P. Hendrik Pretorius

A mathematical model of motion of the heart for use in generating source and attenuation maps for simulating emission imaging

This manuscript documents the alteration of the heart model of the three-dimensional (3D) mathematical cardiac torso (MCAT) phantom to represent cardiac motion. The objective of the inclusion of motion was to develop a digital simulation of the heart such that the impact of cardiac motion on single-photon emission computed tomography (SPECT) imaging could be assessed and methods of quantitating cardiac function could be investigated. The motion of the gated 3D MCATs (gMCAT) heart is modeled using 128 separate and evenly spaced time samples from a blood volume curve approximating an average heart cycle. Sets of adjacent time samples can be grouped together to represent a single time interval within the heart cycle. Maximum and minimum chamber volumes were selected to be similar to those of a normal healthy person while the total heart volume stayed constant during the cardiac cycle. Myocardial mass was conserved during the cardiac cycle and the bases of the ventricles were modeled as moving towards the static apex. The orientation of the 3D MCAT heart was changed during contraction to rotate back and forth around the long axis through the center of the left ventricle (LV) using the end systolic time interval as the time point at which to reverse direction. Simple respiratory motion was also introduced by changing the orientation of the long axis of the heart to represent its variation with respiration. Heart models for 24 such orientations spanning the range of motion during the respiratory cycle were averaged together for each time sample to represent the blurring of the heart during the acquisition of multiple cardiac cycles. Finally, an option to model apical thinning of the myocardium was included. As an illustration of the application of the gMCAT phantom, the gated heart model was evaluated by measuring myocardial wall thickening. A linear relationship was obtained between maximum myocardial counts and myocardial thickness, similar to published results. Similar results were obtained for full width at half maximum (FWHM) measurements. With the presence of apical thinning, an apparent increase in counts in the apical region compared to the other heart walls in the absence of attenuation compensation turns into an apparent decrease in counts with attenuation compensation. The apical decrease was more prominent in end systole (ES) than end diastole (ED) due to the change in the partial volume effect. These observations agree with clinical trends. It is concluded that the gMCAT phantom can be used to study the influence of various physical parameters on radionuclide perfusion imaging.

An investigation of the estimation of ejection fractions and cardiac volumes by a quantitative gated SPECT software package in simulated gated SPECT images.

BackgroundThe purpose of this investigation was to determine the accuracy of the estimation of ejection fractions (EFs) and left ventricular volumes from a commercially available soft-ware package (Quantitative Gated SPECT[QGS]) as a function of different true EFs, count level in the acquisitions, severity and location of perfusion defects, increasing hepatic, activity, and modified wall motion.Methods and ResultsThe dynamic mathematic cardiac-torso digital phantom was used to create three-dimensional source and attenuation maps representing the distribution of a technetium-99m-labeled cardiac perfusion agent in the chest. Three hearts with varying end-systolic volumes were used to investigate different EFs. Perfusion defects were created as localized uptake within selected portions of the cardiac walls, scaled to the desired fraction of the normal wall uptake, and subtracted from the normal distribution. The hepatic uptake was increased up to five times of the normal heart uptake to investigate the influence of a “hot” liver. Alteration a lateral wall motion was also investigated. A three-dimensional projector that included the influence of distance-dependent spatial resolution and nonuniform attenuation was then used to create projection images. The projections were scaled to the desired acquisition count level, and Poisson noise was added. Automatic determination of EF slightly overestimated the true EF for normal count levels by 3% to 7% of the true EF and underestimated the true EF by up to 9% for very low count levels for 180-degree reconstructions. The accuracy for determining the volumes was not as high as for the EFs (an average error of 12% was observed). The calculated EFs were relatively accurate for perfusion defects of 50% or less. When perfusion defects exceeded 50%, extracardiac counts were included in the heart contours causing larger underestimations of EF. With removal of the extracardiac counts, the EFs increased. With a hepatic uptake of two or more times the heart uptake, no meaningful EF could be obtained. Either drawing a single region of interest for every slice or use of the manual mode with constrain option could remarkably improve the estimation. The accuracy of the calculation of EF and volumes for the heart with stationary wall was fairly high but decreased significantly when coupled with perfusion defects.ConclusionIt is concluded that the QGS program evaluates the functional parameter of EF accurately. The biggest limitations occurred in determining the appropriate cardiac contour if areas with very high extracardiac counts were present in the heart slices, and when a greater than 50% decrease occurred in uptake for perfusion defects.

