Ultrafast Magnetic Resonance Imaging Research Articles

Transdiaphragmatic leakage of ascites in cirrhotic patients: Evaluation with ultrafast gradient echo MR imaging and intraperitoneal contrast enhancement

The utility of intraperitoneal instillation of Gd-DTPA and ultrafast MR imaging in the diagnosis of transdiaphragmatic leakage of ascites was assessed in two cirrhotic patients with right-sided pleural effusion. Contrast-enhanced serial snapshot-FLASH reliably demonstrated the communication between the peritoneal and the pleural cavities in both patients. In one case, a right-sided diaphragmatic defect could be exactly localized.

Ultrafast imaging of brain tumors

Ultrafast magnetic resonance (MR) imaging allows the acquisition of anatomic images of the brain in a fraction of a second. Recently developed imaging techniques have the potential to increase the specificity of information provided by MR imaging. MR imaging techniques that are based on ultrafast scanning have enabled the collection of functional as well as anatomic information concerning tumor microvasculature. These techniques promise to increase knowledge of the tissue characteristics of human brain tumors, their progress, and their responses to therapy.

Usefulness of ultrafast magnetic resonance imaging in healed myocardial infarction

The value of ultrafast magnetic resonance imaging (MRI) in the assessment of dynamic contrast enhancement and myocardial perfusion abnormalities was evaluated in 20 patients with healed myocardial infarction, who also underwent 2-dimensional echocardiography. At baseline and after bolus injection of the paramagnetic contrast agent gadolinium-diethylenetriaminepentaacetic acid (DTPA) (0.04 mmol/kg body weight), single-level short-axis MRI was performed every third RR interval with an acquisition time of 500 ms. Myocardial signal intensities were measured in transmural myocardial regions of interest. After gadolinium-DTPA injection, infarcted and normal myocardium demonstrated a signal intensity enhancement of 50 and 134%, respectively (p < 0.001). A signal intensity of normal relative to infarcted myocardium increased from 1.25 ± 0.22 (SD) before to 1.91 ± 0.41 after gadolinium-DTPA (p < 0.001). The rate of signal increase in the infarcted and normal myocardium was 5.17 ± 2.22 and 18.99 ± 9.96 s −1 (p < 0.001), respectively. Ultrafast MRI using gadolinium-DTPA bolus administration clearly identifies myocardial perfusion abnormalities in patients with healed myocardial infarction. The infarct site on MRI corresponded with the location of wall motion asynergy determined by echocardiography. It is concluded that gadolinium-DTPA-enhanced ultrafast MRI provides noninvasive assessment of myocardial perfusion in patients with proven coronary artery disease.

Real-time dynamics of an extravascular magnetic resonance contrast medium in acutely infarcted myocardium using inversion recovery and gradient-recalled echo-planar imaging.

The purposes of this study are to evaluate the first-pass profile of gadolinium-BOPTA/Dimeg (Gd-BOPTA/Dimeg) during its transit through hearts subjected to acute myocardial infarction, and to delineate these infarcted regions by the use of ultrafast magnetic resonance imaging (MRI). Regional ischemia was induced in anesthetized rats by occluding the left coronary artery. Imaging parameters for single shot EPI included TE, 10 mseconds; AT, 33 mseconds; and 64 x 64-pixel matrix. Consecutive images were obtained every 1 to 2 seconds over a 30-second period. After approximately two images, Gd-BOPTA/Dimeg was injected intravenously (0.05 and 0.25 mmol/kg). Gd-BOPTA/Dimeg (0.05 mmol/kg), with inversion recovery EPI, produced a substantial increase in signal intensity of right and then left ventricular blood. Normally perfused myocardium also was enhanced, but not the acutely infarcted region. Clear delineation of the infarcted region as negatively enhanced "cold spots" persisted for at least 20 seconds. Gd-BOPTA/Dimeg (0.25 mmol/kg) with standard gradient-recalled EPI produced a different profile of signal intensity changes. Signal intensities of ventricular blood and normal myocardium were greatly reduced, leaving the infarcted zone as a positively enhanced "hot spot." Delineation of the infarcted region persisted for 6 to 8 seconds. The infarcted zone detected with MRI corresponded to that observed at autopsy. Regions of acute myocardial infarction can be detected as negatively enhanced "cold spots" or positively enhanced "hot spots" by studying the first-pass dynamics of Gd-BOPTA/Dimeg through hearts with regional ischemia by use of single shot EPI.

Assessment of intracardiac shunts with gadolinium-enhanced ultrafast MR imaging.

