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J R Brookeman

Publications and source records attributed to J R Brookeman.

At least 19 recordsLinked to original sources
PubMed1992

The design of pulse sequences employing spatial presaturation for the suppression of flow artifacts.

The use of spatial presaturation to suppress the signal, and therefore also the artifacts, from flowing blood has become an important tool in the arsenal of techniques to suppress pulsatile flow artifacts in magnetic resonance images. However, a detailed theoretical analysis of the behavior of these flow artifact suppression pulses and of the important aspects of implementing suppression pulses in combination with particular imaging sequences has yet to be presented. In this paper we present a general theoretical framework to describe the flow artifact suppression technique. This analysis addresses the following four major issues: (1) the spin washout characteristics of the imaging sequence, (2) the interference between the flow signal suppression pulses and the imaging sequence, (3) the flow velocity range for a single application of the suppression pulse, and (4) the total flow velocity range for a suppression pulse repeated with a constant time interval between applications of the pulse. The predictions of our theoretical model are confirmed by experimental measurements made with stationary and flow phantoms. The results of this investigation provide guidelines for the design of flow artifact suppression pulse sequences and, in addition, should aid in the future development and refinement of the spatial presaturation technique as applied to flow signal suppression.

Artifacts↗
PubMed1992

Shaping the signal response during the approach to steady state in three-dimensional magnetization-prepared rapid gradient-echo imaging using variable flip angles.

A theoretical algorithm for shaping the signal response during the approach to steady state in three-dimensional magnetization-prepared rapid gradient-echo (3D MP-RAGE) pulse sequences has been developed and implemented. This algorithm derives the flip angle series required to produce specifically chosen time evolutions of the signal intensities during the data acquisition segment of 3D MP-RAGE sequences. Theoretical predictions for the cases of unshaped, uniform, and mono-exponential decay signal responses were quantitatively validated with a doped-water phantom on a 1.5-T whole-body imager and in all cases there was excellent agreement between the theoretical and experimental values. The effects of RF inhomogeneities and eddy currents on the signal response shaping were also investigated. To demonstrate the potential utility of the technique, the signal response shaping algorithm was applied to a T1-weighted 3D MP-RAGE sequence to derive the acquisition flip angle series which theoretically yields the maximum white matter/gray matter signal difference (WGSD) consistent with the chosen response shape. Images obtained from a healthy volunteer using this variable flip angle sequence were compared with 3D RF-spoiled steady-state gradient-echo images obtained in the same total imaging time. The 3D MP-RAGE images demonstrated a 41% increase in the WGSD-to-noise ratio. These initial very promising results indicate that with further refinement to eliminate some intensity artifacts, the variable flip angle 3D MP-RAGE technique may, with respect to certain image properties, provide considerable improvements over currently available 3D gradient-echo imaging techniques.

Algorithms↗
PubMed1991

Magnetization prepared rapid gradient-echo (MP-RAGE) MR imaging of the liver: comparison with spin-echo imaging.

We have implemented an MR technique that employs a rapid gradient echo sequence, preceded by magnetization preparation pulses to provide T1- and T2-weighted tissue contrast. With this technique, which can be identified as a member of a new family of pulse sequences, generically named Magnetization Prepared RApid Gradient Echo (MP-RAGE), very short repetition times are used, allowing acquisition times of less than one second and images virtually free of motion-induced artifacts during quiet respiration. Fifteen patients with known liver lesions (metastases, hemangiomas, and cysts) were examined using T1- and T2-weighted 2-dimensional MP-RAGE sequences, and the images were compared with conventional T1- and multi-echo T2-weighted spin-echo (SE) sequences. Signal difference-to-noise ratios (SD/Ns) of the lesions were calculated for all pulse sequences using corresponding axial images and were normalized for voxel volume. The mean normalized SD/Ns of the MP-RAGE sequences were generally comparable to those for the SE sequences. In addition, there were no noticeable respiratory artifacts on the MP-RAGE images whereas these were clearly present on the T2-weighted SE images and to a lesser degree on the T1-weighted SE images. It is concluded that the MP-RAGE technique could become an important method for evaluating the liver for focal disease.

Cysts↗
PubMed1990

Three-dimensional magnetization-prepared rapid gradient-echo imaging (3D MP RAGE).

A new three-dimensional imaging technique which is applicable for 3D MR imaging throughout the body is introduced. In our preliminary investigations we have acquired high-quality 3D image sets of the abdomen showing minimal respiratory artifacts in just over 7 min (voxel size 2.7 X 2.7 X 2.7 mm3), and 3D image sets of the head showing excellent gray/white contrast in less than 6 min (voxel size 1.0 X 2.0 X 1.4 mm3).

Abdomen↗
PubMed1988

High-resolution 1H NMR spectral signature from human atheroma.