CHAPTER 22 – Attenuation, Scatter, and Spatial Resolution Compensation in SPECT

This chapter explains methods of correcting for complicating factors in the single-photon emission computed tomography (SPECT) image reconstruction process, namely, attenuation and scatter. SPECT imaging is not ideal. Inherent in SPECT imaging are degradations that distort the projection data. This chapter focuses on three such degradations and the compensation for them. The first is attenuation— in order for a photon to become part of a measured projection, it must escape the body, the second source of degradation is the inclusion of scatter in the projections, and the third source of degradation is the finite, distance-dependent spatial resolution of the imaging system. It is noted that, solely compensating for attenuation is not enough to improve SPECT image quality to its fullest extent. Instead, attenuation, scatter, resolution, correction of patient, physiological motion, and changes in localization during the course of acquisition, can impact image quality, and thus combined compensation is required.

Introduction to the physics of molecular imaging with radioactive tracers in small animals

Recent advances have greatly enhanced the three‐dimensional (3D) imaging of radioactive tracers in living animals. this article introduces the physics of imaging behind the imaging methods. The article first discusses the selection of the radiation emitted from the tracer and then the process of tomographic reconstruction or how 3D images are made from imaging around the outside of the animal. The technique of single photon emission computed tomography (SPECT) in which the detection of one X‐ray or gamma ray at a time is employed for image formation is then described. Finally, positron emission tomography (PET) which relies on the simultaneous detection of the pair of gamma‐rays formed when the positron annihilates is presented. J. Cell. Biochem. Suppl. 39: 221–230, 2002.

A flexible multicamera visual‐tracking system for detecting and correcting motion‐induced artifacts in cardiac SPECT slices

Patient motion is inevitable in SPECT and PET due to the lengthy period of time patients are imaged. The authors hypothesized that the use of external-tracking devices which provide additional information on patient motion independent of SPECT data could be employed to provide a more robust correction than obtainable from data-driven methods. Therefore, the authors investigated the Vicon MX visual-tracking system (VTS) which utilizes near-infrared (NIR) cameras to stereo-image small retroreflective markers on stretchy bands wrapped about the chest and abdomen of patients during cardiac SPECT. The chest markers are used to provide an estimate of the rigid-body (RB) motion of the heart. The abdomen markers are used to provide a signal used to bin list-mode acquisitions as part of correction of respiratory motion of the heart. The system is flexible in that the layout of the cameras can be designed to facilitate marker viewing. The system also automatically adapts marker tracking to employ all of the cameras visualizing a marker at any instant, with visualization by any two being sufficient for stereo-tracking. Herein the ability of this VTS to track motion with submillimeter and subdegree accuracy is established through studies comparing the motion of Tc-99m containing markers as assessed via stereo-tracking and from SPECT reconstructions. The temporal synchronization between motion-tracking data and timing marks embedded in list-mode SPECT acquisitions is shown to agree within 100 ms. In addition, motion artifacts were considerably reduced in reconstructed SPECT slices of an anthropomorphic phantom by employing within iterative reconstruction the motion-tracking information from markers attached to the phantom. The authors assessed the number and placement of NIR cameras required for robust motion tracking of markers during clinical imaging in 77 SPECT patients. They determined that they were able to track without loss during the entire period of SPECT and transmission imaging at least three of the four markers on the chest and one on the abdomen bands 94% and 92% of the time, respectively. The ability of the VTS to correct motion clinically is illustrated for ten patients who volunteered to undergo repeat-rest imaging with the original-rest SPECT study serving as the standard against which to compare the success of correction. Comparison of short-axis slices shows that VTS-based motion correction provides better agreement with the original-rest-imaging slices than either no correction or the vendor-supplied software for motion correction on, our SPECT system. Comparison of polar maps shows that VTS-based motion-correction results in less numerical difference on average in the segments of the polar maps between the original-rest study and the second-rest study than the other two strategies. The difference was statistically significant for the comparison between VTS-based and clinical vendor-supplied software correction. Taken together, these findings suggest that VTS-based motion correction is superior to either no-motion correction or the vendor-supplied software the authors investigated in clinical practice.