Dye dilutional techniques are widely accepted for the assessment of intracardiac shunts, but current techniques require arterial access or radioisotope injection. Ultrafast (less than 500 msec) magnetic resonance (MR) imaging is ideally suited for the evaluation of an indicator during passage through the heart. Twenty patients were studied, including 13 with shunts. Four-chamber, T1-weighted images were obtained during bolus injection of gadopentetate dimeglumine. A single image was obtained in 420 msec, with repetitive images acquired after each QRS complex. After the contrast material was injected, there was pronounced signal intensity enhancement in the right atrium, followed by the right ventricular cavity, left atrium, left ventricular cavity, and descending aorta. Patients with substantial intracardiac shunts demonstrated early recirculation. First-pass contrast material-enhanced MR imaging is a promising new technique for the rapid assessment of intracardiac shunts. Combined with anatomic and functional MR imaging techniques, it can help provide a comprehensive noninvasive evaluation of suspected intracardiac shunts or provide follow-up in patients with known shunts.

Ultrafast MR imaging of water mobility: animal models of altered cerebral perfusion.

"Single shot" magnetic resonance (MR) diffusion imaging was used to study the details of signal decay curves in experimental perturbations of cerebral perfusion induced by hypercapnia or death. Despite large perfusion increases observed with dynamic susceptibility-contrast MR imaging, no correlation with these changes was seen in either the diffusion coefficient or any other intravoxel incoherent motion (IVIM) model parameters in dog gray matter as arterial carbon dioxide pressure increased. Non-monoexponential signal decay in cat gray matter was seen both before and after death. In addition, cat gray matter demonstrated a steady decrease in the diffusion coefficient after death. These data are strong evidence that the fast component of the non-monoexponential diffusion-related signal decay is not due solely to perfusion. The authors believe that a second compartment of nonexchanging spins, most likely cerebrospinal fluid, accounts for the non-monoexponential decay.

Occlusion and narrowing of the pharyngeal airway in obstructive sleep apnea: evaluation by ultrafast spoiled GRASS MR imaging.

Morphologic abnormalities of the pharyngeal airway are frequently found in patients with obstructive sleep apnea. These structural alterations in the pharyngeal airway can be detected in awake patients by using rapid imaging techniques. Ten patients with clinically proved obstructive sleep apnea had ultrafast spoiled gradient-recalled acquisition in the steady state (GRASS) MR imaging of the pharyngeal airway to determine the presence of occlusions and/or narrowings. Twelve sequential images were obtained at one midsagittal plane and at eight transverse planes through the pharyngeal airway. The scans were obtained at the rate of one image per 1.04 sec while the patient was breathing quietly. Occlusions or narrowings of the pharyngeal airways were detected on MR images in all patients. The site(s) of the occlusions and the site(s) and extent of the narrowings varied. Six patients had occlusions and four had narrowings of one or more sites. This study shows that ultrafast spoiled GRASS MR imaging can be used to evaluate patients with obstructive sleep apnea during tidal breathing and is useful for determining the presence of occlusions and narrowings of the pharyngeal airway.

Dynamic study of the upper airway with ultrafast spoiled GRASS MR imaging.

Dynamic magnetic resonance (MR) imaging of the upper airway was not possible previously because of poor temporal resolution. Recently, a rapid technique has been developed that provides the means to obtain multiple images at different section locations with sufficient image quality and temporal resolution to allow a comprehensive, dynamic study of the upper airway. The authors describe an ultrafast spoiled GRASS (gradient-recalled acquisition in the steady state) pulse sequence for dynamic studies of the upper airway. The authors believe that this procedure has potential for identifying and characterizing upper airway abnormalities such as nonfixed occlusions and/or narrowings that may exist in patients with obstructive sleep apnea.

Undergraduate Medical Ethics Education

<h3>PURPOSE</h3> The purpose of our study was to introduce an ultrafast MR imaging technique of the pharynx as a diagnostic tool for viewing the mechanism of obstruction in patients with obstructive sleep apnea. <h3>METHODS</h3> Six healthy volunteers and 16 patients with obstructive sleep apnea were examined on a 1.5-T whole-body imager using a circular polarized head coil. Ultrafast two-dimensional fast low-angle shot sequences were obtained in midsagittal and axial projections during transnasal shallow respiration at rest, during simulation of snoring, and during performance of the Müller maneuver. All patients underwent physical examination, transnasal fiberoptic endoscopy, and polysomnography. <h3>RESULTS</h3> Five to six images were obtained per second with an in-plane resolution of 2.67 x 1.8 mm and 2.68 x 2.34 mm, allowing visualization of motion of the tongue, soft palate, uvula, and posterior pharyngeal surface. MR findings correlated well with results of clinical examination. The length of obstruction in the oropharynx, which cannot be ascertained by transnasal endoscopy of the pharynx, was clearly visible MR images. Differences between patients with obstructive sleep apnea and healthy subjects in terms of the degree of obstruction in the velopharynx and oropharynx depicted on MR images during the Müller maneuver were highly significant. <h3>CONCLUSION</h3> We believe that ultrafast MR imaging is a reliable noninvasive method for use in the evaluation of obstructive sleep apnea.

