Line scan diffusion imaging

A novel line scan diffusion imaging sequence (LSDI) is introduced. LSDI is inherently insensitive to motion artifacts and high quality diffusion maps of the brain can be obtained rapidly without the use of head restraints or cardiac gating. Results from a stroke study and abdominal diffusion images are presented. The results indicate that it is feasible to use the LSDI technique for clinical evaluation of acute ischemic stroke. In contrast to echo-planar diffusion imaging, LSDI does not require modified gradient hardware and can be implemented on conventional scanners. Thus, LSDI should dramatically increase the general availability of robust clinical diffusion imaging.

Determinants of tissue delivery for 129Xe magnetic resonance in humans

Magnetic resonance imaging using laser-polarized 129Xe is a new technique first demonstrated by Albert et. al. (Nature 370, 1994) who obtained a 129Xe image of an excised mouse lung. This paper describes the factors influencing the accumulation of inhaled, polarized 129Xe in human tissue. The resulting model predicts the 129Xe magnetization in different tissues as a function of the time from the start of inhalation, the tissue perfusion rate and partition coefficient for xenon, and the relevant T1 decay times. The relaxation times of 129Xe in biological tissues are not yet known precisely. Substitution of estimated values for these parameters results in an expected signal-to-noise ratio (SNR) from polarized 129Xe MR in the brain of approximately 2% of the equivalent SNR from proton MR.

Age-related changes in intracranial compartment volumes in normal adults assessed by magnetic resonance imaging

Magnetic resonance (MR) image-based computerized segmentation was used to measure various intracranial compartments in 49 normal volunteers ranging in age from 24 to 80 years to determine age-related changes in brain, ventricular, and extraventricular cerebrospinal fluid (CSF) volumes. The total intracranial volume (sum of brain, ventricular, and extraventricular CSF) averaged 1469 +/- 102 cm3 in men and 1289 +/- 111 cm3 in women. The difference was attributable primarily to brain volume, which accounted for 88.6% of the respective intracranial volumes in both sexes, but was significantly larger in men (1302 +/- 112 cm3) than in women (1143 +/- 105 cm3). In both, the cranial CSF volume averaged 11.4%. Total intracranial volume did not change with age, although the normalized brain volume of both men and women began to decrease after the age of 40 years. This decrease was best reflected by expansion of the extraventricular CSF volume which, after the age of 50 years, was more marked in men than in women. The volume of the cranial CSF, as determined by MR image-based computerized segmentation, is considerably larger than traditionally accepted and resides mostly extraventricularly. Expansion of CSF volume with age provides a good index of brain shrinkage although evolving changes and growth of the head with age tend to confound the results.

Intracranial compartment volumes in patients with enlarged ventricles assessed by magnetic resonance-based image processing

Magnetic resonance image-based computerized segmentation was used to measure the volumes of the brain, gray and white matter components, and to identify regions with prolonged enhancement on T2-weighted imaging, such as periventricular or deep white matter hyperintensities. The authors also determined the volumes of the ventricles and subarachnoid space in control subjects and in patients with: 1) aqueductal stenosis (AS); 2) other causes of obstructive hydrocephalus (OH); 3) Alzheimer's disease (AD); and 4) normal-pressure hydrocephalus (NPH). In AS the volume of the brain was smaller, whereas that of ventricles and subarachnoid cerebrospinal fluid space was larger than that of controls. The decrease in brain volume was due primarily to white matter loss. Although in OH the ventricles were larger, the subarachnoid space was smaller than in controls, presumably due to encroachment by the brain, in which the volume remained unchanged. In AD, loss of both gray and white matter resulted in a smaller brain volume, whereas that of ventricles and subarachnoid space was larger than in controls. In NPH patients, only ventricular volume was greater, whereas all other compartments were similar to controls. The brain normally occupies 87% to 92% of the intracranial volume and consequently, as observed in our patients, relatively small decrements in brain size lead to large increments in ventricular and/or extraventricular volumes. The magnitude of such changes differed markedly among our patient groups, and whether such changes prove useful in clinical assessment and differentiation needs to be determined.

