Can noninvasive studies replace conventional angiography in the preoperative evaluation of carotid stenosis?

Most articles in 1995 demonstrated a sensitivity of 100% and specificities of 90% to 98% in detecting carotid bifurcation stenosis using a combination of MR angiography and ultrasonography, reserving conventional angiography for cases of disparate stenosis with the noninvasive studies. Multiple reviews concluded that noninvasive studies were less morbid than conventional angiography when taking into account the risks of conventional angiography and surgery as well as the morbidity associated with unnecessary surgery (false-positive noninvasive study) or withholding indicated surgery (false-negative noninvasive study). Thus, the current literature suggests that non-invasive studies have indeed reached a point where they can replace conventional angiography in the preoperative evaluation of carotid bifurcation stenosis. It is paramount that each imaging center review its own sensitivity and specificity in detecting surgically significant carotid stenosis with noninvasive studies. High sensitivity and specificity in diagnosing percent carotid stenosis with noninvasive studies can occur today with routine imaging equipment, but it takes a great deal of effort and dedication on the part of the imaging center.

Prospective evaluation of extracranial carotid stenosis: MR angiography with maximum-intensity projections and multiplanar reformation compared with conventional angiography.

OBJECTIVE: The purpose of this prospective study was to compare MR angiography of the carotid artery from the aortic arch through the circle of Willis using maximum-intensity projection (MIP) and multiplanar reformation (MPR) images with intraarterial angiography in the depiction of extracranial carotid atherosclerosis. SUBJECTS AND METHODS: The carotid arteries in 20 patients were studied with MR and intraarterial angiography. MR angiography included two-dimensional (2D) time-of-flight (TOF) sequences from the aortic arch through the skull base and three-dimensional (3D) TOF sequences centered at the carotid bifurcation and multiple overlapping thin slab acquisition (MOTSA) from the skull base to above the circle of Willis. Targeted MIP images of the 2D and 3D TOF MR angiograms through each carotid bifurcation were obtained. Last, MPR images of the 3D TOF MR angiograms at the obliquity that showed the greatest stenosis were obtained. All studies were reviewed in a double-blinded fashion by two neuroradiologists. Caliper measurements of MR angiograms and intraarterial angiograms were made by using North American Symptomatic Carotid Endarterectomy Trial criteria. Global MIPs of the aortic arch and common carotid arteries from the 2D TOF MR angiograms and targeted MIPs of the intracranial carotid arteries from the MOTSA MR angiograms were compared with the intraarterial angiogram and graded as normal, mild, moderate, severely stenotic, or occluded. RESULTS: MPR of the 3D TOF MR angiograms was highly correlated with intraarterial angiograms for both observers 1 and 2 (0.94/p < .001, 0.96/p < .001 [Pearson correlation/p value]). No statistically significant difference between 3D TOF MPR and intraarterial angiography was seen with a paired t-test. With an alpha = 0.05 (5% probability of type 1 error), the power to detect a difference as small as +/- 5% stenosis between 3D TOF MPR and intraarterial angiogram was 80% for observer 1 and 90% for observer 2. Although both MIPs of the 2D and 3D TOF MR angiograms showed high Pearson correlation coefficients (0.83, 0.90) with intraarterial angiography, the paired t-test revealed a statistically significant difference in the estimation of carotid stenosis. Both observers thought the global MIPs of the 2D TOF MR angiogram allowed good to excellent visualization of the common carotid arteries. The aortic arch was seen in 70% of patients; most of the missed cases occurred early in our experience, when the 2D axial images were not placed sufficiently inferior to include the arch. No stenosis of the great vessel origins was seen in this study. All four stenoses of the intracranial internal carotid artery identified with intraarterial angiography were seen with the MOTSA MR angiogram but with a tendency to overestimate stenosis. Only one carotid siphon was thought to show severe stenosis on the MR angiogram. Intraarterial angiography showed a 50% stenosis. CONCLUSION: It is possible to image the entire carotid artery from the aortic arch through the circle of Willis with MR angiography in a clinically acceptable time. MPR of the 3D TOF MR angiogram reliably shows the percentage of carotid stenosis with no statistically significant difference compared with intraarterial angiography. The role of MR angiography in showing lesions in the circle of Willis or the aortic arch is promising, but the limited number of tandem lesions in this study makes it difficult to draw any conclusions.

