Charles M. Anderson

Assessment of carotid artery stenosis by ultrasonography, conventional angiography, and magnetic resonance angiography: Correlation with ex vivo measurement of plaque stenosis

PURPOSE Several studies have investigated the correlation between Doppler ultrasonography (DUS), angiography (CA), and magnetic resonance angiography (MRA) in the evaluation of stenosis of the carotid bifurcation. However, these studies suffer from the lack of a true control-the lesion itself-and therefore conclusions about the diagnostic accuracy of each method remain relative. To determine the absolute accuracy of these modalities, we have prospectively studied lesion size with DUS, MRA, and CA in 28 patients undergoing 31 elective carotid endarterectomies and compared the percent of carotid stenosis determined by each technique to the carotid atheroma resected en bloc. METHODS All patients were evaluated by each modality within 1 month before the thromboendarterectomy. With DUS, stenosis size was determined by standard flow criteria. For angiography and MRA, stenosis was defined as residual lumenal diameter/estimated normal arterial diameter (European Carotid Surgery Trial criteria). At surgery the carotid atheroma was removed en bloc in all patients. Patients in whom the lesion could not be removed successfully without damage were excluded from the study. Stenosis of the atheroma was determined ex vivo with high-resolution (0.03 mm3) magnetic resonance and confirmed by acrylic injection of the specimen under pressure and measurement of the atheroma wall and lumen. RESULTS The measurements of the ex vivo stenosis by high-resolution magnetic resonance imaging correlated closely with the size of stenosis determined by the acrylic specimen casts (r = 0.92). By ex vivo measurement, the lesions were placed in the following size categories: 40% to 59% stenosis (n = 2), 60% to 79% stenosis (n = 6), 80% to 89% stenosis (n = 7), and 90% to 99% stenosis (n = 16). CONCLUSIONS In general, the correlation of measurements of ex vivo stenosis with all modalities was good in these severely diseased arteries, although it was better for DUS (r = 0.80; p < 0.001) and MRA (r = 0.76; p < 0.001) than for CA (r = 0.56; p < 0.05).

Characterization of Prostate Cancer, Benign Prostatic Hyperplasia and Normal Prostates Using Transrectal 31Phosphorus Magnetic Resonance Spectroscopy: A Preliminary Report

We assessed the ability of 31phosphorus (31P) transrectal magnetic resonance spectroscopy to characterize normal human prostates as well as prostates with benign and malignant neoplasms. With a transrectal probe that we devised for surface coil spectroscopy we studied 15 individuals with normal (5), benign hyperplastic (4) and malignant (6) prostates. Digital rectal examination, transrectal ultrasonography and magnetic resonance imaging were used to aid in accurate positioning of the transrectal probe against the region of interest within the prostate. The major findings of the in vivo studies were that normal prostates had phosphocreatine-to-adenosine triphosphate (ATP) ratios of 1.2 +/- 0.2, phosphomonoester-to-beta-ATP ratios of 1.1 +/- 0.1 and phosphomonoester-to-phosphocreatine ratios of 0.9 +/- 0.1. Malignant prostates had phosphocreatine-to-beta-ATP ratios that were lower (0.7 +/- 0.1) than those of normal prostates (p less than 0.02) or prostates with benign hyperplasia (1.1 +/- 0.2, p less than 0.01). Malignant prostates had phosphomonoester-to-beta-ATP ratios (1.8 +/- 0.2) that were higher than that of normal prostates (p less than 0.02). Using the phosphomonoester-to-phosphocreatine ratio, it was possible to differentiate metabolically malignant (2.7 +/- 0.3) from normal prostates (p less than 0.001), with no overlap of individual ratios. The mean phosphomonoester-to-phosphocreatine ratio (1.5 +/- 0.5) of prostates with benign hyperplasia was midway between the normal and malignant ratios, and there was overlap between individual phosphomonoester-to-phosphocreatine ratios of benign prostatic hyperplasia glands with that of normal and malignant glands. To verify the in vivo results, we performed high resolution magnetic resonance spectroscopy on perchloric acid extracts of benign prostatic hyperplasia tissue obtained at operation and on a human prostatic cancer cell line DU145. The extract results confirmed the differences in metabolite ratios observed in vivo. We conclude that transrectal 31P magnetic resonance spectroscopy can characterize metabolic differences between the normal and malignant prostate.

Magnetic resonance angiography of the carotid artery combining two- and three-dimensional acquisitions

To assess the agreement between magnetic resonance angiography and conventional angiography in the evaluation of carotid stenosis, 61 carotid arteries of 40 patients were studied by combined two- and three-dimensional magnetic resonance angiography and conventional angiography. Stenosis of the internal carotid artery was categorized as mild, moderate, severe, critical, or complete occlusion. In 42 arteries, the degree of stenosis according to magnetic resonance angiography correlated exactly to that found by conventional angiography. In the remaining 19 carotid arteries, the magnetic resonance angiographic measurement of stenosis differed from the conventional angiographic measurement by only one size category. The Spearman rank correlation coefficient was 0.95 (p < 0.001). This study showed that by combining information from two- and three-dimensional magnetic resonance angiographic studies and making use of the advantages of each method, magnetic resonance angiography was comparable to conventional angiography in determining carotid stenosis. Magnetic resonance angiography tended to demonstrate a higher level of stenosis when there was a discrepancy. These data demonstrate that magnetic resonance angiography is a steadily improving technology. Although additional studies need to be done, it seems clear that magnetic resonance angiography will be an imaging modality comparable in accuracy to conventional angiography.

High resolution cine MRI of vessel distension

Objective We have evaluated the ability of high resolution cine MRI to quantify vessel distension. Materials and Methods Gradient echo imaging was used to acquire high resolution, cardiac-triggered imaging of the ascending aorta in normal volunteers to measure changes in the cross-sectional area of the lumen over the cardiac cycle. The effects of temporal resolution, spatial resolution, and the radiofrequency flip angle on the measurements were investigated. Results High spatial resolution improved the ability to detect changes in the luminal area. High temporal resolution allowed better tracking of maximal vessel distension. A small flip angle reduced the saturation of slow flow observed in imaging acquired using a large flip angle. Conclusion Our study has demonstrated that high spatial resolution and high temporal resolution can substantially improve the measurement of vessel distension. We have overcome problems of intravoxel dephasing and spin saturation by using high spatial resolution, a short echo time with flow compensation, and a small flip angle.

Instrumentation for Magnetic Resonance Angiography

Magnetic resonance angiography (MRA) places high demands on instrumentation capabilities. Magnetic gradient strength capabilities, main magnetic field strength and homogeneity, and eddy current compensation all play a role in determining the quality of the flow studies. In addition, radiofrequency coil design and use is governed by the specific vascular territories of interest. Once the instrumentational and pulse sequence considerations have been optimized, the postprocessing and display of the acquired three-dimensional data sets is of key importance. Great strides have been made in addressing instrumentation needs for MRA, but further improvements are anticipated.