Magnetic resonance imaging therapy. Intraoperative MR imaging

The field of image-guided neurosurgery has developed under the premise that integrating the enhanced discrimination powers of CT scan and MR imaging into the operating room improves the accuracy, safety and precision of neurosurgery, and expand the realm of surgically treatable lesions. Through cooperation between General Electric Corporation and the Brigham and Women's Hospital, and open-configuration MR suite was designed and implemented to attain these goals through real-time, intraoperative MR imaging in a true surgical suite. This system allows intraoperative acquisition of MR images without moving the patient, online image-guided stereotaxy without preoperative imaging, and "real-time" tracking of instruments in the operative field registered to the MR images. The design and implementation of neurosurgery in the open-configuration MR imaging suite are summarized in this article.

Computer-assisted interactive three-dimensional planning for neurosurgical procedures

We have used three-dimensional reconstruction magnetic resonance imaging techniques to understand the anatomic complexity of operative brain lesions and to improve preoperative surgical planning. We report our experience with 14 cases, including intra- and extra-axial tumors and a vascular malformation. In each case, preoperative planning was performed using magnetic resonance imaging-based three-dimensional renderings of surgically critical structures, such as eloquent cortices, gray matter nuclei, white matter tracts, and blood vessels. Simulations, using the interactive manipulation of three-dimensional data, provided an efficient and comprehensive way to appreciate the anatomic relationships. Interactive three-dimensional computer-assisted preoperative simulations provided otherwise inaccessible information that was useful for the surgical removal of brain lesions.

Coherence transfer by isotropic mixing in Carr-Purcell-Meiboom-Gill imaging: implications for the bright fat phenomenon in fast spin-echo imaging

It is well known that when compared to conventional spin-echo (CSE) imaging for equivalent effective echo times, fast spin-echo (FSE) imaging experiments yield higher signal intensities for coupled spin systems, such as that for lipid. One hypothesis put forth for this phenomenon is the removal of scalar coupling-based echo amplitude modulation by the FSE pi pulse train. This would result in the maintenance of signal intensity in the late echoes, with an overall increase in image signal when the multiecho train data is combined to form the image data. It will be shown that in images and spectra obtained from the final echo of a Carr-Purcell-Meiboom-Gill (CPMG) pi pulse train, an increase in signal in coupled spin systems occurs, when compared to conventional single-echo images and spectra at identical echo times. One- and two-dimensional spectroscopy experiments confirm that it is the generation of an isotropic mixing Hamiltonian by the pi pulse train in FSE that is responsible for the increased signal in images of a simple AX system and of corn oil, a model for human fat. This relative increase in signal is due to the maintenance of in-phase magnetization in the coupled spin systems by this Hamiltonian. In CSE, the weak coupling Hamiltonian allows development of antiphase coherences which, in the presence of the line broadening due to the imaging gradients, result in signal loss.

Image-guided surgery in a new magnetic resonance suite: preclinical considerations

Surgical procedures require correct identification of exposed anatomy with concomitant localization amidst contiguous structures. In endoscopic procedures the surgeon is provided a real-time endoscopic view and is prepared with radiologic images. Here we present an overview of a methodology of localization using intraoperatively acquired magnetic resonance (MR) images in preparation for magnetic resonance imaging-guided endoscopic sinus surgery. The methodology centers around a unique prototype imaging device and operating environment. An "open" 0.5 Tesla MR unit has been created that allows complete access to the patient's head and neck while concomitant images are obtained. Illustrative examples of localization technique from cadaver experiments are presented, as well as insights into the host of concerns for anesthesia, equipment, surgical instrumentation, communications, and documentation.

Simultaneous magnetic resonance phase and magnitude temperature maps in muscle

Noninvasive magnetic resonance temperature maps that are used to monitor thermal ablation of tissue are described. In magnetic resonance images, thermally induced proton nuclear magnetic resonance frequency shifts, and changes in the longitudinal relaxation time produce both phase and magnitude changes in the MR signal. Temperature maps with improved sensitivity are derived from the complex-difference nuclear magnetic resonance signal. Bovine muscle specimens were heated with focused ultrasound to model thermal surgery and create a known thermal distribution to test the method. Resulting MR images acquired in 2 s produce temperature maps with 1 min resolution and 2 degrees C temperature sensitivity. The temperature sensitivity was increased by extending the acquisition to 5 s, by decreasing the receiver bandwidth, and increasing the echo time.