Perflubron as an oral contrast agent for MR imaging: results of a phase III clinical trial.

PURPOSE: To assess perflubron for magnetic resonance (MR) imaging in the abdomen and pelvis in a multicenter trial. MATERIALS AND METHODS: MR images were obtained in 127 subjects before and after ingestion of perflubron with T1-, proton-density-, and T2-weighted sequences at 0.38, 1.0, or 1.5 T. Postcontrast images were compared with baseline images, and percentage of additional bowel darkened, distinction of bowel from adjacent tissue, and change in image artifact were graded. RESULTS: Perflubron increased the bowel darkening in over 92% of subjects with all sequences and field strengths. It improved definition of the left lobe of the liver and body and tail of the pancreas in 67%, 29%, and 42% of subjects, respectively, and of the uterus and bladder in 80% and 76%. Abnormal tissue was more conspicuous in 69% of subjects. Highest scores were achieved when the upper abdomen was imaged 5-30 minutes and the pelvis 10-40 minutes after ingestion. No image artifacts or side effects were attributed to perflubron. CONCLUSION: Perflubron is safe, and its efficacy was unaffected by pulse sequences, magnetic field strength, or time delay.

Cerebrovascular magnetic resonance angiography: a critical verification.

Because simultaneous noninvasive noncontrast imaging of cervical and cerebral vasculature and brain is possible with magnetic resonance angiography (MRA) and imaging (MRI), the following study was undertaken from July 1990 to January 1992. One hundred twenty-eight patients were examined with General Electric 1.5 Tesla MRI systems. Axially acquired volumetric three-dimensional time-of-flight MRA with 0.7 mm3 voxel size with regional maximum intensity projection after processing followed a two-dimensional time-of-flight localizing sequence. These two MRA sequences combined with spin-echo parenchymal brain MRI were compared with duplex scans, contrast angiograms, and surgical findings. Blinded readings by a radiologist and vascular surgeon allowed comparison of grades of luminal diameter narrowing (normal, mild, moderate, severe, and occluded) seen on MRA to be compared with those of Doppler and contrast angiography. Excluding 12 nondiagnostically imaged internal carotid arteries (10 MRA) and limiting duplex correlation to within 5 days of the MRA examination allowed critical appraisal of 182 internal carotid arteries. Exact correlation of grade of stenosis was obtained by the radiologist in 136 (74.7%) of 182 arteries and the surgeon in 138 (75.8%) of 182 arteries. Spearman rank correlation analysis found rank correlation coefficients of 0.88 (p < 0.001) and 0.83 (p < 0.001), respectively, for the radiologist and vascular surgeon. Disagreement one category apart was found by the radiologist in 35 studies (19.3%) and the surgeon in 28 studies (15.4%). Two or more grades of disagreement were found by the radiologist in 11 studies (6%) and the surgeon in 16 studies (8.8%). Contrast angiogram-MRA agreement was found in 86% of 36 internal carotid arteries. The degree of stenosis detected by MRA was concordant with surgical findings in 39 of 40 patients. Thus MRA emerges as a useful and accurate method of obtaining cerebrovascular evaluation in clinical practice.

Flow phenomena, classification, and clinical aspects of magnetic resonance angiography.

Both time-of-flight and phase-contrast techniques are currently being employed in producing angiographic MR images of arteries and veins. Both two-dimensional and three-dimensional acquisition methods can be used to display projective vascular images with the maximum intensity projection algorithm, eliminating the need for image subtraction. MR angiography has expanded the armamentarium of MR imaging, especially in the head and neck, and should in some organ systems reduce the need for conventional x-ray catheter angiography.

Clinical experience with routine Gd-DTPA administration for MR imaging of the brain.

A retrospective analysis of intracranial magnetic resonance (MR scans of 514 patients who underwent nonselective Gd-DTPA enhanced MR imaging was performed to determine the efficacy of this protocol for Gd-DTPA administration. This report reviews the frequency and clinical significance of abnormally enhancing areas that were entirely undetectable on precontrast images or would have been missed without the retrospective knowledge of enhancement. Fifty-seven patients (11% of the 514 patients studied) showed enhancing lesions, with 16 (3.1% of the total) of these patients demonstrating one or more lesions identifiable only on postcontrast images. Of those 16 patients, 8 had other focal abnormalities on precontrast studies, whereas the remaining 8 (1.6% of the total) had negative precontrast studies. The new diagnoses affected clinical management directly in five patients and in another nine contributed potentially significant information. Considerations regarding a selective versus nonselective protocol for Gd-DTPA administration for intracranial MR imaging and the use of clinical information to augment these protocols are discussed.

