A novel k-space trajectory measurement technique.

A new k-space trajectory measurement technique is proposed and demonstrated. This technique measures the k-space trajectory, in seconds, using only a few readout lines, using phase values of acquired MR signals. As a result of the technique's efficiency, k-space trajectory measurement using patient data becomes possible. The utility of this techniques is demonstrated in phantom and human studies at 4.1 T.

Blur reduction of conventional film-based tomograms for pre-surgical evaluation of potential mandibular implant sites.

The usefulness of motion-based cross-sectional tomography to evaluate osseous support and adjacent anatomical structures for dental implant placement is limited by the inherent blurring in these images. The goal of this study was to develop a method to remove blurring while permitting accurate dimensional analysis of the potential implant site. Defined regions (anterior, cuspid, premolar, molar) on two preserved human mandibles were imaged using cross-sectional linear tomography. Algorithms were developed as a personal computer application to remove the blur and to aid in identification of the cortical plate borders and the mandibular canal. The data set of eight tomograms was digitized and the blur reduced with the developed algorithm. An operator measured the height and width of the mandible on each original tomogram and each deblurred tomogram in triplicate. Method error was calculated as the difference between direct caliper measurements of the respective skull regions and image measurements of height and width for both the original digitized tomograms and the deblurred tomograms. Method error using the original images (height: -2.72 +/- 2.15 mm; width: -0.58 +/- 1.36 mm) compared to the deblurred tomograms (height: -0.58 +/- 1.16 mm; width: 0.37 +/- 0.59 mm) was significantly greater for both height (t-test level of significance, P = 0.0047) and width (t-test level of significance, P = 0.0001). These findings suggest that the method developed may greatly improve the ability of clinicians to accurately assess the implant site using cross-sectional film-based linear tomograms.

A model-based four-dimensional left ventricular surface detector.

The authors have developed a general model-based surface detector for finding the four-dimensional (three spatial dimensions plus time) endocardial and epicardial left ventricular boundaries. The model encoded left ventricular (LV) shape, smoothness, and connectivity into the compatibility coefficients of a relaxation labeling algorithm. This surface detection method was applied to gated single photon emission computed tomography (SPECT) perfusion images, tomographic radionuclide ventriculograms, and cardiac rotation magnetic resonance images. Its accuracy was investigated using actual patient data. Global left ventricular volumes correlated well, with a maximum correlation coefficient of 0.98 for magnetic resonance imaging (MRI) endocardial surfaces and a minimum of 0.88 for SPECT epicardial surfaces. The average absolute errors of edge detection were 6.4, 5.6. and 4.6 mm for tomographic radionuclide ventriculograms, gated perfusion SPECT, and magnetic resonance images, respectively.

Quantification of three-dimensional left ventricular segmental wall motion and volumes from gated tomographic radionuclide ventriculograms.

Tomographic radionuclide ventriculograms may be used for three-dimensional wall motion analysis. We propose that automatic quantification of these images is possible, and here we describe the implementation and validation of a method to perform this task. Automatic computer methods were developed to locate the left ventricular (LV) endocardial surfaces in all time frames of the cardiac cycle. Global, regional, and local motion and volume were computed. Results were displayed using three-dimensional graphics. The methods were validated using phantom, canine, and human studies. Actual phantom values correlated well with experimentally determined volumes, y = 1.01x + 1.29ml, r = 0.99. In the canine model, the LV endocardial surfaces were located to within an average of 1.9 mm and 3.7 mm at end-diastole and end-systole, respectively. Areas of obvious wall motion abnormalities in automatically processed patient studies corresponded well with angiographically documented coronary artery disease. End-diastolic and end-systolic volumes computed automatically from single photon emission computed tomography averaged errors of 9% and 38%, respectively, when compared with contrast ventriculographic volumes. These results indicate that it is possible to automatically identify the left ventricular endocardial surface in gated tomographic radionuclide ventriculograms. The location of these surfaces corresponds well with the location of implanted endocardial markers, and global volume computed from these surfaces corresponds well with known volumes.

Feature detection in 3-d medical images using shape information.

Three methods for identifying the left ventricular apex in 3-D medical images of the heart called gated blood pool tomograms were investigated. The first method assumed a known orientation and positioning of the entire blood pool. The second and third methods used shapes described by quadratic surfaces, which are invariant to position and orientation. The first method performed best when the blood pool was accurately oriented, but as expected, could not handle blood pools in the wrong orientations. The quadratic surface methods performed well whether or not the blood pool was accurately oriented. The best quadratic surface method predicted the x, y, z value of the apex with correlations of 0.97, 0.98, 0.99.

Quantification of intracerebral steal in patients with arteriovenous malformation.

