Intraoperative dynamic MRI: localization of sites of brain tumor recurrence after high-dose radiotherapy

In patients with malignant astrocytomas or metastatic brain disease treated with high-dose radiotherapy, conventional imaging methods may not adequately distinguish recurrent tumor from radiation change. We used a fast spoiled gradient refocusing technique in the open-configuration intraoperative MR system to assess the rate of regional enhancement of the treated tumor bed and to localize specific sites for pathologic sampling to determine whether gadolinium uptake correlated with histologic data. Twenty-four patients were studied. Fourteen of 15 patients with areas of early enhancement had recurrent tumor present in histologic samples, and 8 of the remaining 9 patients had only reactive changes. Dynamic MRI was predictive of recurrent tumor (P < .0005, Fisher exact test and P < .002, Student t test). We conclude that dynamic MRI in the open-bore magnet is a promising method for localizing potential sites of active tumor growth in patients treated for malignant astrocytomas and metastatic brain lesions.

MR-guided endoscopic sinus surgery

We describe an interactive, intraoperative imaging-guided method for performing endoscopic sinus surgery (ESS) within a vertically open MR system. The procedure was performed with intraoperative imaging using a 0.5-T magnet with a 56-cm vertical gap. Interactive control of imaging planes was accomplished by optical tracking with two infrared light-emitting diodes mounted on an aspirator probe. The probe's position defined the location of the orthogonal imaging planes. Twelve patients with varying degrees of sinus disease underwent ESS with MR imaging guidance. Patients had acute and chronic sinusitis, nasal polyposis causing airway obstruction, or tumor requiring tissue biopsy. All procedures were performed with the patients under general anesthesia. The integration of endoscopy with optical tracking and intraoperative interactive imaging allowed localization of anatomic landmarks during ESS. No complications were encountered.

Line scan diffusion imaging: characterization in healthy subjects and stroke patients

OBJECTIVE

Our objective was to evaluate a new scanning method, MR line scan diffusion imaging, and assess the apparent diffusion coefficient in the brains of healthy subjects and stroke patients.

SUBJECTS AND METHODS

Line scan diffusion imaging without cardiac gating or head restraints was implemented on low- (0.5 T) and medium- (1.5 T) field-strength scanners with conventional hardware. Diffusion-weighted images were obtained in six healthy subjects and eight stroke patients. Unidirectional diffusion encoding was used for fast localization of stroke lesions. For further characterization, orthogonal diffusion encoding was applied, and the trace of the apparent diffusion coefficient was calculated. Single-shot diffusion-weighted echoplanar imaging served as the reference standard. For healthy subjects, imaging was repeated four times on each scanner. Mean and relative precision of the apparent diffusion coefficient trace values were calculated for each pixel. In stroke lesions and adjacent normal tissue, apparent diffusion coefficient trace values were determined.

RESULTS

In the 108 scans obtained, line scan diffusion imaging proved to be robust, virtually free of artifact (independent of slice location and orientation), reproducible, and rapid for localization of a stroke. Scan time for 14 slices at 7-mm thickness was 8 min at 0.5 T and 7 min at 1.5 T. Image qualities with line scan diffusion imaging and single-shot diffusion-weighted echoplanar imaging were comparable. At 1.5 T, precision was essentially the same for line scan diffusion imaging (4.3%) and echoplanar imaging (4.7%). With line scan diffusion imaging at 0.5 T and 1.5 T, normal paraventricular apparent diffusion coefficient trace values averaged 0.71 microm2/msec, and with echoplanar imaging these values averaged 0.69 microm2/msec. In acute lesions apparent diffusion coefficient trace values were low, and in chronic lesions these values were high.

CONCLUSION

Line scan diffusion imaging on low- and medium-field-strength MR scanners equipped with conventional hardware was reliable and practical for measuring brain apparent diffusion values, which can be applied to the early diagnosis, and hence timely management, of stroke.

