Imaging near metal with a MAVRIC-SEMAC hybrid.

The recently developed multi-acquisition with variable resonance image combination (MAVRIC) and slice-encoding metal artifact correction (SEMAC) techniques can significantly reduce image artifacts commonly encountered near embedded metal hardware. These artifact reductions are enabled by applying alternative spectral and spatial-encoding schemes to conventional spin-echo imaging techniques. Here, the MAVRIC and SEMAC concepts are connected and discussed. The development of a hybrid technique that utilizes strengths of both methods is then introduced. The presented technique is shown capable of producing minimal artifact, high-resolution images near total joint replacements in a clinical setting.

Cardiac volumetry. Comparison of echoplanar and conventional cine-magnetic resonance data-acquisition strategies.

RATIONALE AND OBJECTIVES: Ejection fraction (EF) measurements obtained using conventional cine-magnetic resonance imaging (MRI) are accurate but time-consuming. With echoplanar imaging (EPI), these data can be acquired much faster. In this study, EF and cardiac output (CO) measurements based on EPI data are compared with those measurements based on cine-MRI images. METHODS: Twelve subjects were examined on a 1.5-T imager equipped with a special EPI gradient system. The entire heart was imaged with contiguous axial 10-mm sections using cine-MRI and EPI techniques. With cine-MRI, 20 frames were acquired over 256 cardiac cycles; with EPI, 24 frames were obtained over four RR intervals using an electrocardiogram-triggered four-shot acquisition strategy. Ejection fraction and CO were calculated based on the summation of the individual end-systolic and end-diastolic volumes. Ejection fraction and CO measurements based on the two different data sets were compared. RESULTS: Multishot EPI was 50 times faster than cine-MRI. The short acquisition time permitted breath-hold imaging. The high temporal (16 to 24 frames/RR interval) and spatial resolution (1.56 x 1.56 mm in plane) of the multishot EPI images enabled delineation of the ventricular lumen at end-systole and end-diastole in a fashion similar to cine-MRI. Echoplanar imaging EF and CO measurements correlated well with cine-MRI EF measurements, with correlation coefficients of 0.96 and 0.94, respectively. The 95% confidence interval of the EF measurement differences between the two techniques was narrow, ranging from -5.2 to 5.7 EF percentage points. CONCLUSIONS: Accurate volumetric EF and CO measurements are possible based on ultrafast multishot EPI data sets as part of an integrated MRI-based cardiac evaluation.

Magnetic resonance imaging of benign spinal lesions simulating metastasis: role of diffusion-weighted imaging.

In an attempt to avoid unnecessary therapy, this article demonstrates benign vertebral body lesions that mimic metastatic disease in cancer patients with back pain. The magnetic resonance imaging features that aid in differential diagnosis are demonstrated. In addition, the value of diffusion-weighted spinal imaging to further aid in distinguishing benign from malignant disease is described.

Acquisition and evaluation of tagged magnetic resonance images of the human left ventricle.

Magnetic resonance myocardial tagging was used to noninvasively analyze the complicated contraction pattern of the human cardiac left ventricle. The tagging and imaging sequence was optimized to obtain three to four double-angulated short-axis views during systole. The image contrast between labeled and unlabeled tissue was sufficient to apply a semiautomatic image evaluation procedure. In accordance with the invasively achieved findings of other groups, the measurements indicate a wringing motion of the left ventricle, with a clockwise twist at the heartbase and a contrary rotation at the apical level.

Non invasive measurement of myocardial motion using magnetic resonance tagging.

Magnetic resonance imaging has become a key modality within the modern radiology department. In addition to the conventional slice images, three dimensional angiographic images, and quantitative flow measurements can be obtained. More recently methods have been developed which enable one to visualize the motion of objects through the application of magnetic resonance markers or tags. Generally the myocardium appears homogeneous with respect to magnetic resonance imaging. Thus at best the displacement of the myocardial surfaces can be observed, but rotational and shear motions are not discernable. With magnetic resonance tagging this all changing. Tagging involves modifying the ability of the tissue to produce a nuclear magnetic resonance signal, in a spatially dependent manner. Typically the pattern is a grid of reduced signal intensity. The tagging grid is applied prior to the heart contraction. Then images are made during systole and diastole. The rotation and shear of the myocardium can be inferred from the distortions of the tagging grid. Here our work in this field is reviewed.