Eleven patients with angiographically and/or pathologically proved arteriovenous malformations (AVMs) were studied using dynamic, single-photon-emission computed tomography (DSPECT). Quantification of regional cerebral blood flow in structurally normal areas remote from the AVM disclosed areas of decreased flow compared with normal controls in eight of 11 patients examined. Areas of hypoperfusion correlated with altered function as manifested by epileptogenic foci and impaired cognitive function. Dynamic, single-photon-emission computed tomography provides a noninvasive technique to monitor quantitatively hemodynamic changes associated with AVMs. Our findings suggest that such changes are present in the majority of patients with AVMs and that they may be clinically significant. The potential application of regional cerebral blood flow imaging by DSPECT in the management of patients with AVMs is discussed.

Normal distribution of regional cerebral blood flow measured by dynamic single-photon emission tomography.

Regional CBF (rCBF) was measured quantitatively using the inert-gas washout technique with xenon-133 and single-photon emission computed tomography. Tomographic data were reconstructed by filtered back projection, and flow was calculated according to the double-integral method. Ninety-seven subjects ranging in age from 20 to 59 years received a single examination; eight of these received a second examination within 1 h of the first; seven others received a second examination separated from the first by 1-10 days. Transverse-section images were obtained at 2, 6, and 10 cm above and parallel to the canthomeatal line (CML). Cortical gray matter flows were obtained from 12 brain regions in the slice 6 cm above the CML, and cerebellar and inferior cerebral gray matter flows were obtained from 4 regions in the slice 2 cm above the CML. Mean gray matter flow was 72 +/- 12 ml/min/100 g, with highest flows in the parietal lobes and visual cortex. No significant differences in rCBF occurred when a second study followed the first by 30 min to 10 days. Right-sided rCBF was slightly higher than left in all regions except frontal and parietal lobes where there was no difference. Flow was higher in women than in men and declined mildly with age for both sexes (slope = -0.33 ml/min/100 g/year; p less than 0.05).

Regional multiparameter estimation from tomographic diffusible tracer clearance curves: modification of the double-integral method.

Measurement of regional CBF in transverse section using inert, diffusible tracers can be carried out using a double-integral form of the Kety-Schmidt equation. An implementation of this form proposed by Kanno and Lassen (the K-L estimator) is utilized by the Tomomatic 64, a dynamic single-photon computed tomography system that records washin-washout data during and following inhalation of 133Xe gas. Advantages of the algorithm include noninvasive calibration of the input function and excellent depiction of ischemia: disadvantages are sensitivity to errors in input function delay (delta), the inability to estimate the partition coefficient (lambda) (hence, only the clearance index, k, is estimated), and a noise sensitivity proportional to k. A modification of the method is proposed that not only accounts for delta variations, but also provides an estimate of relative lambda (hence, the perfusion, f, is estimated). The proposed estimator is shown to be robust in the presence of noise with error variances equal to or better than those with the K-L estimator, yet estimates of both relative f and lambda are provided by the modification. New simulation results implicate the Compton scatter fraction as a major contributor in the overestimation of white matter perfusion values using both the K-L and proposed estimators, and illustrate the need for hardware and software scatter fraction reduction and control.

Error analysis of the double-integral method for calculating brain blood perfusion from inert gas clearance data.

A single-photon dynamic computer-assisted tomograph (DSPECT) has been built and is currently being used to evaluate regional cerebral blood perfusion in patients and volunteers. A computer simulation of the system was created to analyze the effects of data collection, Poisson noise, attenuation compensation, and the reconstruction technique now employed in the DSPECT. Several methods of attenuation compensation were used to generate perfusion images from both ideal and noisy data. The results indicate that the mean perfusion is calculated to within 10.4% accuracy for all perfusion rates in a region of interest if attenuation correction is used. Without attenuation correction, perfusions are underestimated by as much as 27%. The three correctors tested have different effects on the calculated perfusion value, depending on the location of the region of interest in the picture. The algorithm introduces random noise that is proportional to both the random error in the input data and the perfusion rate. Air-curve delay errors result in inaccuracies in the final perfusion picture that are proportional to perfusion rate. Physiological values (0.8-1.5) of the partition coefficient cause overestimation of both gray (0-34%) and white (7-67%) matter perfusion values. Compton scatter and collimator effects were not addressed in this study.

Attenuation Correction for SPECT: An Evaluation of Hybrid Approaches.

