for optimal coregistration are somewhat different than the rotated transmission images after shifting to optimize coregistration for the following reasons:
However, these differences in appearance of the rotated transmission images have no impact on measurements of the misregistration area since we have used the ratio between the misregistered area and the whole heart area, expressed as a percentage as illustrated in Tables 1 and 3.
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Since attenuation–emission misregistration was the single most significant predictor of artifactual defects, multivariate stepwise linear regression analysis was used to identify predictors of misregistration, shown in Table 4. Diaphragm displacement between the rest and stress images is the strongest predictor of attenuation–emission misregistration (P = 0.001, CI = 0.158–0.630). BMI was also a good predictor of misregistration (P = 0.005, CI = 0.202–1.124). Surprisingly, small whole heart area in the horizontal plane at rest was a significant predictor of misregistration, suggesting that smaller hearts are more prone to attenuation–emission misregistration (P = 0.004, CI = –0.144 to –0.028). The Pearson correlation coefficient between the whole heart area and attenuation–emission misregistration in the horizontal plane was significant and negative (r = –0.554, P < 0.001), suggesting that larger hearts are expected to have less misregistration and smaller hearts more misregistration.
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30.0 kg/m2). Diaphragmatic displacement between each of the 3 initial resting sequential attenuation scans was measured. After assuming the supine position in the scanner, substantial upward diaphragm displacement occurred mostly between the first 2 attenuation scans over the initial 10 min with less upward displacement during the third attenuation scan, over a period of 20 min. The upward diaphragm displacement between the initial 2 resting attenuation scans for the obese group was significantly larger compared with that of the other weight groups (P = 0.027 and P = 0.041, respectively), shown in Table 5.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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Correction of misregistration artifacts altered the PET report of virtually every patient in which such a correction was made in one or more of the following ways: (a) the final diagnosis in the PET report of early coronary atherosclerosis or equivalent description using other words such as "mild deposition of cholesterol in the coronary arteries consistent with age and risk factors" to avoid inappropriate insurance liability; (b) the extent or severity of coronary atherosclerosis in the PET report, for either diffuse disease based on the longitudinal base-to-apex perfusion gradient or severity of flow-limiting stenosis; and (c) the qualitative and quantitative assessment of changes in severity of perfusion changes on follow-up PET images as a guide to treatment or revascularization procedures.
The main independent predictors of artifactual defects in myocardial perfusion PET are (a) quantitative misregistration of attenuation and emission images both at rest and after dipyridamole, (b) heart size (area) in the horizontal plane at rest, and (c) the BMI. The single most significant predictor of attenuation–emission misregistration was downward diaphragm displacement after dipyridamole in addition to heart area and BMI. The fact that downward diaphragm displacement after dipyridamole strongly predicted attenuation–emission misregistration but did not reach statistical significance in predicting artifactual defects suggests that the downward diaphragm displacement interacts with other factors to cause misregistration artifactual defects. Misregistration of the attenuation and the emission images and the associated artifactual defects can be corrected by manually shifting the emission images over the attenuation image to achieve optimal coregistration of the cardiac free lateral wall and re-reconstructing the images using the shifted emission data.
Mechanisms Underlying Attenuation–Emission Misregistration
Our data can be explained by the following synthesis. Systolic contraction causes the root of the aorta and base of the heart to descend and the cardiac apex to recoil forward, up and to the left as in Figure 5, striking the inside of the chest wall where it is felt as an apical impulse. With tachycardia, the heart is in this recoil position for a longer time than that at resting heart rates. Therefore, during the interval of data acquisition for the dipyridamole PET image in which heart rate increases, the heart has a different average position during tachycardia as compared with the normal resting heart rate.
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Dipyridamole typically causes a feeling of chest fullness and labored breathing associated with tachypnea so that the diaphragm is in a lower position for a longer time than that during normal slow breathing. As the diaphragm shifts inferiorly after dipyridamole, the mediastinum also narrows as illustrated in Figure 6E and Figure 7B.