Superficial- and deep-tissue temperature increases in anesthetized dogs during exposure to high specific absorption rates in a 1.5-T MR imager.

Superficial- and deep-tissue heating was measured in five dogs during high-specific-absorption-rate radiofrequency (RF) irradiation to see whether significant temperature changes could be produced by a 1.5-T clinical magnetic resonance imager. The RF power output employed was 6.3 times that required for routine imaging. Temperature probes were placed in both deep and superficial tissues, and temperatures were recorded before, during, and after exposure. In each dog, there was a linear temperature increase of several degrees during RF exposure; the maximal average change was 4.6 degrees C in the urinary bladder. The temperature increase was slightly greater in deep tissues than in superficial tissues. The calculated specific absorption rate, based on the temperature change, averaged 7.9 W/kg for all five dogs. These findings argue for continued caution in the design and operation of imagers capable of high specific absorption rates, particularly when they are used for imaging infants or patients with altered thermoregulatory capability.

Horner syndrome occurring as a complication of chest tube placement.

A case of Horner syndrome occurring secondary to the high insertion of a chest tube is reported. Horner syndrome from this cause can easily be avoided. The tip of the chest tube should be kept at or below the third posterior rib unless the clinical situation dictates otherwise.

Classification and investigation of artifacts in magnetic resonance imaging.

Many MR artifacts are not understandable in terms of previous imaging experience. A variety of artifacts is presented here; their origins and elimination are discussed.

Magnetic resonance applications in atherosclerotic vascular disease.

Magnetic resonance imaging of the cardiovascular system offers great promise in the detection and characterization of the anatomic, physiologic, and biochemical consequences of atherosclerosis. This review will focus on the potential applications of MRI for evaluating atherosclerosis of the abdominal aorta and iliofemoral vessels.

Peripheral vascular disease: correlation of MR imaging and angiography.

The capability of magnetic resonance (MR) imaging for detecting aortic, iliac, and femoral stenoses and occlusions was evaluated. Multisection spin-echo studies at 0.35 tesla were obtained of the infrarenal aorta to the femoral bifurcation in 24 patients, all of whom had undergone intraarterial angiography within 14 days of imaging. Transaxial MR images were compared with the angiograms. Arterial stenoses and occlusions in these vessels detected by MR imaging correlated with angiographic findings in 91% of the instances. Protrusional atherosclerotic plaques and occlusions and stenoses in the aortoiliac region were demonstrated accurately on MR images; complications of previous vascular surgery, such as aneurysms at sites of previous anastomoses or endarterectomy, were also identified. Due to the limited spatial resolution, MR images failed to demonstrate some femoral stenoses. MR imaging may be used for evaluation of aortoiliac vascular disease and for follow-up study after surgical revascularization. However, the limited spatial resolution, noncomposite display of the aortoiliofemoral circulation, and lack of evaluation of peripheral runoff provided by current MR imaging techniques militate against its replacing angiography prior to vascular intervention.

Magnetic resonance imaging of acute myocardial infarction using a nitroxyl spin label (PCA).

The effects of an intravenously administered nitroxyl spin label (PCA) on the magnetic resonance imaging (MRI) appearance and relaxation times of acute canine myocardial infarctions were studied. Twenty-four hours after ligation of the left anterior descending coronary artery (LAD), animals were either sacrificed immediately (three dogs) or injected with 3.0 mmol/kg of PCA prior to sacrifice (six dogs). The PCA group dogs were sacrificed at either 5 minutes postinjection (three dogs) or 15 minutes postinjection (three dogs). Magnetic resonance imaging (0.35 T) using spin-echo techniques demonstrated high signal intensity in the infarct relative to normal myocardium in all three groups. In the control group, the T1 and T2 relaxation times were longer in infarcted compared with normal myocardium, but only the measure in T2 reached statistical significance (P less than .05). PCA produced infarct-avid T1 shortening in the six dogs that received it. Contrast in the group sacrificed at 15 minutes postcontrast administration was greater than that in the control group due to T1 shortening in the infarct. Thus, PCA produces differential effects on normal and infarcted myocardium. Between 5 and 15 minutes after IV administration, it causes greater changes in the infarct due to prolonged retention in this region.

