A.J. Da Silva

Iterative SPECT Reconstruction Using Matched Filtering for Improved Image Quality

SPECT images reconstructed from low count studies suffer either from high noise or poor resolution. We have developed an iterative reconstruction with matched filtering (IRMF) to control image noise while maintaining higher image resolution. IRMF involves filtering the measured projection and re-projection during iterative reconstruction with the same low-pass filter before the two are compared to generate an error projection. Another low-pass filter can be applied to the error projection before it is backprojected to update the current activity distribution estimate. The method is validated with a cardiac phantom filled with a clinical distribution of Tc-99m. A 1-second-per-frame scan was acquired to mimic a single gated segment. The image was reconstructed using ordered-subset expectation-maximization (OSEM) algorithm with depth-dependent resolution recovery. Reconstructions of similar spatial resolution with post-reconstruction Butterworth filtering (OSEM+F) and with matched filtering are compared visually and via standard deviation (SD) and signal-to-noise ratio (SNR) measurements. Results: Images reconstructed with IRMF show strong noise suppression in both the myocardium and background areas as compared to those reconstructed with OSEM+F. The SD in the background is reduced by ~30%, and the SNR is improved by ~100%. IRMF significantly improves image quality by suppressing noise in low count SPECT studies while maintaining higher image resolution.

An ultra high resolution ECG-gated myocardial imaging system for small animals

Standard radionuclide imaging systems are of limited use due to their inability to resolve structures in small animals that represent an increasingly important model for the study of cardiovascular disease. The authors are developing an imaging system incorporating a scintillation camera with a pinhole collimator to acquire cardiac gated images of the mouse heart. Through a simulation study, the authors found that the effective diameter of the pinhole was unaffected by the scatter component of photon penetration through the pinhole insert. The authors have performed a myocardial perfusion study with 0.68 FWHM resolution on a normal 25 gram mouse gated at over 400 beats per minute. The authors have demonstrated that it is possible to obtain cardiac-gated, myocardial perfusion images of mice at submillimeter spatial resolution.

Design and utility of a small animal CT/SPECT system

Recent developments in the genetic engineering of small animals have motivated us to design an in vivo dual-modality CT/SPECT system that can be used to localize and quantify uptake of single-photon radiotracers in mice. The CT system includes a 75-W x-ray tube and a CCD camera, while the radionuclide imager incorporates a CsI(TI) scintillator coupled to a Si photodiode array with interchangeable pinhole collimators. These devices are mounted on a slip-ring gantry to image a 40-mm diameter cylindrical volume, where both modalities share a common field of view without moving the table on which the animal lies horizontally. The calculated modulation transfer function of the radionuclide system exceeds 10% at 1.01p/mm. The spatial resolution of the CT system is 0.1 mm with at scan time of 15 min and a contrast resolution of 1% for soft tissue. The high-resolution CT image can be used to provide a priori information to correct both the visual quality and the quantitative accuracy of the SPECT image. This imaging system and technique is designed for in vivo functional assessments of cancer and cardiovascular disease in small animals.

CT-PET image fusion using the 4D NCAT phantom with the purpose of attenuation correction

The purpose of this study is to improve CT-based attenuation correction of 3D PET data using an attenuation map derived from non-rigidly transformed 3D CT data. Utilizing the 4D NURBS-based cardiac-torso (NCAT) phantom with a realistic respiratory model based on high-resolution respiratory-gated CT data, we develop a method to non-rigidly transform 3D CT data obtained during a single breath hold to match that of 3D PET emission data of the same patient obtained over a longer acquisition time and many respiratory cycles. For patients who underwent 3D CT and PET (transmission and emission) studies, the 3D anatomy of the NCAT phantom was first fit to that revealed through automatic segmentation of the 3D CT data. From the 3D PET emission data, a second body outline was segmented using an automatic algorithm. Using the 4D NCAT respiratory model, the morphed 3D NCAT phantom was transformed such that its body outline provided the best match with that obtained from the 3D PET emission data. The other organs of the NCAT followed the corresponding transformations provided by the 4D respiratory model. The transformations were then applied to the 3D CT image data to form the attenuation map to be used for attenuation correction. For eight preliminary sets of patient data, the NCAT respiratory model allowed excellent registration of the 3D CT and PET transmission data as visually assessed by 3 independent observers. Minor registration errors occurred near the diaphragm and lung walls. The 4D NCAT phantom with a realistic model of the respiratory motion was found to be a valuable tool in a non-rigid warping method to improve CT-PET image fusion. The improved fusion provides for a more accurate CT-based attenuation correction of 3D PET image data.