Monitoring and visualization techniques for MR-guided laser ablations in an open MR system

Our purpose was to develop temperature-sensitive MR sequences and image-processing techniques to assess their potential of monitoring interstitial laser therapy (ILT) in brain tumors (n = 3) and liver tumors (n = 7). ILT lasted 2 to 26 minutes, whereas images from T1-weighted fast-spin-echo (FSE) or spoiled gradient-recalled (SPGR) sequences were acquired within 5 to 13 seconds. Pixel subtraction and visualization of T1-weighted images or optical flow computation was done within less than 110 msec. Alternating phase-mapping of real and imaginary components of SPGR sequences was performed within 220 msec. Pixel subtraction of T1-weighted images identified thermal changes in liver and brain tumors but could not evaluate the temperature values as chemical shift-based imaging, which was, however, more affected by susceptibility effects and motion. Optical flow computation displayed the predicted course of thermal changes and revealed that the rate of heat deposition can be anisotropic, which may be related to heterogeneous tumor structure and/or vascularization.

Computer-assisted intra-operative magnetic resonance imaging monitoring of interstitial laser therapy in the brain: a case report

Hardware and software for a customized system to use magnetic resonance imaging (MRI) to noninvasively monitor laser-induced interstitial thermal therapy of brain tumors are reported. An open-configuration interventional MRI unit was used to guide optical fiber placement and monitor the deposition of laser energy into the targeted lesion. T1-weighted fast spin echo and gradient echo images were used to monitor the laser tissue interaction. The images were transferred from the MRI scanner to a customized research workstation and were processed intraoperatively. Newly developed software enabled rapid (27-221 ms) availability of calculated images. A case report is given showing images which reveal the laser-tissue interaction. The system design is feasible for on-line monitoring of interstitial laser therapy. © 1998 Society of Photo-Optical Instrumentation Engineers.

Evaluation of intracranial cysts by intraoperative MR

Eleven patients with intracranial cystic collections were evaluated in the open-bore intraoperative MR system. In each case, the cystic collection or the surrounding cerebrospinal fluid (CSF) space was injected with .02 to .5 cc of .5 mol/l gadopentetate dimeglumine. Serial imaging was performed using T1-weighted imaging. In seven patients, free communication was demonstrated between the cystic collection and the surrounding CSF spaces. In four cases, the cyst did not communicate with the CSF; two of these were drained in the intraoperative MR system with reduction in symptoms. One patient developed an aseptic meningitis 10 days after the study, which was successfully treated with steroids; no other complications were noted. We conclude that the communication of intracranial cystic collections with the cisterns and ventricles can be safely and effectively elucidated with gadolinium injection in the intraoperative MR system.

Hypohydration effects on skeletal muscle performance and metabolism: a 31P-MRS study

The purpose of this study was to determine whether hypohydration reduces skeletal muscle endurance and whether increased H+ and Pi might contribute to performance degradation. Ten physically active volunteers (age 21-40 yr) performed supine single-leg, knee-extension exercise to exhaustion in a 1.5-T whole body magnetic resonance spectroscopy (MRS) system when euhydrated and when hypohydrated (4% body wt). 31P spectra were collected at a rate of one per second at rest, exercise, and recovery, and were grouped and averaged to represent 10-s intervals. The desired hydration level was achieved by having the subjects perform 2-3 h of exercise in a warm room (40 degrees C dry bulb, 20% relative humidity) with or without fluid replacement 3-8 h before the experiment. Time to fatigue was reduced (P < 0.05) by 15% when the subjects were hypohydrated [213 +/- 12 vs. 251 +/- 15 (SE) s]. Muscle strength was generally not affected by hypohydration. Muscle pH and Pi/beta-ATP ratio were similar during exercise and at exhaustion, regardless of hydration state. The time constants for phosphocreatine recovery were also similar between trials. In summary, moderate hypohydration reduces muscle endurance, and neither H+ nor Pi concentration appears to be related to these reductions.

Volumetric evaluation of the thalamus in schizophrenic male patients using magnetic resonance imaging

BACKGROUND

The thalamus, an important subcortical brain region connecting limbic and prefrontal cortices, has a significant role in sensory and cortical processing. Although inconsistently, previous studies have demonstrated neuroanatomical abnormalities in the thalamus of schizophrenic patients.

METHODS

This structural magnetic resonance imaging study, based on segmentation of contiguous coronal 1.5-mm images, compared thalamic brain volumes of 15 chronic, male schizophrenic patients with 15 normal controls matched on age, sex, handedness, and parental socioeconomic status.

RESULTS

There were no significant differences between patients and controls in thalamic volumes, right or left, adjusted for total brain volume; however, there were significantly different correlations of thalamic volumes with prefrontal white matter and lateral ventricles among patients, but not among controls. Thalamic volumes among patients were also significantly correlated with bizarre behavior, hallucinations, and thought disorder.

