John C. Engdahl

The effect of scintillator manipulation on a thick crystal gamma detector

When a NaI(Tl) based gamma camera is used for high energy single photon imaging or positron imaging, a relatively thick scintillation crystal is required to provide enough stopping power for impinging gamma rays. However, it causes worsening of the image quality due to the wider scintillated light spread and parallax error. To address the image quality degradation problem with a thick crystal, we investigated the effects of crystal manipulation on detector performance. The methods include drilling holes, cutting grooves and making pyramid shapes on the gamma ray entrance surface of the crystal. It has been proven throughout this investigation that the proposed method can improve intrinsic spatial/energy resolution, DOI (depth of interaction) decodeability and uniformity. This is done by proper controlling of the optical characteristics of scintillated light photons. Among the tested methods, entrance side groove cutting found to be the most promising method in terms of improving detector performance and providing cost effective manufacturability as well

Investigation on a bar detector with 3D position decoding scheme

Simulation studies were performed to find optimum design parameters for a bar detector allowing 3D position decoding of impinging gamma rays. This design incorporates surface treatment, system geometry, photo-sensors, scintillators, and a more robust positioning algorithm. The bar detector simulated consists of a long scintillation crystal, ranged from 5 cm to 20 cm, with a relatively small cross section (4times4 mm2), coupled to two photo sensors, one on each end. Statistical based positioning scheme has been developed to improve positioning accuracy. The method maps event characterization vectors to the associated position based on chi square error. Our simulation shows that a 5cm long bar detector can achieve ~1mm FWHM spatial resolution for a 511 keV gamma photon. The performance improvement of the new positioning scheme over a linear estimator were over 70%. Grounded surface with reflector found to be the most optimum surface condition with respect to the steepest and linear correlation of photo sensor signals. Experimental measurements are in progress to validate the simulation results

Analysis of the impact of CT based attenuation correction, scatter compensation and 3D-beam modeling on SPECT image quality

Non uniform photon attenuation and Compton scatter degrade nuclear medicine SPECT images by removing true counts or adding unwanted counts, respectively. For quantitative SPECT, iterative reconstruction algorithms, such as OSEM, allow 3D collimator modeling and compensation of the effects of scatter and attenuation. In this work, we investigate, through ROI and SPM analysis of phantom and computer simulation studies, how a combination of compensation strategies affects image quality and quantitative accuracy. Phantoms were imaged with a SPECT and a CT scanner. With these acquisitions, semi-quantitative analysis was performed for the following reconstruction strategies: OSEM-3D without attenuation and scatter compensation; OSEM-3D CTAC (with attenuation correction); and OSEM-3D with attenuation and scatter compensation (CTACSC). For the simulated dataset, the SNR was 50.5 with CTACSC compared to 27.5 without any compensation and OSEM-3D CTACSC produced reconstructed images with contrast within 0.23% of the true image with a standard error of 21 counts. Without compensation, the error increases to 2382 counts. The implementation and design of the OSEM-3D CTACSC approach proved effective, with improved visual quality and quantitative accuracy of the SPECT images.

Integrated approach to brightness- and contrast-invariant detection of the heart in SPECT imaging

In this paper we propose an appearance-based method to heart detection by principal component analysis (PCA). In contrast to conventional methods of PCA-based training, there is no brightness and contrast normalization since such normalization is usually based on maximum and minimum intensity values and is very sensitive to noises. We propose to integrate the normalization procedure into the detection phase. This is achieved by projecting the intensity-transformed image (with unknown scale and shift parameters) onto the eigen-images and minimizing the error of fit. This leads to a set of equations on both the intensity transformation parameters and the projection coefficients. By using the least-squares method, these equations can be easily solved for the scale and shift parameters. After an initial detection of heart positions is conducted, robust fitting of the heart trajectory is used to correct any detection errors. Besides, we also propose an eigen-image re-orthonormalization method for multiple resolution detection without extra training on multiple scales.

Automatic quantification of liver-heart cross-talk for quality assessment in SPECT myocardial perfusion imaging

In the single-photon emission computed tomography (SPECT), it is highly desirable to provide physicians with a measure of the strength of the liver-heart cross talk as a means of assessing the quality of the images, so that appropriate actions can be taken to avoid false diagnosis. Liver-heart cross talk is an phenomenon in which the liver count interferes with the heart count in 3D reconstruction, which generates artifacts in the reconstructed images. In this paper, we propose an automatic method for quantification of such liver-heart cross talk. The system performs heart detection followed by non-heart organ segmentation and quantification of their activities. An appearance-based approach is applied to find the heart center in each image, with invariance to image intensity and contrast. Then heart and non-heart activities are quantified in each image. A measurement formula is proposed to compute the amount of liver-heart cross talk as a function of the size of the non-heart activity regions, of the strengths of the heart and non-heart activities, and of the distance of the non-heart regions to the heart. The method has been tested on 150 patient studies of different isotopes and acquisition types, with very promising results.

Variable-length correlation method for the correction of body motions and heart creep in the SPECT myocardial perfusion imaging

Body motion and heart upward creep are among the most frequent sources of artifacts in the single-photon emission computed tomography (SPECT) of nuclear medicine. This paper provides a new method for automatic correction of such motions. Under the formulation of a variable length correlation, abrupt body motions, gradual body motions, and heart upward creep are corrected in sequential passes. An affine transformation is used to compensate for changes in heart appearance caused by varying view angles in image acquisition. Additionally, a method is proposed for automatic exclusion of non-cardiac organs. The effectiveness of the method has been demonstrated with experimental results.