Ultrafast interleaved gradient-echo-planar imaging on a standard scanner.

Ultra-fast imaging traditionally implies either echo-planar imaging on specially developed gradient systems, or very short repetition time gradient-echo imaging on standard magnetic resonance imaging scanners. An alternative strategy for very fast imaging with conventional whole-body scanners is discussed here. The technique is a hybrid, whereby the advantages of gradient-echo imaging and echo-planar imaging are combined. It is here denoted interleaved gradient-echo-planar imaging. It is not a single specific measurement sequence, but rather a continuum of sequences whereby multiple excitations with multiple gradient-echos are employed. The power of this fast imaging approach is that one has much more flexibility toward the optimization of the measurement sequence with respect to imaging time, T2 relaxation, gradient power, resolution, image distortion, and signal-to-noise ratio. In vivo human heart images, acquired in 110 ms, and with a resolution of 2.5 mm, have been obtained with a standard whole-body scanner.

Interleaved echo planar phase contrast angiography.

Technical aspects of a new rapid 2D multiplanar phase contrast angiographic sequence are presented. The sequence uses multiple excitation echo planar imaging concepts. The advantage of using multiple excitations is that the resolution can be increased over that obtainable with single shot echo planar imaging. By combining phase contrast measurements with echo planar imaging, the measurement time is considerably reduced compared with conventional phase contrast magnetic resonance angiography. The present implementation still requires some improvements before being suitable for clinical applications. Future embodiments could, however, permit angiograms of vessels subject to respiratory motion.

On self-refocusing pulses for rapid gradient-echo imaging.

Self-refocusing slice selective radio frequency pulses could potentially be used to reduce imaging times in rapid gradient-echo sequences. However, in practice, the improvement is likely to be no greater than about 20%, and at the expense of much increased radio frequency power requirements.

A one-shot lactate-editing sequence for localized whole-body spectroscopy.

A pulse sequence is presented for performing localized lactate-edited proton spectroscopy in the whole-body environment. The sequence is optimized for relatively low field strength (1.5-T) clinical measurements. Characteristics of this method of lactate editing, which makes it more suitable for clinical applications than previous methods, include the following: The "one shot" nature of the method makes it relatively insensitive to patient motion; water suppression is high as all 90 degrees pulses are binomial pulses; and very narrow band rf pulses, such as used in high-field lactate-editing sequences, but which are necessarily very long at low fields, are not required. Further, this lactate editing scheme can be very easily combined with localized spectroscopic measurements.

Localized double-quantum filter and correlation spectroscopy experiments.

Problems with in vivo proton spectroscopy include strong water and lipid signals, and the very complicated structures of the spectra. Multiple-quantum experiments can be designed to overcome these difficulties. Using a volume selective refocusing technique, one can demonstrate how multiple-quantum experiments can be performed in a spatially localized manner. Rather than trying to integrate a localization procedure into a particular spectroscopy experiment itself, the approach here is to leave the basic experiment intact and to add a volume selective "block" to the end of the sequence. This "block" must, of course, preserve the integrity of the particular experiment. The volume selective refocusing method, presented here, does just this for a wide variety of proton NMR experiments. The advantage of this approach is that the analysis of the particular experiment is not changed. This is very important considering the complexity involved. Using this procedure, two very elementary multiple-quantum experiments have been performed on a 1.5-T whole-body scanner. These experiments are a localized double-quantum filter experiment (for water suppression) and a localized two-dimensional correlation spectroscopy experiment.

Spectral baseline correction using CLEAN.