CONCLUSIONS

Findings suggest that connectivity between thalamic nuclei and prefrontal cortical areas are abnormal in chronic male schizophrenic patients. In addition, ventricular enlargement may be, in part, due to subtle reduction in thalamic volume and/or in volume of thalamocortical and corticothalamic fibers secondary to thalamic abnormalities. Finally, correlations with positive symptomatology underscore the role of the thalamus in gating or filtering of sensory information and coordination of cortical processing.

Interactive magnetic resonance imaging-guided biopsy in the head and neck: initial patient experience

Because of its excellent soft tissue resolution, magnetic resonance imaging (MRI) can optimize image guidance for interventional and surgical procedures. Notably, needle biopsy of head and neck lesions has been used for years, deeper lesions often requiring some form of image guidance. The closed space of diagnostic MRI scanners proves cumbersome for intervention. The authors report on the first head and neck image-guided biopsies performed in a new, investigational "open configuration" intraoperative MRI scanner. Vertical space between the scanner's upright coils gives access to the patient while imaging; image acquisition is as fast as 2 sec/image. Biopsies were performed on seven patients (parotid, parapharyngeal space, second cervical vertebra); five specimens were diagnostic. Both general anesthesia and intravenous sedation were used. The procedures were without complications. Imaging provided definition of anatomy to direct needle placement. Access to the patient allowed for both percutaneous and transoral approaches. The environment of the open magnet is well suited for biopsy of the head and neck, and near real-time intraoperative MRI has promise for guiding more complex head and neck procedures. Further study should optimize the quality of the images and the interactibility of localization and targeting and fully utilize MRI's three-dimensional imaging capabilities.

White matter changes with normal aging

We evaluated brain tissue compartments in 72 healthy volunteers between the ages of 18 and 81 years with quantitative MRI. The intracranial fraction of white matter was significantly lower in the age categories above 59 years. The CSF fraction increased significantly with age, consistent with previous reports. The intracranial percentage of gray matter decreased somewhat with age, but there was no significant difference between the youngest subjects and the subjects above 59. A covariance adjustment for the volume of hyperintensities did not alter the foregoing results. The intracranial percentage of white matter volume was strongly correlated with the percentage volume of CSF. The finding of a highly significant decrease with age in white matter, in the absence of a substantial decrease in gray matter, is consistent with recent neuropathologic reports in humans and nonhuman primates.

MRI study of cavum septi pellucidi in schizophrenia, affective disorder, and schizotypal personality disorder

OBJECTIVE

A cavum between the septi pellucidi may reflect neurodevelopmental anomalies in midline structures of the brain. The authors examined cavum septi pellucidi in subjects with schizophrenia, affective disorder, and schizotypal personality disorder and in normal subjects.

METHOD

Thirty schizophrenic patients (15 chronic, 15 first-episode), 16 patients with affective disorder (first-episode), 21 patients with schizotypal personality disorder, and 46 normal subjects were evaluated with magnetic resonance imaging. Cavum septi pellucidi was assessed by counting the number of 1.5-mm slices containing cavum septi pellucidi.

RESULTS

The presence or absence of cavum septi pellucidi did not differentiate among groups. However, the prevalence of abnormal cavum septi pellucidi (i.e., cavum septi pellucidi contained on four or more slices) was 30.4% for schizophrenic patients (36.4% for chronic, 25.0% for first-episode), 20.0% for patients with affective disorder, 18.8% for patients with schizotypal personality disorder, and 10.3% for normal subjects. When the authors used the Nopoulos et al. criteria for rating cavum septi pellucidi, which omitted borderline cases with cavum septi pellucidi on three slices, the prevalence of abnormal cavum septi pellucidi increased to 35.0% for schizophrenia (40.0% for chronic, 30.0% for first-episode), 25.0% for affective disorder, 27.3% for schizotypal personality disorder, and 13.0% for normal subjects. There was a statistically significant difference in ratings between schizophrenic and normal subjects.

CONCLUSIONS

The results suggest that alterations in midline structures during the course of neurodevelopment may play a role in the pathogenesis of schizophrenia.

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.

