Alastair J. Martin

Aneurysm Growth Occurs at Region of Low Wall Shear Stress Patient-Specific Correlation of Hemodynamics and Growth in a Longitudinal Study

Background and Purpose— Evolution of intracranial aneurysmal disease is known to be related to hemodynamic forces acting on the vessel wall. Low wall shear stress (WSS) has been reported to have a negative effect on endothelial cells normal physiology and may be an important contributor to local remodeling of the arterial wall and to aneurysm growth and rupture. Methods— Seven patient-specific models of intracranial aneurysms were constructed using MR angiography data acquired at two different time points (mean 16.4±7.4 months between the two time points). Numeric simulations of the flow in the baseline geometries were performed to compute WSS distributions. The lumenal geometries constructed from the two time points were manually coregistered, and the radial displacement of the wall was calculated on a pixel-by-pixel basis. This displacement, corresponding to the local growth of the aneurysm, was compared to the time-averaged wall shear stress (WSSTA) through the cardiac cycle at that location. For statistical analysis, radial displacement was considered to be significant if it was larger than half of the MR pixel resolution (0.3 mm). Results— Mean WSSTA values obtained for the areas with a displacement smaller and greater than 0.3 mm were 2.55±3.6 and 0.76±1.5 Pa, respectively (P<0.001). A linear correlation analysis demonstrated a significant relationship between WSSTA and surface displacement (P<0.001). Conclusions— These results indicate that aneurysm growth is likely to occur in regions where the endothelial layer lining the vessel wall is exposed to abnormally low wall shear stress.

Whole-heart steady-state free precession coronary artery magnetic resonance angiography.

Current implementations of coronary artery magnetic resonance angiography (MRA) suffer from limited coverage of the coronary arterial system. Whole‐heart coronary MRA was implemented based on a free‐breathing steady‐state free‐precession (SSFP) technique with magnetization preparation. The technique was compared to a similar implementation of conventional, thin‐slab coronary MRA in 12 normal volunteers. Three thin‐slab volumes were prescribed: 1) a transverse slab, covering the left main (LM) artery and proximal segments of the left anterior ascending (LAD) and left circumflex (LCX) coronary arteries; 2) a double‐oblique slab covering the right coronary artery (RCA); and 3) a double‐oblique slab covering the proximal and distal segments of the LCX. The whole‐heart data set was reformatted in identical orientations. Visible vessel length, vessel sharpness, and vessel diameter were determined and compared separately for each vessel. Whole‐heart coronary MRA visualized LM/LAD (11.7 ± 3.4 cm) and LCX (6.9 ± 3.6 cm) over a significantly longer distance than the transverse volume (LM/LAD, 6.1 ± 1.1 cm, P < 0.001; LCX, 4.2 ± 1.2 cm, P < 0.05). Improvements in visible vessel length for RCA and LCX in the whole‐heart approach vs. their respective targeted volumes were not significant. It is concluded that the whole‐heart coronary MRA technique improves visible vessel length and facilitates high‐quality coronary MRA of the complete coronary artery tree in a single measurement. Magn Reson Med 50:1223–1228, 2003.

Safety, efficacy, and functionality of high-field strength interventional magnetic resonance imaging for neurosurgery

OBJECTIVE Interventional magnetic resonance imaging (MRI) allows neurosurgeons to interactively perform surgery using MRI guidance. High-field strength (1.5-T) imaging permits exceptional observation of intracranial and spinal pathological features. The development of this technology and its application to a variety of neurosurgical procedures are described. METHODS We report on the first 101 cases that were treated in the interventional MRI unit (between January 1997 and September 1998). These cases included 39 brain biopsies, 30 tumor resections, 9 functional neurosurgical cases, 8 cyst drainages, 5 laminectomies, and 10 miscellaneous cases. Patients ranged in age from 14 months to 84 years (median, 43 yr); 61 patients were male and 40 were female. Intraoperative functional techniques that were used to influence surgical decision-making included magnetic resonance spectroscopy, functional MRI, magnetic resonance angiography and venography, chemical shift imaging, and diffusion-weighted imaging. All surgery was performed using MRI-compatible instruments within the 5-gauss line and conventional instruments outside that line. RESULTS All 39 brain biopsies yielded diagnostic tissue. Of the 30 tumor resections, 24 (80%) were considered radiographically complete. The incidence of serious complications was low and was comparable to that associated with conventional operating rooms. One patient developed a Propionibacterium acnes brain abscess 6 weeks after surgery and another patient experienced Staphylococcus aureus scalp cellulitis after a brain biopsy, yielding an infection rate of less than 2%. No clinically significant hemorrhage was observed in immediate postoperative imaging scans, although one patient developed a delayed hematoma after a thalamotomy. One patient experienced a stroke after resection of a hippocampal tumor. No untoward events were associated with MRI-compatible instrumentation or intraoperative patient monitoring. CONCLUSION High-field (1.5-T) interventional MRI is a safe and effective technology for assisting neurosurgeons in achieving the goals of surgery. Preliminary results suggest that the functional capabilities of this technology can yield data that can significantly influence intraoperative neurosurgical decision-making. The rates of serious complications, such as infection, associated with this new technology were low.