Implementation and applications of a combined CT/SPECT system

Combined radionuclide/radiographic (e.g., SPECT/CT, PET/CT) imaging systems are being developed for correlation of structure and function, primarily for assessment of patients with cancer. Another important aspect of these approaches is the possibility of using anatomical data from CT to derive patient-specific compensations for perturbations in the radionuclide data. For example, a patient-specific attenuation map can be derived from CT and incorporated into an iterative reconstruction algorithm to correct the radionuclide image for photon attenuation. In addition, the geometry, location, and configuration of anatomical regions can be determined using CT, from which recovery coefficients or other geometrical factors can be derived to compensate the radionuclide data for errors caused by the limited spatial resolution of radionuclide images. The use of CT to derive correction factors for these perturbations allows combined X-ray/radionuclide imaging techniques to achieve a high degree of accuracy in the absolute quantitation of radiopharmaceuticals.

CT-based attenuation correction in PET image reconstruction for the Gemini system

The Gemini system is a combined CT/PET imaging system newly developed by Philips Medical Systems. It has a unique open gantry design that allows for variable separation between the CT and PET gantries. The Gemini system incorporates CT-based attenuation correction (CT-AC) into a three-dimensional row-action maximum likelihood algorithm (RAMLA) for PET image reconstruction. It uses several unique techniques to achieve high accuracy while reducing patient X-ray dose. These new techniques include (1) using low-dose CT protocols to obtain CT images with adequate quality and quantitation for CT-AC while keeping patient X-ray dose low; (2) using a CT truncation compensation technique to improve the accuracy of CT-AC; and (3) using a generalized model for the conversion of CT images to attenuation maps at 511 keV. In this paper, we report the workflow and performance of Gemini CT-AC using phantom and patient studies. For comparison, attenuation maps obtained from PET transmission scans are also used for attenuation correction (TX-AC). Both phantom and patient studies show that PET images with CT-AC have image quality equivalent to or better than those with TX- AC.

Study of the effect of statistical fluctuations on defect detectability at clinical count levels in cardiac SPECT

Cardiac SPECT using Tl-201 suffers from low count statistics. Any statistical studies concerning the evaluation of a reconstruction algorithm, acquisition parameters, diagnostic confidence, etc., for clinical applications are impacted by the difficulty of obtaining data with multiple noise realizations. For this work, we acquired list-mode data of a Tl-201 cardiac phantom with very high counts in three configurations-with an anterior defect, an inferior defect, and no defect. The list-mode data were repartitioned to obtain statistically independent multiple data sets all with the same, clinically relevant noise level. Images were reconstructed from each of the resulting data sets using an iterative algorithm with attenuation correction. Reconstructed images were examined by four human observers, as well as analyzed quantitatively. The ability of observers to differentiate between normal scans and scans with defects varied substantially among datasets. There was correlation between the measured defect detectability and the visual assessment. The fact that the visibility of defects and the uniformity of normal scans varied significantly from one data set to the next, even when both were acquired at the same time, under identical conditions, indicates that the low statistics levels at clinical doses can have a measurable effect on diagnostic confidence.

The effect of attenuation on lesion detection in PET oncology

Using simulation and patient studies, we compare the lesion detection in whole-body PET oncology with and without performing attenuation correction (AC and non-AC). In addition, we discuss the prediction of lesion distortion in non-AC PET images. In all of the simulation studies, the OSEM algorithm is used to reconstruct 2D noise-free data. Lesions of different size, density, and uptake are simulated in the lungs. Lesions with the density of soft tissue and different uptake are simulated in the mediastinum and abdomen. The patient studies are performed using an ADAC dual-head hybrid SPECT-PET system. The lesion-to-background ratio (LBR) in the reconstructed images is used for quantitative analyses. Simulation studies and a total of 40 patient studies show that the LBR in AC images is higher in lungs but lower in the mediastinum and abdomen than in non-AC images. In addition, simulation studies show that, in non-AC images, for given lesion and lung uptakes, the LBR increases when the lesion density decreases and spherical lesions appear to be elongated in the direction with minimal attenuation and squeezed in the direction with maximal attenuation.