Demonstration of potential noninvasive ultrasound brain therapy through an intact skull

Utilization of therapeutic ultrasound in the brain has been seriously limited by the commonly accepted view that these exposures would require that a piece of the skull bone be removed to allow the ultrasound beam to propagate into the brain. In this paper, the feasibility of delivering ultrasound therapy through the intact skull was studied. Sonications were performed through a piece of human skull with focused transducers at 0.248, 0.559, 1.0 and 1.68 MHz. The skull attenuated and distorted the field; however, a sharp focal spot was created at frequencies of 1 MHz or lower. At the higher frequency, the focus was destroyed. To investigate the feasibility of compensating for the ultrasound field distortion caused by the bone, phased array experiments were performed. Two arrays with 64 elements, operating at 0.6 MHz and 1.58 MHz, were used in these experiments. The phase shifts caused by the skull were measured for each element of the arrays and then compensated for by phase-control circuitry. These phase corrections allowed a sharp focus to be generated at both frequencies. Finally, tissue destruction was induced by using pulsed sonication through a piece of human skull in a rabbit brain in vivo at the frequency of 0.559 MHz. In summary, the results showed that transcranial delivery of therapeutic ultrasound into the brain may be feasible.

Applicability and efficiency of near-optimal spatial encoding for dynamically adaptive MRI

Adaptive near-optimal MRI spatial encoding entails, for the acquisition of each image update in a dynamic series, the computation of encodes in the form of a linear algebra-derived orthogonal basis set determined from an image estimate. The origins of adaptive encoding relevant to MRI are reviewed. Sources of error of this approach are identified from the linear algebraic perspective where MRI data acquisition is viewed as the projection of information from the field-of-view onto the encoding basis set. The definitions of ideal and non-ideal encoding follow, with nonideal encoding characterized by the principal angles between two vector spaces. An analysis of the distribution of principal angles is introduced and applied in several example cases to quantitatively describe the suitability of a basis set derived from a specific image estimate for the spatial encoding of a given field-of-view. The robustness of adaptive near-optimal spatial encoding for dynamic MRI is favorably shown by results computed using singular value decomposition encoding that simulates specific instances of worst case data acquisition when all objects have changed or new objects have appeared in the field-of-view. The mathematical analysis and simulations presented clarify the applicability and efficiency of adaptively determined near-optimal spatial encoding throughout a range of circumstances as may typically occur during use of dynamic MRI.

Open-configuration MR imaging, intervention, and surgery of the urinary tract

The open-configuration MR imaging system provides new applications both in diagnosis and in therapy of conditions in the urinary tract. In addition to conventional imaging, the open configuration permits MR imaging of patients in many positions. This has already been shown to be useful in imaging the pelvis during voiding, where a sitting position allows urodynamic evaluation. The lithotomy position can be used for imaging the prostate, which also permits procedural access. The ultimate purpose of the interventional MR imaging suite is to integrate therapeutic tools and techniques with MR imaging. From surgical planning through specialized imaging systems with minimally invasive surgical applications, new methods are being developed and implemented. This new field of image-guided therapy will require extensive clinical development and evaluation for applications in the urinary tract. This will require a large concentrated interdisciplinary effort of surgeons, radiologists, computer scientists, engineers, and physicists. Successful integration of basic research and clinical work will result in a number of cutting-edge technologies with direct clinical application in the urinary tract. Initial projects have included biopsies, endoscopies, and real-time procedural control of high-temperature and cryogenic ablations. It is anticipated that the current surge in image-guided interventions will motivate even more research activity in this field, and will ultimately define the role of MR imaging guidance in urologic intervention and surgery.

Quantitative magnetic resonance imaging of brain development in premature and mature newborns