Investigation of intraoperative brain deformation using a 1.5-T interventional MR system: preliminary results

All image-guided neurosurgical systems that the authors are aware of assume that the head and its contents behave as a rigid body. It is important to measure intraoperative brain deformation (brain shift) to provide some indication of the application accuracy of image-guided surgical systems, and also to provide data to develop and validate nonrigid registration algorithms to correct for such deformation. The authors are collecting data from patients undergoing neurosurgery in a high-field (1.5 T) interventional magnetic resonance (MR) scanner. High-contrast and high-resolution gradient-echo MR image volumes are collected immediately prior to surgery, during surgery, and at the end of surgery, with the patient intubated and lying on the operating table in the operative position. Here, the authors report initial results from six patients: one freehand biopsy, one stereotactic functional procedure, and four resections. The authors investigate intraoperative brain deformation by examining threshold boundary overlays and difference images and by measuring ventricular volume. They also present preliminary results obtained using a nonrigid registration algorithm to quantify deformation. They found that some cases had much greater deformation than others, and also that, regardless of the procedure, there was very little deformation of the midline, the tentorium, the hemisphere contralateral to the procedure, and ipsilateral structures except those that are within 1 cm of the lesion or are gravitationally above the surgical site.

Brain biopsy using high-field strength interventional magnetic resonance imaging

OBJECTIVE Lesions within the brain are commonly sampled using stereotactic techniques. The advent of interventional magnetic resonance imaging (MRI) now allows neurosurgeons to interactively investigate specific regions, with exquisite observational detail. We evaluated the safety and efficacy of this new surgical approach. METHODS Between January 1997 and June 1998, 35 brain biopsies were performed in a high-field strength interventional MRI unit. All biopsies were performed using MRI-compatible instrumentation. Interactive scanning was used to confirm accurate positioning of the biopsy needle within the region of interest. Intraoperative pathological examination of the biopsy specimens was performed to verify the presence of diagnostic tissue, and intra- and postoperative imaging was performed to exclude the presence of intraoperative hemorrhage. Recently, magnetic resonance spectroscopic targeting was used for six patients. RESULTS Diagnostic tissue was obtained in all 35 brain biopsies and was used in therapeutic decision-making. Histological diagnoses included 28 primary brain tumors (12 glioblastomas multiforme, 9 oligodendrogliomas, 2 anaplastic astrocytomas, 2 astrocytomas, 1 lymphoma, and 1 anaplastic oligodendroglioma), 1 melanoma brain metastasis, 1 cavernous sinus meningioma, 1 cerebral infarction, 1 demyelinating process, and 3 cases of radiation necrosis. In all cases, magnetic resonance spectroscopy was accurate in distinguishing recurrent tumors (five cases) from radiation necrosis (one case). No patient sustained clinically or radiologically significant hemorrhage, as determined by intraoperative imaging performed immediately after the biopsy. One patient (3%) suffered transient hemiparesis after a pontine biopsy for investigation of a brain stem glioma. Another patient developed scalp cellulitis, with possible intracranial extension, 3 weeks after the biopsy; this condition was effectively treated with antibiotic therapy. Three patients were discharged on the day of the biopsy. CONCLUSION Interventional 1.5-T MRI is a safe and effective method for evaluating lesions of the brain. Magnetic resonance spectroscopic targeting is likely to augment the diagnostic yield of brain biopsies.