Diminishing the impact of the partial volume effect in cardiac SPECT perfusion imaging

The partial volume effect (PVE) significantly restricts the absolute quantification of regional myocardial uptake and thereby limits the accuracy of absolute measurement of blood flow and coronary flow reserve by SPECT. The template-projection-reconstruction method has been previously developed for PVE compensation. This method assumes the availability of coregistered high-spatial resolution anatomical information as is now becoming available with commercial dual-modality imaging systems such as SPECT/CTs. The objective of this investigation was to determine the extent to which the impact of the PVE on cardiac perfusion SPECT imaging can be diminished if coregistered high-spatial resolution anatomical information is available. For this investigation the authors introduced an additional parameter into the template-projection-reconstruction compensation equation called the voxel filling fraction (F). This parameter specifies the extent to which structure edge voxels in the emission reconstruction are filled by the structure in question as determined by the higher spatial-resolution imaging modality and the fractional presence of the structure at different states of physiological motion as in combining phases of cardiac motion. During correction the removal of spillover to the cardiac region from the surrounding structures is performed first by using reconstructed templates of neighboring structures (liver, blood pool, lungs) to calculate spillover fractions. This is followed by determining recovery coefficients for all voxels within the heart wall from the reconstruction of the template projections of the left and right ventricles (LV and RV). The emission data are subsequently divided by these recovery coefficients taking into account the filling fraction F. The mathematical cardiac torso phantom was used for investigation correction of PVE for a normal LV distribution, a defect in the inferior wall, and a defect in the anterior wall. PVE correction resulted in a dramatic visual reduction in the impact of extracardiac activity, improved the uniformity of the normally perfused heart wall, and enhanced defect visibility without undue noise amplification. No significant artifacts were seen with PVE correction in the presence of mild (one voxel) misregistration. A statistically significant improvement in the accuracy of the count levels within the normal heart wall was also noted. However, residual spillover of counts from within the myocardium creates a bias in regions of decreased wall counts (perfusion defects/abnormal wall motion) when the anatomical imaging modality does not allow definition of templates for defects present in the heart during emission imaging.

Effect of filtering on the detection and localization of small Ga-67 lesions in thoracic single photon emission computed tomography images.

Tumor detection can be significantly affected by filtering so determining an optimal filter is an important aspect of establishing a clinical reconstruction protocol. The purpose of this study was to identify the cut-off frequency of a Butterworth filter used in a filtered backprojection (FBP) reconstruction that maximized the detection and localization accuracy of 1 cm spherical lesions in Ga-67 citrate, thoracic SPECT images. Image quality was evaluated by means of a localization receiver operating characteristic (LROC) study using computer simulated images. Projection data were generated using the mathematical cardiac-torso digital phantom with a clinically realistic background source distribution. The images were reconstructed using FBP with multiplicative Chang attenuation correction and fifth-order Butterworth filtering. The cut-off frequencies considered were 0.25, 0.32, 0.47, and 0.79 cm(-1) for the case of three-dimensional (3D) post-filtering and 0.25, 0.32, and 0.47 cm(-1) for two-dimensional (2D) post-filtering. The images were read by three research scientists and one board certified nuclear medicine clinician. The area under the LROC curve and the localization accuracy for all test conditions were compared using Scheffés multiple comparisons test. It was found that 3D post-filtering using filters with cut-off frequencies of 0.32 and 0.47 cm(-1) resulted in the highest lesion detectability and localization accuracy. These two test conditions did not differ significantly from each other but were significantly better (p<0.05) than all of the 2D, and the 3D 0.79 cm(-1) cut-off frequency cases.

