Evaluation of 4D reconstruction methods for gated cardiac SPECT imaging in obese patients

The purpose of this study is to evaluate 4D reconstruction methods for the processing of gated cardiac single photon emission computed tomography (SPECT) images from obese patients. Gated SPECT on obese patients is extremely noisy and often clinically useless; it is hypothesized that 4D reconstruction methods may help. The methods compared are the ordered-subsets expectation-maximization (OS-EM) algorithm with a 3D Gaussian filter, OS-EM with a 3D Gaussian combined with a time-domain Butterworth filter, and the rescaled block-iterative maximum a posteriori (RBI-MAP) algorithm with Gibbs priors for spatial and time-domain smoothing. Clinical gated SPECT data were used to derive a table of Tc-99m tetrofosmin activity uptake ratios. Moderately and morbidly obese male and female phantom models were created for the 4D NURBS-based Cardiac Torso (NCAT) phantom, and mild and severe motion defects were generated in addition to a normal heart model. A blood pool phantom study enabled optimization of reconstruction parameters for the methods so they result in similar noise statistics in the heart. Poisson noise was added to the projection data (including the effects of detector response, attenuation and scatter) generated from the phantoms. The noisy phantom and patient projection data were reconstructed with the three methods, and imported onto the clinical workstations, to be analyzed with the Quantitative Gated SPECT (QGS) software. Quantitative parameters (chamber volumes) were recorded for the phantom and patient data. Statistical analysis led to the conclusion that OS-EM with 4D filtering was markedly different, a result confirmed in the normal phantom models, with better quantitation. Visually, RBI-MAP appeared to result in smoother, more realistic cardiac motion. A preference study was performed with four physicians who read the patient images using QGS and rated them on a 7-point scale to indicate which method most improved their confidence in the diagnoses. The one-way ANOVA showed no significant difference in preference for the processing methods. The conclusion is that the choice of reconstruction method may make more of a difference in patients with greater heart motion, and that the OS-EM method with 4D filtering may have an advantage over the other methods when it comes to LV chamber volume quantification

The association of rate pressure product (RPP) and myocardial perfusion imaging (MPI) findings: A preliminary study

Introduction: The product of heart rate and systolic blood pressure, termed as rate-pressure product (RPP), is a very reliable indicator of myocardial oxygen demand and is widely used clinically. There have been previous attempts to describe the relationship between RPP and the onset of pain in angina pectoris. The current study aimed to evaluate the association between RPP results and scan findings.Materials and methods: In total, 497 patients with suspected coronary artery disease (CAD) underwent gated, single-photon emission computed tomography (SPECT) imaging with dipyridamole, exercise, or dobutamine stress, and were included in this study. Baseline and maximum heart rate (HR), systolic blood pressure (SBP), diastolic blood pressure (DBP), and electrocardiogram (ECG) results were recorded. The rate-pressure product (RPP) was calculated as the product of heart rate and systolic arterial pressure for both baseline and maximum measures. The difference between the RPP max and the basal RPP is known as the RPP reserve. Researchers also obtained semi-quantitative analyses of myocardial perfusion imaging (MPI), using gated software, demographic information, risk factors of CAD, and pretest likelihoods of CAD using nomograms.Result: Four hundred and ninety-seven cases, including 426 patients with dipyridamole stress, 59 with exercise stress, and 12 with dobutamine stress, underwent myocardial perfusion imaging. Scan results were positive in 194 (45.5) and negative in 232 (54.5) patients with dipyridamole stress. In patients with exercise stress, the scan was positive in 24 (40.7) cases and negative in 35 (59.3) cases. In dobutamine stressed patients, the scan was positive in 6 (50) cases and negative in the 6 remaining cases. Dipyridamole stress resulted in a significant difference between HR at rest and at maximum (28.95 ± 24.53, p-value<0.0001), between systolic BP at rest and maximum (6.75 ± 12.50, p-value<0.0001) and between diastolic BP at rest and maximum (1.45 ± 5.80; p-value<0.0001). There was a significant correlation between sum stress scores (SSS) and reserved RPP (r= -0.12, p-value<0.001) which, in dipyridamole patients, was r=-0.18, p-value=0.0001). In addition, there was a significant association between reserved RPP and risk of CAD (p-value<0.001). In the patients with dipyridamole stress, the ejection fraction (EF) change (odds ratio =0.92; 95 CI: 0.86-0.98; p=0.01), reserve RPP (odds ratio =1.00; 95 CI: 1.00-1.00; p=0.04), risk of CAD (odds ratio =5.80; 95 CI: 3.21-10.50; p<0.0001) and age (odds ratio =0.94; 95 CI: 0.89-0.98; p=0.01) were associated significantly with MPI results, using multiple logistic regressions.Conclusion. The study demonstrated that RPP is associated with MPI findings using gated SPECT imaging with dipyridamole stress. However, to confirm this preliminary result, further studies are mandatory. © The Author(s) 2012