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Using the early resting measured attenuation data for the dipyridamole emission image may cause misregistration as illustrated in Figure 6. Using the late measured attenuation after dipyridamole for the dipyridamole emission scan reduces misregistration artifacts in stress emission images since the attenuation structures best approximate those during stress emission image acquisition. However, using the late attenuation after dipyridamole for the resting emission scan may then cause attenuation–emission misregistration and artifactual defects in the rest emission scan, as illustrated in Figure 7.
The correlation between misregistration and BMI may be explained by several potential mechanisms. Greater BMI indicates more visceral fat having greater effects on the diaphragm position associated with greater attenuation–emission misregistration and more artifactual defects. In addition to these complex position changes, the amount of fat around the heart also has significant effects on attenuation–emission misregistration. In obese people, the fat around the heart causes the borders of the attenuation image to be bigger than the borders of the heart. Therefore, the left mediastinal border of the attenuation image extends farther leftward than the myocardium and thereby produces a "halo" of attenuation-corrected space around the heart that further complicates attenuation correction. The association of small hearts with more attenuation–emission misregistration is likely due to the relatively poor resolution of PET compared with small hearts such that even slight absolute misregistration causes a relatively large percentage of the heart to be misregistered.
Lateral cardiac recoil and diaphragmatic displacement have different effects on attenuation–emission superposition depending on whether the heart is positioned vertically as in thinner subjects or horizontally as in heavier people. For a horizontal heart as occurs in heavier people, recoil and diaphragm displacement tend to cause vertical misregistration with an anterolateral artifact, whereas in a vertical heart the artifact is more lateral. In thin people, misregistration artifacts are more likely due to cardiac recoil and downward diaphragmatic displacement after dipyridamole. In heavier people, these artifacts are more likely due to slow plastic upward displacement of the diaphragm and visceral fat on assuming the supine position plus later the same above changes after dipyridamole.
z-Axis misaligment along the long axis of the scanner is important as our illustration shows. However, as illustrated in Figures 5– 7, the long axis and the left border of the heart are slanted leftward and downward so that horizontal shifting of the images in the in-plane acquisition tomographic views also corrects for vertical misalignment of attenuation and emission images as our data demonstrate. z-Axis shifting along the long axis of the scanner in 3D PET acquisition without in-plane septa is easier than in 2D PET acquisition with in-plane septa; however, the scatter from other organs in 3D PET is a major problem that substantially degrades cardiac images in our experience.
Solutions for Attenuation–Emission Misregistration
These complex changes in the attenuation structures of the thorax preclude predicting attenuation–emission misregistration artifacts in any individual. However, an attenuation–emission imaging sequence can be chosen to reduce misregistration artifacts that can also be recognized, routinely assessed on clinical images, and corrected. In our experience, the sequence of a single initial resting attenuation scan followed by resting and dipyridamole emission scans has the highest probability of attenuation–emission misregistration artifacts. For a one-attenuation scan protocol, the sequence of rest and dipyridamole emission scans followed by a late measured attenuation scan starting approximately 5–10 min after the dipyridamole emission scan has fewer misregistration artifacts since the late attenuation scan better approximates the average attenuation structures over the period of emission imaging.
Other solutions include separate measured attenuation scans for rest and for dipyridamole as we now currently do for more reliable results despite slightly prolonging the study. However, we also check every image with any defect for correct attenuation–emission coregistration using the acquisition tomographic shift software and shift the emission images to achieve optimal coregistration and reconstruct the emission images with shifted data if needed.
In our experience a short SAC attenuation scan immediately after dipyridamole frequently causes attenuation–emission misregistration for several complex interacting reasons. The attenuation structures during this brief SAC acquisition do not approximate the average of the changing attenuation structures during emission scanning or we do not adequately understand the SAC method yet. Another approach to attenuation correction in PET may combine CT and PET scanning that also reduces the study time (23–25). However, the same potential problems may be exaggerated by the very brief CT attenuation acquisition that does not approximate the average of the dynamically changing attenuation structures present during the more prolonged emission data collection. Further studies of the combined PET/CT approach are needed.