Dynamic T1-weighted magnetic resonance imaging of interstitial laser photocoagulation in the liver: observations on in vivo temperature sensitivity

BACKGROUND AND OBJECTIVE

Magnetic resonance imaging's (MRI) potential to monitor interstitial laser photocoagulation (ILP) has been previously demonstrated and is further tested here with improved spatial and temporal resolution.

STUDY DESIGN/MATERIALS AND METHODS

In vivo experiments employed fiber-delivered 1,064 nm light (3.0 W, 150 sec) in six rabbit livers monitored under T1-weighted FSE MRI as 1 image/10 sec and a 3 mm thick 8 cm FOV. Image signal intensities (SI) were compared with temperatures (T) at 7, 10, and 15 mm from the fiber.

RESULTS

Data showed 33 degrees C < T < 60 degrees C. SI did not vary inverselinearly with T; changes in the tissue altered the MRI signal interfering with the SI changes due to temperature.

CONCLUSION

MRI cannot map SI-derived temperatures over the entire treatment site. The role of MRI's temperature sensitivity must be coordinated with organ- and dose-specific tissue changes.

A clinical, noninvasive, MR imaging-monitored ultrasound surgery method

A noninvasive method of tissue ablation that is guided and monitored with magnetic resonance (MR) imaging has been developed. The method uses sharply focused ultrasound transducers of different focal lengths to induce a localized temperature elevation during a short exposure (1-20 seconds). A hydraulic, computer-controlled positioning device moves the transducer in an MR imager. The positioner is built into a standard cradle in the imager. The system includes cavitation detection and power monitoring circuitry for patient safety. The target volume is outlined with cross-sectional MR images obtained immediately before sonication. By means of the software, the focus is moved to ablate the volume defined with the images. The temperature elevation during the exposure is monitored by means of the proton resonance frequency shift with fast gradient-echo sequences, and the necrosed volume is demonstrated with T2-weighted fast spin-echo images. This method has been extensively tested in in vivo animal experiments and is now undergoing clinical trial.

Dynamic T1-weighted magnetic resonance imaging of interstitial laser photocoagulation in the liver: observations on in vivo temperature sensitivity

BACKGROUND AND OBJECTIVE

Magnetic resonance imaging's (MRI) potential to monitor interstitial laser photocoagulation (ILP) has been previously demonstrated and is further tested here with improved spatial and temporal resolution.

STUDY DESIGN/MATERIALS AND METHODS

In vivo experiments employed fiber-delivered 1,064 nm light (3.0 W, 150 sec) in six rabbit livers monitored under T1-weighted FSE MRI as 1 image/10 sec and a 3 mm thick 8 cm FOV. Image signal intensities (SI) were compared with temperatures (T) at 7, 10, and 15 mm from the fiber.

RESULTS

Data showed 33 degrees C < T < 60 degrees C. SI did not vary inverselinearly with T; changes in the tissue altered the MRI signal interfering with the SI changes due to temperature.

CONCLUSION

MRI cannot map SI-derived temperatures over the entire treatment site. The role of MRI's temperature sensitivity must be coordinated with organ- and dose-specific tissue changes.

Potential adverse effects of high-intensity focused ultrasound exposure on blood vessels in vivo

The aim of the study was to evaluate the potential adverse effects of high intensity ultrasound exposure on blood vessels during noninvasive focused ultrasound surgery. A hydraulic MR-compatible positioning device was used to manipulate a focused ultrasound transducer (frequency 1.49 MHz, f-number = 0.8) in an MRI scanner. The system was used to sonicate a branch of the femoral artery and vein of 19 rabbits (26 thighs) in vivo at intensity levels above the threshold for transient cavitation; i.e., between 4400 and 8800 W cm-2 with multiple 1 s pulses stepped across the vessels (step size = 0.7 mm). The vessels were located and followed by MR angiography. In 13 rabbits, x-ray angiograms were also performed after the animals were euthanized. The results demonstrated that the 1 s high-intensity exposures caused the arteries to constrict at all exposure levels tested. At the intensity of 5800 W cm-2 and above, the MRI angiogram immediately after the sonications showed no flow. The x-ray angiograms (1-2 h later) showed that the blood vessels were open, but constricted to about 50% or less of their diameter. Both the MR and x-ray angiograms showed that the vessel diameters relaxed toward their initial diameter during the first week after sonication. In five cases, hemorrhage or vessel rupture was caused by the sonication. This study demonstrates that short, high-intensity focused ultrasound exposure can cause vessel spasm and hemorrhage when transient cavitation is present. This condition should be avoided during noninvasive focused ultrasound surgery.

