PubMed Health⌕ Search

Biomedical subjects

R V Mulkern

Publications and source records attributed to R V Mulkern.

At least 91 records · Page 5Linked to original sources
PubMed1989

Response to and control of destructive energy by magnetic resonance.

Magnetic resonance imaging techniques can be used to control and monitor the deposition of destructive energy. The authors evaluated the feasibility of phosphorus-31 magnetic resonance spectroscopy for the control, monitoring, and prediction of the three-dimensional extent of tissue destruction during interstitial laser surgery. Characteristic metabolic changes were demonstrated within the lesion and in the adjacent normal tissue during the deposition of thermal energy.

Adenosine Triphosphate↗
PubMedDate not supplied

Comparing the FAISE method with conventional dual-echo sequences.

The FAISE (fast-acquisition interleaved spin-echo) technique consists of a hybrid rapid-acquisition relaxation-enhanced (RARE) sequence combined with a specific phase-encode reordering method. Implemented on a 1.5-T unit, this multisection, high-resolution technique permits convenient contrast manipulation similar to that of spin-echo imaging, with selection of a pseudo-echo-time parameter and a TR interval. With a TR of 2 seconds, eight 256 x 256 images are obtained in 34 seconds with either T2 or proton-density weighting. A direct comparison between FAISE and spin echo for obtaining T2-weighted head images in healthy subjects indicates that FAISE and spin-echo images are qualitatively and quantitatively similar. Image artifacts are more pronounced on "proton-density" FAISE images than on the T2-weighted FAISE images. T1 contrast can be obtained with inversion recovery and short TR FAISE images. Preliminary temperature measurements in saline phantoms do not indicate excessive temperature increases with extended FAISE acquisitions. However, extensive studies of radio-frequency power deposition effects should be performed if the FAISE technique is to be fully exploited.

Artifacts↗
PubMedDate not supplied

T2-weighted thin-section imaging with the multislab three-dimensional RARE technique.

A novel three-dimensional (3D) RARE (rapid acquisition with relaxation enhancement) sequence was implemented on a clinical imager. In this technique, multiple slabs are excited in the same way as in the multisection spin-echo sequence, and each slab is further phase encoded into eight sections along the section-slab direction. With a 16-echo RARE sequence, 128 excitations cover the 256 X 256 X 8 3D k space. With a TR of 2,500 msec, 10 slabs can be excited sequentially at each TR, yielding 80 sections in 5 minutes. Slabs were overlapped to give contiguous sections after discarding of the aliased sections at slab edges. This relatively fast sequence makes contiguous thin-section T2-weighted imaging possible, an impractical achievement with the much longer spin-echo method. Compared with 3D Fourier transform gradient-echo imaging, the sensitivity of 3D RARE sequences to magnetic susceptibility is reduced. The clinical potential of T2-weighted 3D imaging is illustrated with high-resolution brain, spine, and temporomandibular joint images.

Brain↗
PubMedDate not supplied

Fast spin-echo studies of contrast and small-lesion definition in a liver-metastasis phantom.

A liver-metastasis model was used to study the ability of fast spin-echo (FSE) imaging to show small lesions (1 pixel in diameter) relative to conventional spin-echo imaging. FSE images of the liver-metastasis phantom were acquired with various phase-encode reordering schemes to manipulate T2 contrast. The imaging time for multisection acquisitions was 27 seconds for FSE imaging and 6 minutes 48 seconds for conventional spin-echo imaging. Computer simulations were performed to determine how the point spread function varies with the different phase-encoding orders in FSE imaging. Contrast-to-noise ratios and signal profiles of the lesions were measured as a function of the effective TE and lesion size. Experimental results and theoretical simulations showed that T2-weighted FSE imaging provides high contrast and good edge definition even for small lesions. The results indicate that FSE imaging may become a powerful method for the early detection of liver metastases.

Computer Simulation↗
PubMedDate not supplied

Rapid fat/water assessment in knee bone marrow with inner-volume RARE spectroscopic imaging.

An inner-volume modification of a spectroscopic imaging technique based on RARE (rapid acquisition with relaxation enhancement) sequences is shown to provide, in less than 1 minute, spectra suitable for fat/water peak-area integration from selected imaging columns containing more than 1,000 5 x 5 x 1-mm voxels. A limitation is the unavoidable T2 weighting of the spectral peaks, which influences estimations of true fat/water composition from the spectra. Studies obtained in the knees of healthy volunteers demonstrate the ability of the technique to enable characterization of the two major proton peaks in bone marrow. Data collected at two or more effective TEs are suitable for extrapolation of fat/water estimates to zero TE.

Adult↗
PubMedDate not supplied

Tissue temperature monitoring for thermal interventional therapy: comparison of T1-weighted MR sequences.