Limitations of Study
An expected criticism of this study is lack of coronary arteriography to prove that the image abnormalities were artifactual and are not due to CAD. Our study has demonstrated artifactual defects due to attenuation–emission misregistration on resting images that improve or disappear on dipyridamole images in which the attenuation–emission registration is corrected. The disappearance of resting image abnormalities on stress images after dipyridamole is not consistent with flow-limiting coronary artery stenosis as justification for coronary arteriography. For assessing the less severe perfusion abnormalities seen in early preclinical coronary atherosclerosis, coronary arteriography cannot be justified. Furthermore, based on >1,000 patients with coronary arteriograms, we have previously published the diagnostic value of mild-to-moderate regional PET perfusion defects and of the base-to-apex longitudinal perfusion gradient associated with diffuse nonobstructive coronary atherosclerosis before significant localized stenosis develops (1–3). By demonstrating specific attenuation–emission misregistration in the same region as the abnormality on the PET images and normalization of the images when the misregistration is corrected, we believe that our results do not require coronary arteriographic confirmation. Moreover, we have shown the longitudinal base-to-apex perfusion gradient to be a significant marker of diffuse CAD before significant flow-limiting coronary artery stenoses occur (3). This observation combined with the well-documented gross inaccuracies of visual arteriographic interpretation could be viewed as PET providing a more reliable reference standard for severity of coronary disease than the coronary arteriogram as used now clinically, provided attenuation–emission images are correctly coregistered.
A second criticism of the study is the absence of complete information on diaphragmatic displacement on assuming the supine position or after dipyridamole in the 3 different imaging protocols reported here. Traditionally, the cardiac PET imaging protocols involved only a single initial resting measured attenuation scan followed by rest and dipyridamole emission scans. For patients studied using this traditional single resting attenuation image before May 2000, we recognized a potential problem but had no knowledge of the causes or solutions. By May 2000, at the beginning of the 1,177 patients in this study, we had discovered the attenuation–emission misregistration problem and its empiric correction by shifting the emission images to obtain coregistration but without understanding the mechanisms. With further experience we have developed multiattenuation emission image sequences to study the mechanisms and solutions as reported here.
As a consequence of this study, we now routinely use a protocol with the following image acquisition sequence: a resting measured attenuation, resting emission, dipyridamole emission, and a postdipyridamole measured attenuation. Our study demonstrates a concept where the details of thresholding, viewing, and coregistering emission and attenuation images may vary depending on the scanner, the radionuclide used, the reconstruction technique, resolution settings, and the goal of cardiac PET in each case.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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These artifactual defects due to attenuation–emission misregistration typically (a) are anterolateral or lateral but do not correspond to the typical course of any of the coronary distribution territories in comparison with a precise myocardial perfusion map, (b) are associated with visual and quantifiable misregistration of superimposed attenuation and emission images, and (c) disappear when the misregistration is corrected by manually shifting the emission image over the attenuation image until optimally superimposed with the image normalizing on re-reconstruction using shifted emission data. Multiattenuation rest and dipyridamole imaging sequences with routine review and coregistration of superimposed attenuation and emission images substantially improve PET images for identifying mild abnormalities of relative CFR due to early nonobstructive coronary atherosclerosis as the basis for intense, life-long pharmacologic and lifestyle management.
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FOOTNOTES |
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For correspondence or reprints contact: K. Lance Gould, MD, Weatherhead PET Center for Preventing and Reversing Atherosclerosis, University of Texas Medical School, 6431 Fannin St., Room 4.256MSB, Houston, TX 77030.
E-mail: k.lance.gould{at}uth.tmc.edu
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONREFERENCES |
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