Nuclear magnetic resonance relaxation parameters of muscle in malignant hyperthermia-susceptible swine

RATIONALE AND OBJECTIVES

The malignant hyperthermia (MH) syndrome involves characteristic changes in metabolism that have a significant effect on the postmortem properties of muscle tissue. We investigated the 1H nuclear magnetic resonance (MR) relaxation parameters of muscle tissue from normal (MH-) and malignant hyperthermia-susceptible (MH+) swine taken before their death to identify whether postmortem metabolic differences would be accompanied by changes in relaxation parameters.

METHODS

Muscle samples from 63 MH-susceptible swine were characterized by proton MR relaxation times (T1 and T2). The water content, pH, conducting, and the "subjective meat" quality were assessed in order to differentiate the meat into one of three groups: normal, pale-soft-exudative, and dark-firm-dry.

RESULTS

The T2 relaxation of MH- and MH+ muscles was significantly different, but no significant changes were detected in the water content and corresponding T1 relaxation times.

CONCLUSION

These data suggest that MH-susceptible swine can be distinguished from normal swine using nuclear MR relaxometry. The results of relaxation measurements and the multiexponential analysis of the T2 curves suggest a shift between the different water compartments during MH that might predict the increased water loss of muscle tissue after death.

Adaptive segmentation of MRI data

Intensity-based classification of MR images has proven problematic, even when advanced techniques are used. Intrascan and interscan intensity inhomogeneities are a common source of difficulty. While reported methods have had some success in correcting intrascan inhomogeneities, such methods require supervision for the individual scan. This paper describes a new method called adaptive segmentation that uses knowledge of tissue intensity properties and intensity inhomogeneities to correct and segment MR images. Use of the expectation-maximization (EM) algorithm leads to a method that allows for more accurate segmentation of tissue types as well as better visualization of magnetic resonance imaging (MRI) data, that has proven to be effective in a study that includes more than 1000 brain scans. Implementation and results are described for segmenting the brain in the following types of images: axial (dual-echo spin-echo), coronal [three dimensional Fourier transform (3-DFT) gradient-echo T1-weighted] all using a conventional head coil, and a sagittal section acquired using a surface coil. The accuracy of adaptive segmentation was found to be comparable with manual segmentation, and closer to manual segmentation than supervised multivariant classification while segmenting gray and white matter.

Caudate, putamen, and globus pallidus volume in schizophrenia: a quantitative MRI study

Basal ganglia structures have been reported to be abnormal in schizophrenia. However, while component structures of the basal ganglia are functionally differentiated, there have been no evaluations of their separate magnetic resonance imaging (MRI) volumes with small voxel (1.5 mm3) spoiled gradient-recalled acquisition in steady state techniques and multi-plane assessments. We examined MRI scans from 15 male, right-handed, neuroleptic-medicated schizophrenic patients and 15 age-, handedness-, and gender-matched normal volunteers. Compared with normal subjects, schizophrenic patients showed enlarged volumes: 14.2% for total basal ganglia, 27.4% for globus pallidus, 15.9% for putamen, and 9.5% for caudate. Increased volumes, especially of the caudate, were associated with poorer neuropsychological test performance on finger tapping and Hebb's Recurring Digits. These findings indicate abnormalities throughout all basal ganglia structures in at least a subgroup of schizophrenic patients.

Estimation of average flow in ungated 3D phase contrast angiograms

Three dimensional (3D) phase contrast angiograms contain velocity data, which is discarded after the reconstruction of the projections. In extension to earlier work on velocity quantification with ungated 2D phase data, this paper shows that a useful estimate of the average velocity and flow rate can be extracted from ungated 3D phase contrast angiograms. Simulations and experiments in a phantom and in vivo were performed. For pulsatile flow and strong spin saturation, an over-estimation of the flow rate at the net in-flow end of the imaging volume and underestimation at the net out-flow end was observed. Imaging at lower RF tip angles yielded flow rates close to the correct value within the entire imaging volume. In contrast to ungated 2D experiments, the flow rates determined by repeated 3D experiments showed no variation.