For thermal interventional therapy, near real-time monitoring of temperature changes in the treated area is desirable. In this study, various fast T1-weighted magnetic resonance (MR) imaging protocols were compared to determine the sensitivity and resolution of signal intensity for temperatures within the range of 36 degrees C-66 degrees C in gel phantoms and in vitro porcine liver specimens. The results showed that a T1-weighted fast spin-echo sequence with a TR of 100 msec had better temperature sensitivity and resolution than other sequences with comparable temporal resolutions. The longer imaging times required for fast spin-echo sequences with a TR of 300 msec did not improve temperature sensitivity. The methods introduced to evaluate temperature sensitivity and resolution should prove useful in selecting appropriate MR protocols for monitoring thermal treatment modalities such as interstitial laser therapy, focused ultrasound therapy, or radio-frequency heating.

Animals↗
PubMedDate not supplied

Bone marrow characterization in the lumbar spine with inner volume spectroscopic CPMG imaging studies.

The authors describe new magnetic resonance (MR) spectroscopic and postprocessing methods for characterizing major proton peaks and their spectral T2 values in many small voxels throughout extensive regions of bone marrow within the adult lumbar spine. The techniques are based on spectroscopic interrogation of 128 voxels along columns oriented through the spine at eight TE values. Mean fat content measurements, based on quantification of the proton peaks of water and saturated fat (-CH2-)n, corrected for T2 decay, ranged from 40% to 60%. The mean T2 value of the lipid peak, 113 msec +/- 21, was significantly longer (P < .001) than that of water (71 msec +/- 14). The techniques combine features of MR spectroscopy and imaging most suited for spatially efficient coverage of bone marrow at spectral resolutions sufficient for intra-voxel fat or water content measurements. The methods introduced provide a practical, quantitative means for characterizing vertebral marrow in diseases affecting marrow cellularity.

Adult↗
PubMedDate not supplied

Silicone-fat differentiation in the breast: exploiting the bright-fat phenomenon in fast spin-echo MR imaging.

Selective suppression of silicone or fat with chemical shift-selective (CHESS) pulses is difficult because of the small chemical shift difference between the primary lipid signal and the primary silicone signal at 1.5 T. Differentiation of these chemically distinct species is, however, an important clinical task in assessing implant rupture and silicone migration in breast tissue. A method uniquely suited for silicone-fat differentiation with fast spin-echo (FSE) sequences is reported. It is based on the dependence of fat signal on echo spacing in FSE imaging and results show that it may provide a clinically robust method for silicone-fat differentiation in magnetic resonance imaging of the breast.

Adipose Tissue↗
PubMedDate not supplied

Line scan imaging of brain metabolites with CPMG sequences at 1.5 tesla.

A line scan Carr-Purcell-Meiboom-Gill (CPMG) spectroscopic imaging sequence has been implemented on a standard 1.5 T clinical scanner to map metabolite signals at multiple echo times from voxels along selected tissue columns through the brain. The CPMG multiecho spectroscopic image data sets are used to estimate brain metabolite T2 decay parameters in a group of healthy volunteers and in one tumor patient. Inherent trade-offs between T2 decay, spectral resolution, and echo spacing prove to be important limiting factors. In particular, separate quantitation of choline and creatine resonances at 1.5 T was not achieved in the present implementation. However, the ability to collect data sets suitable for T2 decay analyses of combined choline and creatine resonances and N-acetyl aspartate resonances in under 10 minutes may prove of clinical utility in the study of brain pathology.

Adolescent↗
PubMedDate not supplied

MR imaging of the female pelvic floor in the supine and upright positions.

The goal of this study was to determine whether a .5-T open configuration magnet system could be used to evaluate the female pelvic floor support structures and their functional changes in the upright and supine positions. We evaluated five normal volunteers with full bladders in the supine and sitting positions. Multiple measurements were obtained, including distance between symphysis and urethra, bladder neck to fixed pubococcygeal line, and posterior urethrovesical angle. The pelvic floor was evaluated for integrity of the urethra, vagina, and supporting ligaments. High quality, interpretable images were obtained for all five patients in both positions. Most of the pelvic floor structures were stable, with the exception of the posterior urethrovesical angle, which increased in the sitting position. We conclude that the vertically open configuration magnet system shows promise for evaluation of the female pelvic floor, including urinary stress incontinence and prolapse.

Adult↗
PubMedDate not supplied

Multi-echo 31P spectroscopic imaging of ATP: a scan time reduction strategy.

Spectroscopic imaging of 31P metabolites and adenosine triphosphate (ATP) in particular with multiple spin echoes may prove useful for reducing data acquisition times. The usual T2 decay processes that degrade multi-echo spectroscopic imaging methods, however, are further compounded by J-coupling modulations in the case of ATP. We determine how these modulations affect multi-echo spectroscopic imaging k-space data and produce systematic spatial misregistrations of the ATP resonances. The specific J-coupling modulations of ATP are determined to identify echo-spacing effects in multi-echo spectroscopic imaging of ATP and to determine appropriate post-processing correction schemes to address the spatial misregistration problem. An in vivo demonstration of the technique that offers a threefold reduction in scan time compared to conventional SI methods is provided and compared with the conventional SI approach.