Baseline distortion in NMR spectroscopy, caused by the "dead time" between signal excitation and detection, makes quantitative interpretation difficult and is aesthetically displeasing. Here the use of the CLEAN algorithm for deconvolving the effect of signal dead time to produce a distortionless baseline is discussed. Unlike other nonlinear spectral estimation techniques, CLEAN is easy to program, easy to use, quite robust, and fast.

Improved myocardial tagging contrast.

Myocardial tagging is a new noninvasive MRI method that allows the study of myocardial motion with high accuracy. However, with conventional tagging techniques tagging contrast is impaired at later heart phases due to longitudinal relaxation. An improved method, called Complementary SPAtial Modulation of Magnetization (CSPAMM), which separates the component of the magnetization with the tagging information from the relaxed component by subtraction of two measurements with first a positive and then a negative tagging grid, is presented. This technique improves the grid contrast and greatly facilitates the automatic evaluation of the myocardial motion. Thus the motion assessment of the entire heart cycle becomes possible. The improvements are documented by numerical simulations and by experiments on phantoms and on human volunteers.

Interleaved echo planar flow quantitation.

A method is presented for rapid blood flow quantitation. It combines multiple shot echo planar imaging principles with phase contrast velocity encoding gradients. Preliminary validation experiments performed on a flow phantom indicate a good correlation with actual flow values. Measurements of pulsatile flow in the ascending and descending aorta, acquired in just 4 s, with a temporal resolution of 50 ms, are demonstrated.

Vascular interventions guided by ultrafast MR imaging: evaluation of different materials.

The ability of MRI to acquire not only anatomical but also functional information makes MRI guided vascular interventions an interesting goal. Recent developments in ultrafast MR imaging sequences such as fast gradient echo or echo planar (EPI) mean that not only real time MRI but also MRI guided vascular interventions are real possibilities for the not too distant future. However, currently available guide wires and catheters are potentially unusable in MRI because they are either ferromagnetic or MRI invisible. In order to find different materials suitable for real time MRI, various devices were examined with fast gradient echo and interleaved EPI pulse sequences. The measurements were performed using a continuously running, pseudo real time MRI system to investigate the dynamic imaging behavior under guide wire insertion. Suggestions are made as how to construct guide wires and catheters, which can be visualized with ultrafast imaging sequences, while not causing prohibitive artifacts or image distortions.

Ultra-high-speed MR imaging.

Conventional magnetic resonance imaging (MRI) has been shown to provide excellent morphological images of the body organs, particularly structures undergoing little physiologic motion. Nevertheless, the clinical usefulness of MRI has been hampered by long acquisition times, high cost of scanning because of limited patient throughput, and image artifacts due to patient motion. With recent technical developments, several ultrafast scanning techniques capable of acquiring images in a breath-hold now find their introduction into clinical use. The system improvements are potentially useful for a vast range of applications hitherto not accessible to MR imaging. Among these are functional brain imaging, realtime imaging of cardiac motion and perfusion, fast abdominal imaging, improved MR angiography, and potentially real-time monitoring of interventional procedures. Whereas some ultrafast techniques can be performed on conventional scanners, echo-planar imaging, the fastest currently available data acquisition strategy, requires specially designed hardware. This article provides an overview of the technical advances in ultrafast MRI and discusses potential applications and the possible future impact on body scanning.

Optimization of contrast timing for breath-hold three-dimensional MR angiography.