Interactive MR-guided biopsy in an open-configuration MR imaging system

PURPOSE

To describe new techniques for percutaneous biopsy with use of an open-configuration magnetic resonance (MR) imaging system with integrated frameless stereotaxic guidance tools.

MATERIALS AND METHODS

In 28 patients, biopsy was performed in which the image plane was interactively controlled by the position of a hand-held probe attached to the biopsy needle. An icon integrated into the image was used to guide needle advancement in three planes orthogonal to the needle. In vitro measurements of spatial accuracy were also performed.

RESULTS

Diagnostic tissue was retrieved in 25 of 28 patients. The system was most accurate near the isocenter with a maximum measured error of 3.1 mm within a sphere of radius 2.5 cm about the isocenter.

CONCLUSION

MR-guided biopsy with a frameless stereotaxic technique is safe and accurate. Image feedback is near real time, and the procedure is interactive. These techniques may be used to perform MR-guided biopsies and to place probes for MR-guided therapies.

The evolution of multiple sclerosis lesions on serial MR

PURPOSE

To characterize temporal changes in signal intensity patterns of multiple sclerosis lesions on serial MR.

METHODS

T1-, T2-, proton density-, and contrast-enhanced T1-weighted MR was performed on five patients with relapsing-remitting multiple sclerosis at least 22 times in the course of 1 year.

RESULTS

Forty-three enhancing lesions and 1 new lesion that never showed enhancement were detected and followed for periods ranging from approximately 4 weeks to 1 year (total of 702 time points). At first detection the center of new lesions was brighter than the periphery (20 of 24 new lesions on proton density-weighted and 19 of 23 new lesions on contrast-enhanced images). On contrast-enhanced images, ring hyperintensity was predominant at time points later than 29 days. As lesions aged, a residual rim of "nonenhancing" hyperintensity often was noted on contrast-enhanced images. Some older lesions (> 1 year) showed similar appearance on unenhanced T1-weighted images. On proton density-weighted images ring hyperintensity was most frequent 2 to 4 months after lesion detection. The estimated average duration of gadopentetate dimeglumine enhancement was 1 to 2 months.

CONCLUSIONS

A lesion evolution pattern relevant to MR was inferred. We believe that specific information about the histopathologic evolution of a lesion may be extracted not only from contrast-enhanced but also from nonenhanced serial MR. Assessment of drugs targeting specific phases of lesion evolution could benefit from quantitative pattern analysis of routine MR images.

Increased rate of P300 latency prolongation with age in schizophrenia. Electrophysiological evidence for a neurodegenerative process

BACKGROUND

The latency of the P300 event-related potential is prolonged in disorders associated with neural damage and degeneration and also becomes prolonged in the course of neural changes that accompany aging. We tested whether the rate of P300 latency increase with age was greater in male schizophrenic patients than in normal subjects because a steeper slope in schizophrenia would suggest a progressive neurodegenerative process. We also evaluated a subset of these subjects for changes in brain volumes as determined by magnetic resonance imaging.

METHOD

The P300 component was elicited during an auditory "oddball" paradigm and was recorded from 47 male patients with chronic schizophrenia whose mean age at onset was 22.4 years and from 47 age-, handedness-, and gender-matched control subjects. The relation of P300 latency and amplitude to age within each group was evaluated using correlation and regression analyses. Brain volumes determined via magnetic resonance imaging were evaluated by quantitative volumetric analyses of images acquired with three-dimensional Fourier transform and double echo-spin echo-pulse sequences.

RESULTS

The slope of P300 latency on age was steeper for schizophrenic patients than for normal control subjects at midline frontal and central electrode sites. The slope of N100 latency did not differ, implying that the P300 differences were not likely to be due to peripheral hearing loss or damage affecting the initial stages of neural processing. Posterior superior temporal gyrus gray matter volume determined via magnetic resonance imaging significantly diminished with age on the left side in patients with schizophrenia but not on the right side or in controls; these slopes were not, however, statistically significantly different from each other.

CONCLUSIONS

These findings provide preliminary evidence that male patients with chronic schizophrenia experience a neurodegenerative process that becomes evident in adulthood and is reflected by the rate of change of P300 latency with age. Whether this process is due to the primary effects of schizophrenia or is secondary to factors associated with schizophrenia's chronic course and treatment remains a question for future investigation.