Equipment requirments to facilitate contrast‐enhanced MR imaging

During the past decade, MR equipment and imaging techniques have experienced unprecedented development. Significant improvements have been made in image quality, enhancing conventional contrast agent studies. In addition, the development of new applications, such as MR angiography, has expanded the role of existing agents. A major benefit for contrast agent research has been the development of ultrafast MR imaging. This ability to provide information on contrast agent dynamics will aid in the detection and characterization of neoplastic and other disease states. While the gradient-echo techniques may provide adequate temporal resolution for many applications, echo planar imaging will enable the assessment of perfusion for an entire organ, which is critical in the heart or when the location of the pathology is unknown a priori. It is also likely that continued development in MR contrast agents will have a synergistic impact on pulse sequences, e.g., MR diffusion imaging. The growth in functional MR imaging will be based in large measure on the continued interactive development of imaging strategies and magnetopharmaceutials.

Differentiation between hemangiomas and metastases of the liver with ultrafast MR imaging: preliminary results with T2 calculations.

We studied the efficacy of T2 measurements at high field strength in distinguishing between liver hemangiomas and hepatic metastases when an ultrafast (single-excitation) MR imaging technique is used. Fourteen patients with known liver tumors were imaged in a 2.0-T prototype ultrafast MR scanner with a spin-echo (infinite TR and TE of 30-340 msec) pulse sequence. Each image was obtained with a total data acquisition time of 20 msec. T2 calculations for hepatic metastases (n = 6) showed a mean of 79.3 +/- 13.5 msec, whereas hemangiomas (n = 8) showed a T2 of 139.8 +/- 18.8 msec (p less than .0001). T2 values of lesions had a smaller relative standard deviation than previously reported, and the range of T2 values of hemangiomas (119-181 msec) and metastases (68-103 msec) did not overlap. Our preliminary results suggest that T2 calculations with ultrafast MR imaging may be useful for differentiating hemangiomas from metastases. We hypothesize that T2 values obtained from ultrafast MR images are more reliable than those obtained from conventional MR images, primarily because of the elimination of T1 information and effects of motion on image signal intensity.

Hepatic Focal Nodular Hyperplasia

The utility of gadolinium-enhanced ultrafast MR imaging in the diagnosis of focal nodular hyperplasia (FNH) of the liver was assessed in one patient. Contrast-enhanced serial snapshot-FLASH with a frequency of 30 images/min demonstrated the typical hemodynamic appearance of FNH known from CT studies.

Factors in myocardial “Perfusion” imaging with ultrafast MRI and Gd‐DTPA administration

Ultrafast magnetic resonance imaging (MRI) and first pass observation of an interstitial contrast agent are currently being used to study myocardial perfusion. Image intensity, however, is a function of several parameters, including the delivery of the contrast agent to the interstitium (coronary flow rate and diffusion into the interstitium) and the relaxation properties of the tissue (contrast agent concentration, proton exchange rates, and relative intra- and extracellular volume fractions). In this study, image intensity during gadopentetate dimeglumine (Gd-DTPA) administration with T1-weighted ultrafast MR imaging was assessed in an isolated heart preparation. With increasing Gd-DTPA concentration, the steady-state myocardial image intensity increased but the time to reach steady state remained unchanged, resulting in an increased slope of image intensity change. A range of physiologic perfusion pressures (and resulting coronary flow rates) had insignificant effects on kinetics of Gd-DTPA wash-in or steady-state image intensity, suggesting that diffusion of Gd-DTPA into the interstitium is the rate limiting step in image intensity change with this preparation. Following global ischemia and reperfusion, transmural differences in the slope of image intensity change were apparent. However, the altered steady-state image intensity (due to postischemic edema) makes interpretation of this finding difficult. The studies described here demonstrate that although Gd-DTPA administration combined with ultrafast imaging may be a sensitive indicator of perfusion abnormalities, factors other than perfusion will affect image intensity. Extensive studies will be required before image intensity with this protocol is fully understood.