Most methods that have been proposed for attenuation compensation in single-photon emission computed tomography (SPECT) either rely on simplifying assumptions, or use slow iteration to achieve accuracy. Recently, hybrid methods which combine iteration with simple multiplicative correction have been proposed by Chang and by Moore et al. In this study we evaluated these methods using both simulated and real phantom data from a rotating gamma camera. Of special concern were the effects of assuming constant attenuation distributions for correction and of using only 180 degrees of projection data in the reconstructions. Results were compared by means of image contrast, %RMS error, and a chi-square error statistic. Simulations showed the hybrid methods to converge after 1-2 iterations when 360 degrees data were used, less rapidly for 180 degrees data. The Moore method was more accurate than our modified Chang method for 180 degrees data. Phantom data indicated the importance of using an accurate attenuation map for both methods. The speed of convergence of the hybrid algorithms compared to traditional iterative techniques, and their accuracy in reconstructing photon activity, even with 180 degrees data, makes them attractive for use in quantitative analysis of SPECT reconstructions.

Single-proton tomographic study of regional cerebral blood flow in epilepsy. A preliminary report.

Regional cerebral blood flow (rCBF) may be measured with a single-photon-emission computed tomograph (SPECT) after inhalation of xenon 133. Our SPECT studies of rCBF in a group of 18 patients with seizure disorders, when compared with studies in 32 normal control subjects, have shown enhanced flow to an active seizure focus and ischemia of brain areas in certain subjects between seizures. Thus, SPECT determination of rCBF has demonstrated a number of findings recently observed with positron-emission tomography and may become a useful modality in the study of patients who have epilepsy.

Single-photon tomographic determination of regional cerebral blood flow in epilepsy.

Using a single-photon emission computed tomographic scanner (SPECT) the authors determined regional cerebral blood flow (rCBF) with inhaled xenon-133, a noninvasive procedure. Studies were performed in 40 normal individuals, and these were compared with rCBF determinations in 51 patients with seizure disorders. Although positive results were obtained in 15 of 16 patients with mass lesions, the group of principal interest comprised 25 patients suffering from "temporal lobe" epilepsy. Only one of these had a positive x-ray computed tomogram, but 16 had positive findings on rCBF study. These findings included increased local blood flow in the ictal state and reduced flow interictally.

Attenuation compensation in single-photon emission tomography: a comparative evaluation.

Attenuation of photons in single-photon emission tomography (SPECT) makes three-dimensional reconstruction of unknown radioactivity distributions a mathematically intractable problem. Approaches to approximate SPECT reconstruction range from ignoring the effect of photon attenuation to incorporating assumed attenuation coefficients into an iterative reconstruction procedure. We have developed a computer-based simulation method to assess the relative effectiveness of attenuation compensation procedures. The method was used to study four procedures for myocardial SPECT using an infarct-avid radiopharmaceutical, Tc-99m stannous pyrophosphate. Reconstructions were evaluated by two criteria: overall (sum-of-squares) accuracy, and accuracy of lesion sizing. For moderate- to high-contrast studies there were no significant differences among the reconstructions by either evaluation criterion; for low contrast ratios the iterative method produced lower sum-of-squares criterion; for low contrast ratios the iterative method produced lower sum-of-squares error. We conclude that the additional expense of the iterative method is not justified under the conditions of this study. The approach used here is a convenient tool for evaluating specific SPECT reconstruction alternatives.

A contiguous-slice design for single-photon emission tomography (SPECT).

Recent multislice, single-photon emission tomographic systems produce nonoverlapping transverse-section images, requiring repositioning of the patient and repeated studies to obtain a complete set of cross-sectional data. A complete, overlapping set of transverse-section images can be obtained by designing a collimator with alternating, staggered centerlines that are offset in the slice dimension.

Quantitation of experimental canine infarct size with multipinhole and rotating-slanthole tomography.

Myocardial infarct size was estimated by three methods in a canine model, using Tc-99m pyrophosphate at 24 and 48 hr after coronary ligation. A gamma camera provided anterior, LAO, and lateral views, and was then fitted with multipinhole (MPH) and rotating-slanthole (RSH) collimators for tomographic studies, processed by computer to display frontal sections of the chest. Infarct weight was measured postmortem for comparison. All transmural infarcts were detected by all three imaging techniques. RSH tomography was superior to both MPH tomography and planar imaging for the detection of nontransmural infarction. Infarcts as small as 1.0 g were detected. Estimates of infarct volume measured from RSH slices showed an excellent correlation with infarct weight (r = 0.89) and were reproducible within acceptable limits. Estimates on infarct volume measured from MPH slices demonstrated a significantly poorer correlation with infarct weight (r = 0.48, p less than 0.01). Both tomographic techniques may improve infarct visualization by suppressing overlying activity and increasing contrast between infarct and background, but both produce significant blur artifacts that hamper their utilization by inexperienced observers.