Calibration of the radio frequency field for magnetic resonance imaging.

We have developed and validated the performance of a novel slice selective pulse sequence that allows direct calibration of the RF field using a simple rectangular pulse. The new sequence offers a number of substantial advantages. It operates at steady state and has an accurate calibration response at short repetition times. The slice selection train is insensitive to RF field strength changes caused by patient loading. The issue of patient motion has been addressed in our data collection and analysis routines. The applicability of the method to human scanning has been demonstrated in the automated RF power calibration routine of a commercial imaging system.

Hemodynamics and aneurysm development in vascular allografts.

PURPOSE: Mechanical and immunologic factors may play a role in the development of native arterial and biologic graft aneurysms. We developed an experimental rat aortic allograft aneurysm model in which segments of infrarenal aorta were transplanted between hypertensive and normotensive rats to study these factors in this model. METHODS: Aortic allografts and autografts were inserted into spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rats. Effects of immunologic and antihypertensive therapy were evaluated. Graft diameters were followed up with magnetic resonance imaging and at harvest. Direct-pressure measurements were taken and dp/dtmax (force of ventricular contractions) was calculated before harvest. RESULTS: Autografts remained isodiametric and maintained their histologic architecture. Aneurysmal dilation of transplanted segments occurred in SHR host allografts but not in WKY host allografts. Histologic examination of all allograft specimens noted a rejection reaction characterized by inflammatory cell infiltration and medial smooth muscle cell loss. Antigenic enhancement accelerated aneurysm development in SHR hosts but had no significant effect on WKY hosts. Rates of allograft enlargement and final allograft diameters were similar in antihypertensive treated and untreated SHR hosts. The dp/dtmax in untreated SHR hosts was greatest and differed significantly from that in the WKY rats but only marginally from that in treated SHR hosts. CONCLUSIONS: Immunologic rejection but not abnormal hemodynamics is necessary for development of allograft aneurysm in this model.

Measurement of regional blood oxygenation and cerebral hemodynamics.

An echo planar linewidth mapping technique, Shufflebutt, has allowed temporal measurements of changes in linewidth caused by static inhomogeneities (delta LWSI) and transverse relaxation rate (delta R2) in models of hypoxia and hypercapnia. We demonstrate these changes are due to intravascular susceptibility differences/(delta chi) between the blood and tissue. Contrast agent injections at a delta chi equivalent to that of deoxygenated blood showed a twofold difference between the contrast agent and physiological anoxia values. Hypercapnia decreased both delta LWSI and delta R2 consistent with an increase in blood oxygenation. We attribute these findings to constant oxygen extraction during an increase in blood flow, resulting in less deoxygenated venous blood and thus reduced delta chi. For in vivo perturbations we found that delta R2/delta R2' approximately 0.33, a ratio much different from that measured in whole blood phantoms (delta R2/delta R2' approximately 2). This demonstrates that signal changes in these studies are produced predominantly by dephasing of extravascular protons due to field inhomogeneities produced by intravascular deoxygenated hemoglobin (deoxyHb).

Magnetic resonance imaging of small arteries.

Magnetic resonance imaging (MRI) with receive-only surface coil technology was used to visualize and quantitative luminal diameters of small arteries in the rat. MRI measurements of normal and aneurysmal aortas, over a diameter range of 1-3 mm, were closely correlated with direct measurements made visually at laparotomy: measured differences averaged 0.16 mm, and the least-squares regression line (R2 = 0.97, P < 0.001) compared favorably to the line of equivalence, X = Y. This noninvasive but precise imaging modality demonstrates the potential value of using MRI to evaluate the diameter of small vessels, including the postoperative monitoring of arterial bypass graft patency in peripheral regions.

Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation.

Neuronal activity causes local changes in cerebral blood flow, blood volume, and blood oxygenation. Magnetic resonance imaging (MRI) techniques sensitive to changes in cerebral blood flow and blood oxygenation were developed by high-speed echo planar imaging. These techniques were used to obtain completely noninvasive tomographic maps of human brain activity, by using visual and motor stimulus paradigms. Changes in blood oxygenation were detected by using a gradient echo (GE) imaging sequence sensitive to the paramagnetic state of deoxygenated hemoglobin. Blood flow changes were evaluated by a spin-echo inversion recovery (IR), tissue relaxation parameter T1-sensitive pulse sequence. A series of images were acquired continuously with the same imaging pulse sequence (either GE or IR) during task activation. Cine display of subtraction images (activated minus baseline) directly demonstrates activity-induced changes in brain MR signal observed at a temporal resolution of seconds. During 8-Hz patterned-flash photic stimulation, a significant increase in signal intensity (paired t test; P less than 0.001) of 1.8% +/- 0.8% (GE) and 1.8% +/- 0.9% (IR) was observed in the primary visual cortex (V1) of seven normal volunteers. The mean rise-time constant of the signal change was 4.4 +/- 2.2 s for the GE images and 8.9 +/- 2.8 s for the IR images. The stimulation frequency dependence of visual activation agrees with previous positron emission tomography observations, with the largest MR signal response occurring at 8 Hz. Similar signal changes were observed within the human primary motor cortex (M1) during a hand squeezing task and in animal models of increased blood flow by hypercapnia. By using intrinsic blood-tissue contrast, functional MRI opens a spatial-temporal window onto individual brain physiology.

Compositional changes in vertebral bone marrow during treatment for acute leukemia: assessment with quantitative chemical shift imaging.

A modified Dixon chemical shift imaging technique was used to quantify longitudinal changes in bone marrow that occur during induction chemotherapy in patients with acute leukemia. Results were correlated with those of bone marrow biopsy. Forty-seven quantitative images were obtained with a 0.6-T whole body imager in a total of 11 patients over the course of treatment. Quantitative measures of fat fractions and water and fat component T1 and T2 relaxation times were determined, as well as average relaxation times. Imaging results showed sequential increases in fat fractions among responding patients (n = 9), consistent with biopsy-confirmed clinical remission. In the two patients who later relapsed, sharp decreases in fat fractions were noted. In the two patients who failed to regenerate normal marrow, unchanging, low fat fractions were seen. Water component T1 values reflected posttherapeutic changes in the hematopoietic elements. Quantitative chemical shift imaging proved useful in assessing treatment response in acute leukemia during early bone marrow regeneration and, later, in ascertaining remission or relapse.

Quantitative chemical shift imaging of vertebral bone marrow in patients with Gaucher disease.

To evaluate extent of bone marrow involvement and disease severity in Gaucher patients, results of modified Dixon quantitative chemical shift imaging (QCSI) of the lumbar spine were correlated with quantitative analysis of marrow triglycerides and glucocerebrosides and with quantitative determination of splenic volume at magnetic resonance (MR) imaging. High-field-strength MR spectra of surgical marrow specimens were dominated by a single fat and a water peak, validating use of QCSI. QCSI showed average vertebral marrow fat fractions of 10% +/- 8 in Gaucher patients (normal adult averages, 29% +/- 6). Relaxation times for lipid and water approximated normal averages; bulk T1 values were significantly longer, reflecting decreased marrow fat. Glucocerebroside concentrations were higher in Gaucher marrow and inversely correlated with triglyceride concentrations. Extent of marrow infiltration determined by fat fraction measurements correlated with disease severity measured by splenic enlargement. These results show that as Gaucher cells infiltrate bone marrow and displace normal marrow adipocytes, bulk T1 increases due to the higher T1 of water compared with that of fat. QCSI provides a sensitive, noninvasive technique for evaluating bone marrow involvement in Gaucher disease.