Patellofemoral joint: evaluation during active flexion with ultrafast spoiled GRASS MR imaging.

An ultrafast spoiled gradient-recalled acquisition in the steady state pulse sequence was developed that permits multiple images to be obtained at a temporal resolution suitable for examining the patellofemoral joint during active flexion. This pulse sequence was used to perform kinematic magnetic resonance imaging of patellar alignment and tracking in five healthy subjects and seven patients with a provisional clinical diagnosis of abnormal patellofemoral joints.

Fluoroscopic MR imaging at 0.064 Tesla

The authors developed a system for ultrafast magnetic resonance imaging (MRI) protocols at low field. The system design permits the acquisition of the raw data in the background while the reconstruction and display steps repeat as fast as they can in the foreground. The performance speeds depends partly on the desired use. By collecting raw data at a rate of 20 ms per echo with an echo delay of 9 ms, a complete data cycle for a 128x64 image takes 1.28 s. However, once half of that data is incorporated into the reconstruction, the image appears complete. Using this set of parameters the authors were able to get the rate of the recon/display loop to paint about two times per completed raw data cycle, showing an entirely new image at least once per second with an apparent frame rate of two per second. Interleaving of two or three orthogonal scans reduces the speed of update but provides better information. The authors discuss the system design for rapid scan/recon/display and demonstrate the image quality available at low field strength with scan times below one second.

Study of internal structure of the human fetus in utero by echo-planar magnetic resonance imaging

The ultrafast echo-planar magnetic resonance imaging technology, developed and built in Nottingham, has been used to produce the first snapshot images of the human fetus in utero. The imager, operating at a proton resonance frequency of 22 MHz, produces transaxial views in 64 or 128 milliseconds. These images comprise either 64 × 128 or 128 × 128 pixels with an in-plane resolution of 3 × 3 mm2. The slice thickness is 10 mm. Fetal scans of up to 32 contiguous slices are produced in a few minutes. These have been used to study the internal structure of the uterus and the fetus in a range of cases with gestations ranging from 26 weeks to term. Echo-planar imaging seems particularly suitable as an imaging modality since its high speed obviates image blurring arising from fetal motion.

First-pass cardiac perfusion: evaluation with ultrafast MR imaging.

The authors studied cardiac perfusion by administering gadolinium diethylenetriaminepentaacetic acid (DTPA) in conjunction with an ultrafast imaging technique that produces strongly T1-weighted images. The method consisted of a 180 degrees inversion pulse, followed by a gradient-echo acquisition with a very short repetition time (less than 4 msec). Each image was acquired throughout a small fraction of the cardiac cycle. The method was applied in an isolated perfused rat heart model (acquisition time = 116 msec) and in human subjects without known cardiac disease (acquisition time = 125 msec). Fast, high-resolution images (128 X 128 matrix) were created by combining sequentially acquired small matrixes. After bolus administration of Gd-DTPA in the perfused rat heart model, contrast was pronounced between the nonperfused myocardium and perfused normal myocardium. First-pass wash-in and washout phases of the contrast material were observed in the perfused rat heart model and in human subjects. Results demonstrated the clinical feasibility of first-pass perfusion studies of the heart. The studies can be performed on a conventional whole-body imaging system with standard hardware.

Forty-millisecond MR imaging of the abdomen at 2.0 T.

The ability of an ultrafast magnetic resonance (MR) imaging technique to provide abdominal MR images free of motion artifacts was studied. Individual T2-weighted transverse MR images were acquired in as little as 40 msec on a whole-body system operating at 2.0 T. Clinical evaluation was undertaken with fat-suppressed images in which only protons of water molecules contributed to image signal intensity. The ultrafast MR images were compared with conventional MR images obtained at 0.6 T. In 22 patients and two healthy volunteers, ultrafast MR images were of diagnostic quality and free of motion artifacts. Images obtained at an echo time (TE) of 30 msec (imaging time, 40 msec) had liver signal-to-noise ratios of 56.3 +/- 22.6 (n = 19). Because of a smaller data matrix, ultrafast MR images had soft-tissue interfaces that were less sharp than those of the highest-quality conventional MR images in which no motion artifacts were present. However, ultrafast MR images demonstrated high T2-dependent soft-tissue contrast, and pathologic and normal anatomies were readily detected with both imaging techniques. This ultrafast imaging technique has significant promise in whole-body MR imaging, in which motion artifacts often degrade image quality.

On the theory of the indicator-dilution method for measurement of blood flow and volume.

On the theory of the indicator-dilution method for measurement of blood flow and volume.