The purpose of this study was to determine the influence of various factors (age, weight, breathing, saline flush) on the contrast kinetics of a test bolus injection for the purpose of calculating the scan delay for optimized contrast-enhanced three-dimensional MR angiography. Initially, the test bolus administration was optimized by evaluating the influence of breathing (breathing versus breath-hold) and the administration of a saline flush after the contrast injection (no flush versus flush) on the kinetics of a 4-ml Gd-DTPA test bolus injection in three healthy volunteers. Subsequently, in 33 patients referred for three-dimensional MR angiography of the renal arteries, test bolus kinetics were correlated to age, weight, and heart rate. In addition, the image quality of the three-dimensional MR angiograms was assessed on a four-point scale with regard to vessel visibility. The administration of a saline flush after the contrast injection significantly shortened the first appearance time (14 versus 16 seconds, P < .05), as well as the time to maximal signal intensity (SI) (6 versus 10 seconds, P < .05) and increased both maximum (67 versus 151 seconds, P < .05) and the SI slope (6.4 versus 20.5 seconds, P < .05). Breath-holding was shown to have no significant affect on the test bolus kinetics. No correlation was found between physiologic parameters and test bolus kinetics in the patient group. Image quality was graded as sufficient for diagnostic purposes in 32 of 33 patients. The contrast travel time from injection site to the vascular system under consideration cannot be predicted based on physiologic parameters. This time interval can be reliably and accurately determined by a test bolus injection of a small volume of contrast agent followed by a saline flush during normal breathing.

Menstrual cycle variation of apparent diffusion coefficients measured in the normal breast using MRI.

Recent investigations have shown that tumors may be distinguished from benign lesions in the breast based on differences in apparent diffusion coefficient (ADC) values. The goal of this study was to assess the magnitude of normal variations in the measured ADC of breast parenchyma during the menstrual cycle. Eight healthy female subjects were scanned once a week for 4 weeks, using a diffusion-weighted single-shot fast spin-echo (DW-SSFSE) sequence. The ADC of breast fibroglandular tissue was calculated for each woman at each time point. Results showed a trend of decreased ADC during the second week of the cycle, and increased ADC during the final week. However, no significant influence of menstrual cycle on breast ADC values was identified. The results of this study show that the normal fluctuation of breast ADC is relatively small, and the coefficient of variation was determined to be 5.5% for our group of volunteers during a menstrual cycle. Nonetheless, breast diffusion measurements for tumor differentiation and evaluation of treatment response should be interpreted with consideration of normal variability.

Normal myocardial perfusion assessed with multishot echo-planar imaging.

A new magnetic resonance imaging strategy is presented for accessing myocardial perfusion. Most previous work has relied on using T1-weighted fast gradient-echo imaging to monitor dynamically the signal changes during the passage of a contrast media bolus. However, the gradient-echo approach is limited by an inability to image the entire heart with adequate temporal resolution. This paper focuses on a electrocardiogram-gated multishot echo-planar imaging sequence, using the simple strategy of using the intrinsic T1 weighting produced by a repetition time equal to the heart period. To quantitate the sequence's performance with respect to normal myocardial perfusion, seven volunteers were imaged, each with three different doses of the contrast medium gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA). The first-pass dynamics of the contrast were quantified in 13 regions per heart for each examination. In all volunteers, the complete heart could be covered, with five to seven slices, every two heartbeats. Enhancement was homogeneous throughout the left ventricular myocardium, with an enhancement of approximately 50% for the optimum contrast dose of 0.05 mmol/kg Gd-DTPA.

Technical note--approach to myocardial perfusion with echo planar imaging.

PURPOSE: To evaluate the feasibility of MRI-based myocardial first-pass contrast perfusion imaging with a multi-shot echo planar imaging (EPI) technique. SUBJECTS AND METHODS: A non-sequential (ECG-triggered) gradient echo two-shot EPI acquisition strategy capable of covering the entire heart in contiguous 10-mm sections every two cardiac cycles with an in-plane resolution of 1.56 X 1.56 mm was implemented on a 1.5-T Signa Advantage Scanner equipped with prototype hardware for non-resonant EPI in the transverse plane. The heart of a single volunteer was studied prior to and following the intravenous bolus application of a paramagnetic contrast agent (Gd-DOTA, 0.2 mmol/kg). RESULTS: Twelve contiguous transaxial 10-mm EPI images were obtained every two RR intervals for a total of 40 s. The myocardial contrast perfusion study was technically adequate. Contrast caused a signal loss of 87% in the right and 67% in the left ventricle and 59% in the myocardium. CONCLUSION: First-pass myocardial perfusion imaging with a gradient echo, two-shot echo planar imaging strategy is feasible.