Douglas A. Ortendahl

Magnetic resonance imaging of the TMJ disc in asymptomatic volunteers

Forty-two temporomandibular joints (TMJs) in 21 asymptomatic volunteers were visualized by magnetic resonance imaging (MRI). The subjects, 12 males and nine females, were between 23 and 43 years of age and had no history of TMJ pain, joint noise, limited opening, or previous treatment for TMJ disorder. A cephalometric head-holder was designed to position the TMJ in an accurate and reproducible manner and multisection parasagittal images were obtained perpendicular to the longitudinal axis of the condyle. MR images depicted anterior disc position in 32% of the asymptomatic joints (8/24 males, 5/18 females). Anterior disc position in asymptomatic subjects may be a predisposing factor to TMJ dysfunction or simply an anatomic variant whose prevalence must be considered when evaluating TMJ dysfunction.

The Value of Relaxation Times and Density Measurements in Clinical Mri

The hope that MRI relaxation time signatures would identify tissues, specifically, malignancies, has not been realized. This is due much less to measurement inaccuracies than to a large intrinsic variability and overlaps between malignancies and many benign pathologies. Neither has there been success in predicting relaxation times from basic tissue compositions. Nevertheless, MRI provides a qualitative measure of tissue hydration, and of flow, on the basis of relaxation times. Furthermore, pixel-by-pixel maps of relaxation times have proven useful in understanding the MRI process, in predicting the efficacy of untried techniques, and replace, in many circumstances, the need for acquisition of images with diverse sequencing parameters.

Emission imaging of patients with craniomandibular dysfunction.

Signs and symptoms of craniomandibular dysfunction in 37 patients were compared with the results of corrected cephalometric tomography and an emission imaging protocol consisting of both planar and single photon emission computed tomography (SPECT) (7500 ZLC Orbiter) images. The planar images and the single photon emission computed tomography projection views were processed with a bayesian deblurring algorithm to improve image quality. The correlation of emission imaging with craniomandibular dysfunction, as indicated by temporomandibular joint pain and joint noise, showed a high sensitivity (93%) and a high specificity (86%), whereas the correlation of corrected cephalometric tomography with temporomandibular joint pain and joint noise showed a relatively high sensitivity (89%) but a low specificity (27%). These results indicate that emission imaging is a sensitive and accurate indicator of craniomandibular dysfunction.

Investigation of Partial Flip Angle Magnetic Resonance Imaging

For magnetic resonance imaging (MRI) of the head a TR of about 2.0 seconds has been found to give excellent sensitivity to desease. If a flip angle less than 90/sup 0/ is used for initial excitation, it is possible to maintain sensitivity to disease while reducing TR and thus total imaging time. Simulations based on data from patients with representative pathology show that image contrast is preserved. By acquiring sequences at TR = 0.5 seconds with two different flip angles it is possible to calculate T1 with good accuracy, thus allowing calculated images at arbitrary TR to be produced.

Semiconductor Gamma-Cameras in Nuclear Medicine

The availability of semiconductor detectors, principally High Purity Germanium, permits consideration of new generations of nuclear medicine imaging devices. These are characterized by excellent spatial resolution and by simultaneous multiple radioisotope imaging capabilities. The technology and some of its resultant benefits are presented here.

Automated Tissue Characterization with NMR Imaging

Nuclear magnetic resonance imaging is a rapidly evolving modality which has aroused great excitement in the medical community (1,2). NMR has shown excellent soft tissue contrast without the necessity of intravenous contrast agents. It also has the advantage of using no ionizing radiation.

A Comparison of the Noise Characteristics of Projection Reconstruction and Two-Dimensional Fourier Transformations in NMR Imaging

Human images of excellent quality have been obtained with Nuclear Magnetic Resonance (NMR), using two different techniques: projection reconstruction (PR) and two-dimensional Fourier transformations (2DFT). It has been shown in CT that the reconstruction algorithms will multiply the noise in the projections and change the noise spectrum. A comparison of the noise of the two techniques is presented under the constraint of equal imaging conditions using both computer simulations and data obtained with the UCSF human imager. The computer simulations show that the two techniques produce approximately equal signal to noise values when S/N ~ 15. Below that value small differences are observed. For both simulations and actual data, the 2DFT shows a uniform spatial frequency response while the PR method shows a roll-off at low spatial frequencies. The 2DFT method is shown to be more resistant to artifact formation.

High Resolution Emission Computed Tomography with a Small Germanium Camera

The imaging capabilities of the germanium camera, (high spatial resolution and scatter rejection) combined with processing to recover loss of resolution at depth, have resulted in single-photon emission tomography with significant improvement in spatial resolution when campared to devices based on conventional technology. Multiple radionulides can be imaged, and spatial resolution of 2-4 mm at depth can be realized.

Operating Characteristics of Small Position-Sensitive Mercuric Iodide Detectors

Room temperature semiconductors are potentially attractive candidates for gamma cameras because of their compactness and high spatial resolution. We have evaluated position-sensitive HgI2 detectors for use in high resolution small area gamma cameras. Two 0.5 mm thick detectors resolved by orthogonal strips into elements 1.75 mm on a side and 0.9 mm on a side have been tested. The spatial resolution is 2 mm FWO.1M and 1 mm FWO.1M respectively. Energy resolution is improved by measuring energy from the anode in coincidence with the cathode signal of the detector. An energy resolution of 6.8 keV FWHM at 122 keV and 5.7 keV FWHM at 60 keV has been observed.

Tissue Characterization Using Intrinsic NMR Parameters and a Hierarchical Processing Algorithm

The multiparametric nature of NMR imaging allows the same section to be displayed in many different ways by either adjusting acquisition parameters or calculating additional images, creating a possibly overwhelming data set. The need for this many images may be reduced by creating tissue type maps where each tissue is identified according to its characteristic NMR parameters and painted using a color code. Although T1 and T2 values for normal tissue are reproducible, variability among patients, the wide range of T1,T2 and N(H) values for abnormal tissue and partial volume averaging make the determination of tissue signatures difficult without significant operator intervention. A hierarchical processing algorithm, pyramidal segmentation, allowing simultaneous analysis at multiple levels of spatial resolution provides the basis for a tissue mapping algorithm with significantly reduced operator supervision.