Coronary artery disease due to atherosclerosis takes the lives of approximately 550,000 Americans each year--an enormous toll. Put in economic terms, the cost to the United States alone has been estimated to exceed 60 billion dollars annually. We have found that well-resolved proton (1H) NMR spectra can be obtained from human atheroma (fatty plaque), despite its macroscopic solid appearance. The fraction of the total spectral intensity corresponding to the sharp 1H NMR signals is temperature dependent and approaches unity at body temperature (37 degrees C). Studies of the total lipids extracted from atheroma and cholesteryl esters were conducted to identify the chemical and physical origin of the spectral signature. The samples were characterized through assignment of their chemical shifts and by measurement of their T1 and T2 relaxation times as a function of magnetic field strength. The results suggest that the relatively sharp 1H NMR signals from human atheroma (excluding water) are due to a mixture of cholesteryl esters, whose liquid-crystalline to isotropic fluid phase transition is near body temperature. Preliminary applications to NMR imaging of human atheroma are reported, which demonstrate early fatty plaque formation within the wall of the aorta. These findings offer a basis for noninvasive imaging by NMR to monitor early and potentially reversible stages of human atherogenesis.

Aorta↗
PubMed1988

Identification and 3-D quantification of atherosclerosis using magnetic resonance imaging.

Cardiovascular disease due to atherosclerosis is a leading cause of death in the United States as well as other developed countries. This paper describes the development of image processing, pattern recognition, and graphical display techniques to non-invasively quantify the atherosclerotic disease process using magnetic resonance imaging (MRI). We have demonstrated the ability to identify the soft tissue classes of (1) normal, smooth muscle wall, (2) fatty plaque, (3) complex, fibrous plaque, and (4) calcified plaque. The objective of this work has been to combine functional information, such as plaque tissue type, with structural information, represented by 3-D display of vessel structure, into a single composite display. The results of this work provide a "high information content" display which will aid in the diagnosis and analysis of the atherosclerotic disease process, and permit detailed and quantitative studies to assess the effectiveness of therapies (e.g. changes in diet, exercise and drug administration).

Aorta↗
PubMed1988

Liver metastases: optimization of MR imaging pulse sequences at 1.0 T.

Twenty patients with known liver metastases were examined with magnetic resonance imaging; four different pulse sequences were used and six different images were produced to allow comparison of pulse sequence performance at a 1.0-T field strength. Pulse sequence performance was in each case calculated by measuring contrast-to-noise ratios (C/N) comparing normal liver, metastatic tumors to liver, and background noise. All pulse sequences required approximately the same length of time for data acquisition. Short inversion time inversion-recovery (STIR) sequences yielded the greatest signal difference-to-noise ratio in 17 patients. Phase contrast images produced the greatest contrast ratio in three patients. Although short TR, short TE spin-echo sequences provided the best anatomic detail, in no patient did this sequence yield the greatest signal difference-to-noise ratio in the comparison of liver and tumor. Differences in field strength and in equipment software and hardware may account for the discrepancy between our findings and previously published data.

Humans↗
PubMed1987

High-resolution proton NMR spectra of human arterial plaque.

Well-resolved proton (1H) NMR spectra of solid human arterial plaque can be acquired. Studies have been carried out of human fatty plaque obtained postmortem (ex vivo), the total lipids extracted from human atheroma, and a model mixture of cholesteryl esters whose lipid composition resembles that of human atheroma. In each case, well-resolved 1H NMR spectra were obtained at body temperature (37 degrees C), with little or no underlying broad signal. Such sharp 1H NMR spectra are typical of isotropic fluids, whereas solid and liquid-crystalline materials give rise to much broader spectral lines. The results suggest the sharp 1H NMR spectra of human atheromatous lesions at body temperature are due largely to the presence of intracellular and extracellular droplets of cholesteryl esters in the isotropic liquid phase. These findings provide a necessary basis for use of 1H NMR techniques to image quantitatively the lipid constituents of human atheroma in vivo, and to study their chemical and physical properties.

Adult↗
PubMed1987

Tumor detection with phase-contrast imaging: an evaluation of clinical potential.

The conspicuity of a variety of human tumors outside the liver, with a broad spectrum of histologic types, was evaluated using phase-contrast imaging (proton chemical shift imaging, proton spectroscopy) and the results compared with the data obtained using routine T1- and T2-weighted magnetic resonance (MR) imaging sequences. In all cases, tumors were most conspicuous using the phase-contrast technique. In two instances, the tumors could be discerned only by this method, being undetectable by any routine MR pulse sequence. The use of phase-contrast imaging is recommended as an adjunct to standard MR sequences when evaluating for either the presence or extent of a tumor, regardless of its anatomic location or histologic type.