[Contrast agents in magnetic resonance imaging]. / Les agents de contraste en imagerie par résonance magnétique.

The origin of nuclear magnetic resonance signal is reminded in this paper. Different ways of increasing the contrast in magnetic resonance imaging are presented, especially modifications of tissues relaxation times by the use of paramagnetic ions or nitroxides. The potential of these substances is illustrated by several examples of application in the animal.

Dilute oral iron solutions as gastrointestinal contrast agents for magnetic resonance imaging; initial clinical experience.

Delineation of the gastrointestinal tract in magnetic resonance imaging (MRI) remains a problem. Ferric ammonium citrate is paramagnetic, producing a high MRI signal intensity by virtue of its spin-lattice (T1) relaxation rate enhancement properties. Water is diamagnetic, producing a low MRI signal intensity, especially with short TR and TE times. To compare efficacy for gastrointestinal contrast alteration, ferric ammonium citrate was administered to 18 patients and water was given to 10 patients. Spin-echo imaging at 0.35T was performed after administration of these agents. Ferric ammonium citrate produced high signal intensity within the esophagus, stomach, duodenum, and small intestine that aided in the differentiation of the gastrointestinal tract from adjacent tumors, vessels, and viscera. Delineation of the gut wall was superior using ferric ammonium citrate compared to that produced by water. Delineation of the margins of the pancreas, liver, and kidney from adjacent gastrointestinal tract was also better with ferric ammonium citrate. Optimal distinction between bowel and fat was better with water. Longer TE times (75 to 200 ms) may allow improved contrast between gut and intrabdominal fat using ferric ammonium citrate.

Magnetic field dependence of spin-lattice relaxation enhancement using piperidinyl nitroxyl spin-labels.

We examined the magnetic resonance properties of 12 paramagnetic piperidinyl nitroxyls in water and plasma solutions. Paramagnetic contributions to proton relaxation times were measured using 10.7 and 100 MHz spectrometers. Proton relaxation enhancement from nitroxyls increased with ascending molecular weight, in plasma solutions versus equimolar aqueous solutions, and with measurements at 10.7 MHz compared to 100 MHz. Relaxation rates were observed to approximately double at 10.7 MHz compared to 100 MHz and from water to plasma solutions. The data indicate that proton spin-lattice relaxation enhancement is magnetic field-dependent, and increases using nitroxyls of large molecular weight and with chemical substitutents that increase the microviscosity of solvent water molecules. The development of nitroxyls for diagnostic MRI will be aided by understanding these in vitro physical characteristics and trends.

Enhanced MRI of tumors utilizing a new nitroxyl spin label contrast agent.

Nitroxyl spin labels have been shown to be effective in vivo contrast agents for magnetic resonance imaging (MRI) of the central nervous system, myocardium, and urinary tract. A new pyrrolidine nitroxyl contrast agent (PCA) with better resistance to in vivo metabolic inactivation than previously tested agents was studied for its potential to enhance subcutaneous neoplasms in an animal model. Twenty-two contrast enhancement trials were performed on a total of 15 animals 4-6 weeks after implantation with human renal adenocarcinoma. Spin echo imaging was performed using a .35 T animal imager before and after intravenous administration of PCA in doses ranging from 0.5 to 3mM/kg. The intensity of tumor tissue in the images increased an average of 35% in animals receiving a dose of 3 mM/kg. The average enhancement with smaller doses was proportionately less. Tumor intensity reached a maximum within 15 min of injection. The average intensity difference between tumor and adjacent skeletal muscle more than doubled following administration of 3 mM/kg of PCA. Well-perfused tumor tissue was more intensely enhanced than adjacent poorly perfused and necrotic tissue.

Translational molecular self-diffusion in magnetic resonance imaging. I. Effects on observed spin-spin relaxation.

The reduced T2 (spin-spin) relaxation times (T2obs less than 200 ms) measured on pure fluids on our 0.35T magnetic resonance imagers stimulated an investigation into this phenomenon. The cause for the short T2obs of fluids was found to be translational molecular self-diffusion of hydrogen nuclei through the pulsed slice-selective magnetic gradient in the imagers. Similar reductions in biological tissue T2obs were also attributed to molecular self-diffusion.