David M. Goldhaber

Model-based region-of-interest selection in dynamic breast MRI

Abstract: Magnetic resonance imaging (MRI) is emerging as a powerful tool for the diagnosis of breast abnormalities. Dynamic analysis of the temporal pattern of contrast uptake has been applied in differential diagnosis of benign and malignant lesions to improve specificity. Selecting a region of interest (ROI) is an almost universal step in the process of examining the contrast uptake characteristics of a breast lesion. We propose an ROI selection method that combines model-based clustering of the pixels with Bayesian morphology, a new statistical image segmentation method. We then investigate tools for subsequent analysis of signal intensity time course data in the selected region. Results on a database of 19 patients indicate that the method provides informative segmentations and good detection rates.

Coronary calcium scoring: modelling, predicting and correcting for the effect of CT scanner spatial resolution on Agatston and volume scores

The purpose of this study was to evaluate the impact of spatial resolution on coronary calcium scoring by x-ray CT, to assess the scoring performance of different CT scanners as they are operated in the field and to correct for the effects of CT scanner spatial resolution on coronary calcium scoring. A phantom consisting of five aluminium wires of known diameter in water was used to measure spatial resolution and to assess scoring performance. Fourteen CT scanners (three helical, two dual, two electron-beam and seven multi-detector) from four manufacturers were evaluated, some under different operating conditions. One scanner was monitored over a 3 month period and again 6 months later. Both spatial resolution and image pixel size significantly affect calcium scoring results. Spatial resolution can be measured with a precision of about 2%. Scanner spatial resolution ranged from 1 to 1.7 mm full-width-half-maximum (FWHM), and pixel size from 0.25 to 0.86 mm. Spatial resolution differences introduce systematic scoring differences that range from 38% to 1100% depending on wire size. Significant temporal variations in spatial resolution were observed in the monitored scanner. By correcting all the scanners to the same target spatial resolution, the standard deviation of individual scanners with respect to a mean value (the spread) can be reduced by 25-70% for different wires. In conclusion, scanner spatial resolution significantly affects calcium scoring and should be controlled for. Scanner performance can change over time. Under ideal conditions, CT scanners should be operated with a standard spatial resolution for calcium scoring. When this is not possible, post-processing correction is a viable alternative.

Measurement of tissue temperature by MRI

The authors investigated the possibility of making non-invasive temperature measurements using MRI. Excised samples of beef muscle and brain were imaged with sequences for rapidly measuring the T/sub 1/, T/sub 2/, and proton density as the tissue temperature was varied from 37/spl deg/C to 43/spl deg/C and above. The signal strength varied by as much as 1.7% per /spl deg/C. For the imaging protocol used, the minimum detectable temperature change (1/spl sigma/) in a 1 cm/sup 3/ voxel over a 6 minute time interval was 1/spl deg/C. Some improvement is expected if the measured dependencies of T/sub 1/, T/sub 2/, and proton density on temperature are used to design a sequence that maximizes the change in signal strength with temperature. The signal strength vs. temperature data show some hysteresis, even for samples held below 43/spl deg/C-the maximum temperature to which healthy tissue is generally raised in hyperthermia. If live tissue shows the same behavior, it may be impossible to do true thermometry based on changes in T/sub 1/, T/sub 2/, and density. There may remain the possibility of accurately measuring small temperature changes over a narrow range.<<ETX>>

Ghost imaging in MRI

Needle biopsies and other interventions done under MR Fluoroscopy sometimes do not show the target well, either because the rapid sequence does not have adequate contrast or because a contrast agent may have washed out of the target. In these cases, an image that shows the target can be saved and scaled to match the spatial parameters of the fluoroscopic sequence, and used as a virtual or ghost field upon which the fluoroscopic images are superimposed, thus providing a view of the target, useful for needle pre-localization and for monitoring its progress as it is inserted.

Ghost imaging for targeting breast masses with MR imaging: A phantom study

RATIONALE AND OBJECTIVES The purpose of this study was to test the accuracy of ghost magnetic resonance (MR) imaging for guiding core biopsies of simulated breast masses in a tissue phantom. MATERIALS AND METHODS A tissue MR phantom implanted with 20 grapes as targets was placed into an interventional breast MR coil. The locations of the centers of the targets were determined, recorded, and saved as ghost images. A nonmagnetic phantom needle was constructed to avoid imprecision secondary to magnetic field inhomogeneity and was used to determine the three-dimensional location of the needle tip in the center of each grape on the ghost image. Once the positions were determined, the true needle was placed and biopsy specimens were taken. The needle was inspected for the presence of pulp after each pass. Each grape was inspected to determine the location of the needle track in relation to the center of the grape. The duration of the procedure was recorded. RESULTS All grapes were hit by the biopsy needle, as demonstrated either by pulp within the needle or by a needle track within the grape. Seventeen of the 20 grapes (85%) were hit centrally. Three were sampled eccentrically, up to 5-6 mm from the center. Each biopsy took approximately 1 hour. CONCLUSION These results suggest that ghost imaging may be ideal for needle guidance in core biopsy or preoperative localization, as it extends the period of visibility after a bolus injection of contrast material. Additionally, using a phantom needle for localization appears to overcome imprecision due to magnetic field inhomogeneity of the needle.