Elimination of RF inhomogeneity effects in segmentation.

There are various methods proposed for the segmentation and analysis of MR images. However the efficiency of these techniques is effected by various artifacts that occur in the imaging system. One of the most encountered problems is the intensity variation across an image. To overcome this problem different methods are used. In this paper we propose a method for the elimination of intensity artifacts in segmentation of MRI images. Inter imager variations are also minimized to produce the same tissue segmentation for the same patient. A well-known multivariate classification algorithm, maximum likelihood is employed to illustrate the enhancement in segmentation.

Gamma Knife 3-D dose distribution near the area of tissue inhomogeneities by normoxic gel dosimetry.

The accuracy of the Leksell GammaPlan, the dose planning system of the Gamma Knife Model-B, was evaluated near tissue inhomogeneities, using the gel dosimetry method. The lack of electronic equilibrium around the small-diameter gamma beams can cause dose calculation errors in the neighborhood of an air-tissue interface. An experiment was designed to investigate the effects of inhomogeneity near the paranosal sinuses cavities. The homogeneous phantom was a spherical glass balloon of 16 cm diameter, filled with MAGIC gel; i.e., the normoxic polymer gel. Two hollow PVC balls of 2 cm radius, filled with N2 gas, represented the air cavities inside the inhomogeneous phantom. For dose calibration purposes, 100 ml gel-containing vials were irradiated at predefined doses, and then scanned in a MR unit. Linearity was observed between the delivered dose and the reciprocal of the T2 relaxation time constant of the gel. Dose distributions are the results of a single shot of irradiation, obtained by collimating all 201 cobalt sources to a known target in the phantom. Both phantoms were irradiated at the same dose level at the same coordinates. Stereotactic frames and fiducial markers were attached to the phantoms prior to MR scanning. The dose distribution predicted by the Gamma Knife planning system was compared with that of the gel dosimetry. As expected, for the homogeneous phantom the isodose diameters measured by the gel dosimetry and the GammaPlan differed by 5% at most. However, with the inhomogeneous phantom, the dose maps in the axial, coronal and sagittal planes were spatially different. The diameters of the 50% isodose curves differed 43% in the X axis and 32% in the Y axis for the Z =90 mm axial plane; by 44% in the X axis and 24% in the Z axis for the Y=90 mm coronal plane; and by 32% in the Z axis and 42% in the Y axis for the X=92 mm sagittal plane. The lack of ability of the GammaPlan to predict the rapid dose fall off, due to the air cavities behind or near the lesion led to an overestimation of the dose that was actually delivered. Clinically, this can result in underdosing of lesions near tissue inhomogeneities in patients under treatment.

Evaluation of hemodynamic perfusion MR images.

Perfusion normally refers to the delivery of blood at the level of capillaries. Hemodynamic perfusion magnetic resonance (MR) imaging has been used in clinics for a series of applications including tumor characterization (histological type diagnosis and grading), diagnosis and the follow up of stroke, and several other disorders. For this study a platform to investigate the theoretical basis of perfusion imaging was developed. Using dynamic measurements of contrast agents in magnetic resonance imaging (MRI) relative cerebral blood volume (rCBV), relative cerebral blood flow (rCBF), and mean transit time (MTT) are calculated. These three parameters are quantized, and displayed as color images for diagnostic and follow up studies. The comparative studies in MR perfusion must address issues such as; image registration, region of interest (ROI) selection, threshold identification and quantization of rCBV, rCBF and rMTT. The evaluation process involved the comparison of the diagnostic capabilities of the three perfusion images (rCBV, rCBF, MTT). Digital Substraction angiography was used as the gold standard in these comparisons. The study group comprises 16 patients with the diagnosis of subarachnoid bleeding and intracranial aneurysms. The proposed cerebral MR perfusion analysis system has been accepted by the radiologists as a useful tool for their perfusion studies and clinical evaluation.

Relative dose distribution in gamma knife treatment near tissue inhomogeneties.

The primary goal in this study was to investigate 3-D dose distribution, near the areas of tissue inhomogeneities, in Gamma Knife Radiosurgery with the gel dosimetry. The spherical glass balloon of a diameter of 16 cm filled with the gel forms the homogeneous phantom; and an identical balloon with two corks placed on each side to represent the air cavities forms the inhomogeneous phantom. Dose calibration is performed by irradiating vials at known doses and then utilizing the R2- dose calibration curve. Stereotactic frames and fudicial markers were attached to the phantoms for MR scanning and image processing. Dose distributions from a single shot, using all 201 Cobalt sources, delivered to a known point with identical coordinates, are calculated both in homogeneous and inhomogeneous gel phantoms. In the aspect of dosimetrical quality control, the Gamma Knife planning system predicted dose distribution is compared with the experimental results. In the homogeneous phantom, the gel dosimetry calculated dose distribution is in good agreement with the GammaPlan predicted dose distribution. However, with the inhomogeneous phantom, the dose distribution is spatially different and significant differences in dose levels are observed.