Adipose Tissue↗
PubMedDate not supplied

Evaluation of a simple method for reconstructing asymmetrically sampled echo data.

A simple method for reconstructing echo data with a large degree of sampling asymmetry was investigated. The method is based on the application of an optimized data sampling window before standard Fourier transform magnitude reconstruction. The performance of this "windowed direct" reconstruction was evaluated with theoretical simulations and experimental measurements and compared with that of the half-Fourier method. With the proper selection of data sampling window parameters, highly asymmetric echo data reconstructed with standard Fourier transform magnitude algorithms produced images similar in quality to those reconstructed with the half-Fourier method. Windowed direct reconstruction provides a simple and computationally fast alternative to more sophisticated algorithms and may be particularly applicable for specialized or developmental applications.

Evaluation Studies as Topic↗
PubMedDate not supplied

Rapid three-dimensional T1-weighted MR imaging with the MP-RAGE sequence.

The authors investigated the application of three-dimensional (3D) magnetization-prepared rapid gradient-echo (MP-RAGE) imaging to the acquisition of small (32 x 128 x 256) T1-weighted 3D data sets with imaging times of approximately 1 minute. A theoretical model was used to study the contrast behavior of brain tissue. On the basis of these theoretical results, 3D MP-RAGE sequences were implemented on a 1.5-T whole-body imager. Thirty-two-section 3D data sets demonstrating good signal-to-noise ratios and resolution and strong T1-weighted contrast were obtained in 1 minute. Compared with standard short TR/TE spin-echo sequences with the same imaging times and comparable sequence parameters, the 3D MP-RAGE sequence delivered increases of more than 50% in the white matter/gray matter signal difference-to-noise and white matter signal-to-noise ratios, and provided almost twice as many sections. These sequences may find a clinical role in 3D scout imaging and screening and in patients with claustrophobia or trauma.

Brain↗
PubMedDate not supplied

T2-weighted three-dimensional MP-RAGE MR imaging.

The application of three-dimensional (3D) magnetization-prepared rapid-gradient-echo (MP-RAGE) imaging to the acquisition of T2-weighted 3D data sets has been investigated, with a 90 degrees x-180 degrees y-90 degrees-x pulse set (driven equilibrium) for the T2 contrast preparation. A theoretical model was used to study the contrast behavior of brain tissue. The effects of radio-frequency and static-field inhomogeneities and eddy currents on the T2 contrast preparation and the effects of eddy currents on the gradient-echo acquisition resulted in blurring and intensity banding artifacts. With a multistep gradient preparation, these artifacts could be suppressed. With further development, this technique may yield a clinically practical method for obtaining T2-weighted 3D data sets of relatively large volumes (eg, the whole head) suitable for multiplanar reformatting.

Artifacts↗
PubMedDate not supplied

Selective versus nonselective preparation pulses in two-dimensional MP-RAGE imaging of the liver.

The use of a section-selective preparation pulse in two-dimensional (2D) T1-weighted magnetization-prepared rapid gradient-echo (MP-RAGE) imaging of the liver was investigated. The images were compared with those obtained with a nonselective pulse. The performances of the sequences were evaluated in 11 patients with 12 focal liver lesions, and lesion-liver and lesion-vessel signal difference-to-noise ratios (SD/Ns) were calculated. With the section-selective preparation pulse, small lesions were better differentiated from vessels, and multiple, consecutive images could be obtained at shorter intervals. The mean lesion-liver SD/N was slightly but not significantly greater for images obtained with a selective pulse, while the lesion-vessel SD/N was significantly greater (P less than .01). It is concluded that a section-selective preparation pulse can improve the clinical utility of the 2D MP-RAGE sequence in the evaluation of focal liver disease.

Humans↗
PubMedDate not supplied

Implementation of mixed bandwidth MRI pulse sequences using a single analog lowpass filter.

In many MR imaging situations, such as when imaging certain areas of the brain, substantial increases in the signal-to-noise and contrast-to-noise ratios can be achieved by extending the duration of the data sampling period, or equivalently stated, by reducing the bandwidth of the data acquisition. This technique is particularly applicable for the clinically useful long TR double spin-echo sequence for improving the signal-to-noise and contrast-to-noise ratios for the long TE second echo image. Such a sequence would employ a relatively short data sampling period (wide bandwidth) for the short TE first echo, and a relatively long data sampling period (narrow bandwidth) for the long TE second echo. Implementing such a sequence which uses two data acquisitions of different bandwidths within one sequence repetition can present certain technical problems. In this communication, we describe a method to implement such a mixed bandwidth pulse sequence on a standard commercial whole-body imager without the need for additional software or hardware. This sequence is now in routine clinical use at our institution.

Brain↗