A quantitative evaluation study of four-dimensional gated cardiac SPECT reconstruction

In practice, gated cardiac SPECT images suffer from a number of degrading factors, including distance-dependent blur, attenuation, scatter and increased noise due to gating. Recently, we proposed a motion-compensated approach for four-dimensional (4D) reconstruction for gated cardiac SPECT and demonstrated that use of motion-compensated temporal smoothing could be effective for suppressing the increased noise due to lowered counts in individual gates. In this work, we further develop this motion-compensated 4D approach by also taking into account attenuation and scatter in the reconstruction process, which are two major degrading factors in SPECT data. In our experiments, we conducted a thorough quantitative evaluation of the proposed 4D method using Monte Carlo simulated SPECT imaging based on the 4D NURBS-based cardiac-torso (NCAT) phantom. In particular, we evaluated the accuracy of the reconstructed left ventricular myocardium using a number of quantitative measures including regional bias-variance analyses and wall intensity uniformity. The quantitative results demonstrate that use of motion-compensated 4D reconstruction can improve the accuracy of the reconstructed myocardium, which in turn can improve the detectability of perfusion defects. Moreover, our results reveal that while traditional spatial smoothing could be beneficial, its merit would become diminished with the use of motion-compensated temporal regularization. As a preliminary demonstration, we also tested our 4D approach on patient data. The reconstructed images from both simulated and patient data demonstrated that our 4D method can improve the definition of the LV wall.

Comparison of 18F PET and 99mTc SPECT imaging in phantoms and in tumored mice

Our objective was to compare the performance of a micro-single photon emission computed tomography (micro-SPECT) with that of a micro-positron emission tomography (microPET) in a Her2+ tumored mice using an anti-Her2 nanoparticle radiolabeled with (99m)Tc and (18)F. Camera performance was first compared using phantoms; then a tumored mouse administered the (99m)Tc-nanoparticle was imaged on a Bioscan NanoSPECT/CT, while another tumored mouse received the identical nanoparticle, labeled now with (18)F, and was imaged on a Philips Mosaic HP PET camera. The nanoparticle was radiolabeled with (99m)Tc via MAG(3) chelation and with (18)F via SFB as an intermediate. Phantom imaging showed that the resolution of the SPECT camera was clearly superior, but even with 4 heads and multipinhole collimators, detection sensitivity was 15-fold lower. Radiolabeling of the nanoparticle by chelation with (99m)Tc was considerably easier and safer than manual covalent attachment of (18)F. Both cameras provided accurate quantitation of radioactivity over a broad range. In conclusion, when deciding between (99m)Tc vs (18)F, an advantage rests with the chelation of (99m)Tc over covalent attachment of (18)F, achieved manually or otherwise, but with these small animal cameras, this choice also results in trading lower sensitivity for higher resolution.

The effects of different correction techniques on absolute volume determination with SPECT using a threshold edge detection method.

Quantitation of planar radionuclide images is hampered by structures containing radioactivity which overlie or underlie the organ of interest. The introduction of single photon emission computerized tomography (SPECT) overcame this problem to a large extent and enhanced the contrast of the images. Attenuation of photons, however, degrades the resultant SPECT images and correction methods for photon absorption and scatter were subsequently proposed. The different correction methods have variable effects on the reconstructed images. If threshold techniques are used to quantitate organ volume, i.e., combining pixels with the same percentage of the maximum pixel count in the volume, the selected threshold values which give the most accurate volume determination, will be affected by the specific correction method used. In this study, the effect of various SPECT image correction methods on threshold was investigated. A thorax phantom containing volumes ranging from 30 to 1200 ml was used. Threshold values varying from 45.6% (210 ml without any correction) to 23.7% (1200 ml with a combination of scatter subtraction and attenuation correction) were used to produce correct quantitation when different methods were investigated. A negative correlation was found between threshold and volume. This reduction in threshold was most prominent when scatter and attenuation correction were combined. This study shows that correction methods for attenuation of photons influence the threshold value for volume quantitation and the use of a constant threshold value could lead to underestimation of larger volumes.