Definition in the living premature infant of the anatomical and temporal characteristics of development of critical brain structures is crucial for insight into the time of greatest vulnerability of such brain structures. We used three-dimensional magnetic resonance imaging (3D MRI) and image-processing algorithms to quantitate total brain volume and total volumes of cerebral gray matter (GM), unmyelinated white matter (WM), myelinated WM, and cerebrospinal fluid (CSF) in 78 premature and mature newborns (postconceptional age, 29-41 weeks). Total brain tissue volume was shown to increase linearly at a rate of 22 ml/wk. Total GM showed a linear increase in relative intracranial volume of approximately 1.4% or 15 ml in absolute volume per week. The pronounced increase in total GM reflected primarily a fourfold increase in cortical GM. Unmyelinated WM was found to be the most prominent brain tissue class in the preterm infant younger than 36 weeks of postconceptional age. Although minimal myelinated WM was present in the preterm infant at 29 weeks, between 35 and 41 weeks an abrupt fivefold increase in absolute volume of myelinated WM was documented. Extracerebral and intraventricular CSF was readily quantitated by this technique and found to change minimally. The application of 3D MRI and tissue segmentation to the study of human infant brain from 29 to 41 weeks of postconceptional age has provided new insights into cerebral cortical development and myelination and has for the first time provided means of quantitative assessment in vivo of early human brain development.

Magnetic resonance imaging shows orientation and asymmetry of white matter fiber tracts

Apparent diffusion tensor maps of the human brain were acquired with a magnetic resonance imaging sequence (Gudbjartsson, H., Maier, S.E., Mulkern, R.V., M6rocz, I.A., Patz, S., Jolesz, F.A., Magn. Reson. Med. 36 (1996) 509-519). It was shown that the geometric nature of the apparent diffusion tensors can quantitatively characterize the tissue structure. Display of the orientation and directional uniformity of the water diffusion in the brain demonstrated most of the known major anatomical constituents of human white matter. A comparison of corresponding anatomic regions in the white matter of both hemispheres in 24 healthy volunteers revealed that fiber tracts within the anterior limb of the internal capsule have a significantly higher (P < 0.01) measure of alignment in the right hemisphere. This method offers a unique tool for the in vivo demonstration of neural connectivity in healthy and diseased brain.

Compatible instrumentation for intraoperative MRI: expanding resources

Steadfast progress has been made from biopsy to surgery with interventional MRI (iMRI). Such image-guided interventions require specialized instrumentation due to the unusual elements of the MR environment. Suppliers/manufacturers of MR-compatible instrumentation were few in 1994, but now there are more than 50. We present fundamental issues of MR compatibility and a list of known suppliers/manufacturers.

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).

Calibration of water proton chemical shift with temperature for noninvasive temperature imaging during focused ultrasound surgery

The present work was performed to calibrate water proton chemical shift change with tissue temperature in vivo to establish a method of quantitative temperature imaging during focused ultrasound surgery. The chemical shift change measured with a phase-mapping method using spoiled gradient-recalled acquisition in steady state (SPGR) (TR = 26 msec, TE = 12.8 msec, matrix = 256 x 128) was calibrated with the corresponding temperature elevation (0-50 degrees C, 32-84 degrees C in absolute temperature) measured with a copper-constantan thermocouple (.05-mm-diameter bare wires) in rabbit skeletal muscle (16 animals) under focused ultrasound exposures (10-100 W radiofrequency [RF] power, 20-second sonication). A linear calibration with a regression coefficient of (-8.76+/-.69) x 10(-3) ppm/degrees C (P < .01 [P, significance level]) was obtained. Temperature distributions during a 20-second sonication were visualized every 3.3 seconds with a 2.3-mm3 spatial resolution and 4 degrees C temperature uncertainty.

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.

Real-time interactive three-dimensional segmentation

RATIONALE AND OBJECTIVES

The authors developed a real-time, interactive three-dimensional (3D) segmentation pipeline that uses relatively low-level segmentation operations and provides two-dimensional and 3D visualization through a user-friendly graphical interface.

MATERIALS AND METHODS

The low-level segmentation processes were implemented on a massively parallel computer; the graphical user interface was written with a public domain software toolkit. Since their implementation 2 years ago, these segmentation tools have been applied to approximately 300 computed tomographic and magnetic resonance imaging data sets. Two typical clinical cases are presented to demonstrate their applications.

RESULTS

The entire processing pipeline can be executed in a few seconds. The tools are simple to learn because they involve the use of low-level procedures and a user-friendly graphical interface with a short interactive response time. Segmentation of the bones, aorta, kidneys, and kidney cysts in case 1 could be performed in about 16 minutes. The time needed to segment each organ in case 2 ranged from about 15 minutes for the skin and brain to about 1 minute for the tumor.

CONCLUSION

Satisfactory results can be obtained in a relatively short time with the real-time interactive 3D segmentation system. Operation of the system can be easily learned by a wide variety of nonspecialized users with some medical background.