Adenosine Triphosphate↗
PubMedDate not supplied

Database prescan: a time-efficient alternative to brain MRI autoprescan.

The purpose of this study was to determine the feasibility of database prescan as an alternative to conventional autoprescan in pediatric brain MRI. Autoprescan parameters [receiver levels and transmit gain (TG)] were analyzed prospectively in 236 pediatric brain MRI studies. Paired t test and linear regression analysis were performed to determine predictability of autoprescan parameters by database-generated parameters. Signal-to-noise ratio, image quality, and potential time efficiency of database-generated parameters were assessed. No statistical difference (P = .13) and a high correlation between the TG of the axial fast spin echo (FSE) proton density (PD) and axial FSE T2-weighted sequences (r = .92) was seen. Strong correlations were noted between the TG of the sagittal T1-weighted and the TG of the axial FSE PD (r = .79), axial FSE T2-weighted (r = .81), and contrast-enhanced T1-weighted (r = .78) sequences. The receiver levels did not change significantly between sequences. Quantitative and qualitative analyses revealed no differences in the signal-to-noise ratios of the autoprescan and the database-predicted prescan parameters. Implementation of database prescan could improve time efficiency by 28 to 33%. Autoprescan parameters can be predicted by using database-generated information while preserving the diagnostic image quality of the study. Incorporation of database prescan into commercial MRI systems could improve MRI time efficiency and patient throughput.

Adolescent↗
PubMedDate not supplied

RARE imaging of PCr in human forearm muscles.

Rapid acquisition with relaxation enhancement (RARE) sequences have been used to map the 31P phosphocreatine (PCr) signal in human forearms at 4.7 T. Signal-to-noise levels of approximately 10 were achieved from the major muscle groups in 5.5-minute scan times with a spatial resolution of 4 x 2 x 2 cm3. Exercise caused demonstrable reductions in PCr signal from activated muscles, which correlated with affected muscle groups in T2-weighted proton images. RARE imaging of the PCr signal at 4.7 T is feasible and, with technically achievable improvements in signal-to-noise ratio, should prove useful in studying energy metabolism in muscle and other organs.

Forearm↗
PubMedDate not supplied

MRI of laser-induced interstitial thermal injury in an in vivo animal liver model with histologic correlation.

Laser-induced interstitial thermal therapy (LITT) is a preferred method of minimally invasive therapy. MRI is a noninvasive method by which to monitor the thermal effects of LITT. To properly control such effects, changes in MRI parameters during and after LITT should be correlated with changes in the tissue. T1-weighted fast spin echo (FSE) MRI (1 image/10 seconds) at 1.5 T monitored LITT in vivo in rabbit liver (n = 6) using an interstitial bare delivery fiber (600-microm diameter; 3.0 W; 1,064 nm; 150 seconds). During laser irradiation, MRI signal intensity decreased around the fiber tip; after irradiation, this hypointensity proved reversible and permanent lesions were evident. The lesions had hyperintense margins that were brighter than surrounding normal tissue (P < .001); the tissue in these bright regions was mapped to tissue necrosis characterized by the presence of thermally damaged ghost red blood cells amid generally normal hepatocytes. T1-FSE identified the spatial extent of the LITT lesions.

Animals↗
PubMedDate not supplied

Rapid tip tracking with MRI by a limited projection reconstruction technique.

To rapidly track invasive devices within MRI systems, a novel approach using a limited projection reconstruction technique is presented. Our method exploits the difference between images reconstructed from a limited number of projections and serves to depict the tip of a needle during its advancement. This method was implemented on a standard MRI system with a radial fast-spin-echo sequence and examined in phantom studies. We demonstrated that the proposed method could track the tip every 300 msec and the tip depicted by the present technique was consistently displaced along the needle by a small distance (5 mm).

Feasibility Studies↗
PubMedDate not supplied

Tissue temperature monitoring with multiple gradient-echo imaging sequences.

The inherent sensitivity of multiple gradient-echo sequences to the chemical shift is exploited to rapidly map muscle water frequency shifts caused by ultrasonic heating. The use of multiple echoes is shown to offer several advantages over single gradient-echo approaches previously proposed for temperature measurement. An increase in the effective bandwidth significantly reduces aliasing problems observed with single gradient-echo methods in high temperature applications. Of greater significance is the improved immunity to intrascan motion found for multi-echo versus single echo gradient methods, making the former more attractive for clinical applications. Finally, a sensitivity to the presence of multiple spectral components unavailable with single gradient-echo methods is obtained.

Animals↗