Improvements in Cardiac Spect/CT for the Purpose of Tracking Transplanted Cells

Regenerative therapy via stem cell transplantation has received increased attention to help treat the myocardial injury associated with heart disease. Currently, the hybridisation of SPECT with X-ray CT is expanding the utility of SPECT. This thesis compared two SPECT/CT systems for attenuation correction using slow or fast-CT attenuation maps (mu-maps). We then developed a method to localize transplanted cells in relation to compromised blood flow in the myocardium following a myocardial infarction using SPECT/CT. Finally, a method to correct for image truncation was studied for a new SPECT/CT design that incorporated small field-of-view (FOV) detectors. Computer simulations compared gated-SPECT reconstructions using slow-CT and fast-CT mu-maps with gated-CT mu-maps. Using fast-CT mu-maps improved the Root Mean Squared (RMS) error from 4.2% to 4.0%. Three canine experiments were performed comparing SPECT/CT reconstruction using the Infinia/Hawkeye-4 (slow-CT) and Symbia T6 (fast-CT). Canines were euthanized prior to imaging, and then ventilated. The results showed improvements in both RMS errors and correlation coefficients for all canines. A first-pass contrast CT imaging technique can identify regions of myocardial infarction and can be fused with SPECT. Ten canines underwent surgical ligation of the left-anterior-descending artery. Cells were labeled with 111In-tropolone and transplanted into the myocardium. SPECT/CT was performed on day of transplantation, 4, and 10 days post-transplantation. For each imaging session first-pass perfusion CT was performed and successfully delineated the infarct zone. Delayed-enhanced MRI was performed and correlated well with first-pass CT. Contrast-to-noise ratios were calculated for 111In-SPECT and suggested that cells can be followed for 11 effective half-lives. We evaluated a modified SPECT/CT acquisition and reconstruction method for truncated SPECT. Cardiac SPECT/CT scans were acquired in 14 patients. The original projections were truncated to simulate a small FOV acquisition. Data was reconstructed in three ways: non-truncated and standard reconstruction (NTOSEM), which was our gold-standard; truncated and standard reconstruction (TOSEM); and truncated and a modified reconstruction (TMOSEM). Compared with NTOSEM, small FOV imaging incurred an average cardiac count ratio error greater than 100% using TOSEM and 8.9% using TMOSEM. When we plotted NTOSEM against TOSEM and TMOSEM the correlation coefficient was 0.734 and 0.996 respectively

Post-Acquisition Small-Animal Respiratory Gated Imaging Using Micro Cone-Beam CT

On many occasions, it is desirable to image lungs in vivo to perform a pulmonary physiology study. Since the lungs are moving, gating with respect to the ventilatory phase has to be performed in order to minimize motion artifacts. Gating can be done in real time, similar to cardiac imaging in clinical applications, however, there are technical problems that have lead us to investigate different approaches. The problems include breath-to-breath inconsistencies in tidal volume, which makes the precise detection of ventilatory phase difficult, and the relatively high ventilation rates seen in small animals (rats and mice have ventilation rates in the range of a hundred cycles per minute), which challenges the capture rate of many imaging systems (this is particularly true of our system which utilizes cone-beam geometry and a 2 dimensional detector). Instead of pre-capture ventilation gating we implemented a method of post-acquisition gating. We acquire a sequence of projections images at 30 frames per second for each of 360 viewing angles. During each capture sequence the rat undergoes multiple ventilation cycles. Using the sequence of projection images, an automated region of interest algorithm, based on integrated grayscale intensity, tracts the ventilatory phase of the lungs. In the processing of an image sequence, multiple projection images are identified at a particular phase and averaged to improve the signal-to-ratio. The resulting averaged projection images are input to a Feldkamp cone-beam algorithm reconstruction algorithm in order to obtain isotropic image volumes. Minimal motion artifact data sets improve qualitative and quantitative analysis techniques useful in physiologic studies of pulmonary structure and function

Data registration and fusion for cardiac applications

The registration and fusion of information from multiple cardiac image modalities such as magnetic resonance imaging (MRI), X-ray computed tomography (CT), positron emission tomography (PET) and single photon emission computed tomography (SPECT) has been of increasing interest to the medical community as tools for furthering physiological understanding and for diagnostic of ischemic heart diseases. Ischemic heart diseases and their consequence, myocardial infarct, are the leading cause of mortality in industrial countries. In cardiac image registration and data fusion, the combination of structural information from MR images and functional information from PET and SPECT is of special interest in the estimation of myocardial function and viability. Cardiac image registration is a more complex problem than brain image registration. The non-rigid motion of the heart and the thorax structures introduce additional difficulties in registration. In this thesis the goal was develop methods for cardiac data registration and fusion. A rigid registration method was developed to register cardiac MR and PET images. The method was based on the registration of the segmented thorax structures from MR and PET transmission images. The thorax structures were segmented from images using deformable models. A MR short axis registration with PET emission image was also derived. The rigid registration method was evaluated using simulated images and clinical MR and PET images from ten patients with multivessel coronary artery diseases. Also an elastic registration method was developed to register intra-patient cardiac MR and PET images and inter-patient head MR images. In the elastic registration method, a combination of mutual information, gradient information and smoothness of transformation was used to guide the deformation of one image towards another image. An approach for the creation of 3-D functional maps of the heart was also developed. An individualized anatomical heart model was extracted from the MR images. A rigid registration of anatomical MR images and PET metabolic images was carried out using surface based registration, and the registration of MR images with magnetocardiography (MCG) data using external markers. The method resulted in a 3-D anatomical and functional model of the heart that included structural information from the MRI and functional information from the PET and MCG. Different error sources in the registration method of the MR images and MCG data was also evaluated in this thesis. The results of the rigid MR-PET registration method were also used in the comparison of multimodality MR imaging methods to PET.reviewe