Ling Shao

A generalized model for the conversion from CT numbers to linear attenuation coefficients

We have developed a generalized model for accurate conversion from CT numbers to linear allenuation coefficients (LAC) by introducing a material-dependent conversion factor (CF). When assuming that a material x is a uniform mixture of water and another material A (we call this the water-A assumption in this paper), we show that the conversion from CT number of x (HU/sub x/) to LAC is linear. The slope of the linear function is determined by the attenuation property of material A, namely, its CT number (HU/sub A/) at a given kVp and density (/spl rho//sub A/). This generalized model can be applied to the conversion from CT images to attenuation maps for combined CT/PET and CT/SPECT imaging. When HU/sub x/ is less than zero, we use water-air assumption, otherwise, we us water-cortical bone assumption. These assumptions lead to different slopes for the linear conversion when CT number is below and above 0. In practice, for each CT system, a cylindrical phantom with a small cortical bone cylinder in the center is filled with water and scanned once for each kVp. The CT number of the cortical bone (HU/sub CB/) at each kVp is then measured and used for the conversion. Experiments performed on a Philips CT AURA system show that, for a spongiosa bone sample with known LAC, the errors in LACs converted from CT images at all the kVps are 0% for PET and less than 1.5% for SPECT. Conclusions: The proposed model illustrates the linearity for the conversion from CT numbers to LAC at energy of interest under the water-A assumption. Its application to the conversion from CT images to PET/SPECT attenuation maps is accurate and convenient. In addition, the proposed technique can be used to characterize CT systems by obtaining the effective CT energy at each operating kVp. This allows for absolute attenuation measurement using CT systems instead of the relative measurement given by CT-numbers.

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.

The effects of center of rotation errors on cardiac SPECT imaging

In SPECT imaging, center of rotation (COR) errors lead to the misalignment of projection data and can potentially degrade the quality of the reconstructed images. In this work, we study the effects of COR errors on cardiac SPECT imaging using simulation, point source, cardiac phantom, and patient studies. For simulation studies, we generate projection data using a uniform MCAT phantom first without modeling any physical effects (NPH), then with the modeling of detector response effect (DR) alone. We then corrupt the projection data with simulated sinusoid and step COR errors. For other studies, we introduce sinusoid COR errors to projection data acquired on SPECT systems. An OSEM algorithm is used for image reconstruction without detector response correction, but with nonuniform attenuation correction when needed. The simulation studies show that, when COR errors increase from 0 to 0.96 cm: 1) sinusoid COR errors in axial direction lead to intensity decrease in the inferoapical region; 2) step COR errors in axial direction lead to intensity decrease in the distal anterior region. The intensity decrease is more severe in images reconstructed from projection data with NPH than with DR; and 3) the effects of COR errors in transaxial direction seem to be insignificant. In other studies, COR errors slightly degrade point source resolution; COR errors of 0.64 cm or above introduce visible but insignificant nonuniformity in the images of uniform cardiac phantom; COR errors up to 0.96 cm in transaxial direction affect the lesion-to-background contrast (LBC) insignificantly in the images of cardiac phantom with defects, and COR errors up to 0.64 cm in axial direction only slightly decrease the LBC. For the patient studies with COR errors up to 0.96 cm, images have the same diagnostic/prognostic values as those without COR errors. This work suggests that COR errors of up to 0.64 cm are not likely to change the clinical applications of cardiac SPECT imaging when using iterative reconstruction algorithm without detector response correction.

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.

Transmission attenuation map with measured downscatter correction

As the attenuation correction becomes clinical reality in SPECT, accurate estimation of emission downscatter to transmission during simultaneous emission-transmission acquisition becomes critical. In this study, we proposed a technique that directly measures the downscatter contamination from 140-keV emission photons to the 100-keV transmission window. The technique combines energy window discrimination and spatial window discrimination to separate emission (EM), transmission (TR) and downscattered (DS) photons. A spatial mask (S) aligned with the opposing scanning line source and a gap (G) at each end of S are first defined. Photons detected inside the S belong to TR if their energy fall inside the 100 keV window; those detected outside the S and G belong to either EM if their energy inside the 140 keV window, or DS if they are inside the 100 keV window. The TR is corrected from the downscatter contamination via subtraction a fraction of the DS. The effectiveness of the technique is evaluated based on the uniformity of the ROI in the reconstructed attenuation map from a series of phantom experiments. The technique is compared with another technique using an adjacent energy window measurement. For typical cardiac studies, the uniformity of the transmission map from both techniques are comparable in the heart area, while for the cardiac study with high concentration present in the gall bladder, the uniformity is significantly improved from the proposed technique.

Hybrid parallel-slant hole collimators for SPECT imaging

We propose a new collimator geometry, the hybrid parallel-slant hole geometry (HPS), to improve sensitivity for SPECT imaging with large field of view (LFOV) gamma cameras. A HPS collimator has one segment with parallel holes and one or more segments with slant holes. The collimator can be mounted on a conventional SPECT LFOV system that uses parallel-beam collimators, and no additional detector or collimator motion is required for data acquisition. The parallel segment of the collimator allows for the acquisition of a complete data set of the organs-of-interest and the slant segments provide additional data. In this work, simulation studies of an MCAT phantom were performed with an HPS collimator with one slant segment. The slant direction points from patient head to patient feet with a slant angle of 30/spl deg/. We simulated 64 projection views over 180/spl deg/, and we modeled nonuniform attenuation effect. Images were reconstructed using an OSEM algorithm that incorporated the hybrid geometry. Sensitivity to the cardiac region of the phantom was increased by approximately 50% when using the HPS collimator compared with a parallel-hole collimator. No visible artifacts were observed in the reconstructed images and the signal-to- noise ratio (SNR) of the cardiac walls was improved. Compared with collimators with other geometries, using a HPS collimator has the following advantages: (a) significant sensitivity increase, (b) a complete data set obtained from the parallel segment that allows for artifact-free image reconstruction, and (c) no additional collimator or detector motion. This work demonstrates the potential value of hybrid geometry in collimator design for LFOV SPECT imaging.

The effects of center of rotation errors on SPECT imaging

In SPECT imaging, center of rotation (COR) errors lead to the misalignment of the projection data and can potentially degrade the quality of the reconstructed images. In this work, we study the effects of COR errors on the quality and clinical applications of SPECT images using point source, cardiac phantom, and patient studies. COR errors are simulated in both axial and transaxial directions in the projection data. The reconstructed images are evaluated in terms of point source resolution, image uniformity and lesion-to-background contrast (LBC) in cardiac phantom images, and clinical applications for patient images. It is shown that point source resolution degrades as COR errors increase. Using LEHR collimators. the full-width at half-maximum (FWHM) increases by less than 25% when the COR error increases from 0 to 0.96 cm. COR error of 0.64 cm or larger introduces visible but insignificant nonuniformity to the images of a uniform cardiac phantom. Increase of COR errors from 0 to 0.96 cm can decrease the LBC in the images of an abnormal cardiac phantom by as much as 20%, depending on the direction of COR errors and defect size. For the patient studies with COR errors up to 0.96 cm, images have the same diagnostic/prognostic values as those without COR errors. As a conclusion, for the limit number (n=4) of patient studies, COR errors of up to 0.96 cm are not likely to change the clinical applications of cardiac SPECT images.