Measurement and analysis of brain deformation during neurosurgery

Recent studies have shown that the surface of the brain is deformed by up to 20 mm after the skull is opened during neurosurgery, which could lead to substantial error in commercial image-guided surgery systems. We quantitatively analyze the intraoperative brain deformation of 24 subjects to investigate whether simple rules can describe or predict the deformation. Interventional magnetic resonance images acquired at the start and end of the procedure are registered nonrigidly to obtain deformation values throughout the brain. Deformation patterns are investigated quantitatively with respect to the location an magnitude of deformation, and to the distribution and principal direction of the displacements. We also measure the volume change of the lateral ventricles by manual segmentation. Our study indicates that brain shift occurs predominantly in the hemisphere ipsi-lateral to the craniotomy, and that there is more brain deformation during resection procedures than during biopsy or functional procedures. However, the brain deformation patterns are extremely complex in this group of subjects. This paper quantitatively demonstrates that brain deformation occurs not only at the surface, but also in deeper brain structure, and that the principal direction of displacement does not always correspond with the direction of gravity. Therefore, simple computational algorithms that utilize limited intraoperative information (e.g., brain surface shift) will not always accurately predict brain deformation at the lesion.

Chronic hepatitis: role of diffusion-weighted imaging and diffusion tensor imaging for the diagnosis of liver fibrosis and inflammation.

To determine the diagnostic performance of liver apparent diffusion coefficient (ADC) measured with conventional diffusion‐weighted imaging (CDI) and diffusion tensor imaging (DTI) for the diagnosis of liver fibrosis and inflammation.

Phase-contrast magnetic resonance imaging measurements in intracranial aneurysms in vivo of flow patterns, velocity fields, and wall shear stress: comparison with computational fluid dynamics.

Evolution of intracranial aneurysms is known to be related to hemodynamic forces such as wall shear stress (WSS) and maximum shear stress (MSS). Estimation of these parameters can be performed using numerical simulations with computational fluid dynamics (CFD), but can also be directly measured with magnetic resonance imaging (MRI) using a time‐dependent 3D phase‐contrast sequence with encoding of each of the three components of the velocity vectors (7D‐MRV). To study the accuracy of 7D‐MRV in estimating these parameters in vivo, in comparison with CFD, 7D‐MRV and patient‐specific CFD modeling was performed for 3 patients who had intracranial aneurysms. Visual and quantitative analyses of the flow pattern and distribution of velocities, MSS, and WSS were performed using the two techniques. Spearmans coefficients of correlation between the two techniques were 0.56 for the velocity field, 0.48 for MSS, and 0.59 for WSS. Visual analysis and Bland–Altman plots showed good agreement for flow pattern and velocities but large discrepancies for MSS and WSS. These results indicate that 7D‐MRV can be used in vivo to measure velocity flow fields and for estimating MSS and WSS. Currently, however, this method cannot accurately quantify the latter two parameters. Magn Reson Med 61:409–417, 2009.

Placement of deep brain stimulator electrodes using real-time high-field interventional magnetic resonance imaging

A methodology is presented for placing deep brain stimulator electrodes under direct MR image guidance. The technique utilized a small, skull‐mounted trajectory guide that is optimized for accurate alignment under MR fluoroscopy. Iterative confirmation scans are used to monitor device alignment and brain penetration. The methodology was initially tested in a human skull phantom and proved capable of achieving submillimeter accuracy over a set of 16 separate targets that were accessed. The maximum error that was obtained in this preliminary test was 2 mm, motivating use of the technique in a clinical study. Subsequently, a total of eight deep brain stimulation electrodes were placed in five patients. Satisfactory placement was achieved on the first pass in seven of eight electrodes, while two passes were required with one electrode. Mean error from the intended target on the first pass was 1.0 ± 0.8 mm (range = 0.1–1.9 mm). All procedures were considered technical successes and there were no intraoperative complications; however, one patient did develop a postoperative infection. Magn Reson Med, 2005.

Magnetic Resonance Imaging of Implanted Deep Brain Stimulators: Experience in a Large Series

Magnetic resonance imaging (MRI) is a commonly used and important imaging modality to evaluate lead location and rule out complications after deep brain stimulation (DBS) surgery. Recent safety concerns have prompted new safety recommendations for the use of MRI in these patients, including a new recommendation to limit the specific absorption rate (SAR) of the MRI sequences used to less than 0.1 W/kg. Following SAR recommendations in real-world situations is problematic for a variety of reasons. We review our experience scanning patients with implanted DBS systems over a 7-year period using a variety of scanning techniques and four scanning platforms. 405 patients with 746 implanted DBS systems were imaged using 1.5-tesla MRI with an SAR of up to 3 W/kg. Many of the DBS systems were imaged multiple times, for a total of 1,071 MRI events in this group of patients with no adverse events. This series strongly suggests that the 0.1 W/kg recommendation for SAR may be unnecessarily low for the prevention of MRI-related adverse events.