Open Magnetic Resonance Research Articles

Posterior displacement of the tibia increases in deep flexion of the knee.

Deep knee flexion is important to proper function for some activities and in some cultures, although there are large posterior forces during high knee flexion. Most of what we know about posterior restraint and stability, however, has not been determined from deep flexion and without distinguishing motion in the medial and lateral compartments. We therefore evaluated (1) the difference in posterior displacement between the medial and lateral compartments at a commonly used flexion angle of 90 degrees ; (2) that of deeply flexed knees at 135 degrees ; and (3) the difference in kinematics in the medial and lateral compartments. We analyzed posterior stability in 21 normal knees using interventional open magnetic resonance imaging (MRI) system. When manual posterior stress was applied, the posterior displacements of the tibia were 0.6 mm/2.1 mm (medial/lateral) at 90 degrees and 0.6 mm/3.6 mm at 135 degrees . The posterior aspect of the femoral medial condyle moved 7.5 mm anteriorly with knee flexion, whereas the lateral condyle moved 1.3 mm anteriorly. The contact point of the lateral compartment moved 9.2 mm posteriorly with knee flexion, whereas the contact point of the medial compartment moved 2.3 mm anteriorly. Posterior displacement was larger in the lateral compartment at both flexion angles with manual posterior stress. As the knees flexed from 90 degrees to 135 degrees , posterior displacement became larger in the lateral compartment. Cruciate-retaining total knee arthroplasty (TKA) or posterior cruciate ligament (PCL) reconstruction surgery should aim to achieve stability on the medial side and a few millimeters of laxity at the lateral side at 90 degrees flexion with increasing laxity only on the lateral side in deep flexion.

Evaluation of Magnetic Resonance Imaging-Compatible Needles and Interactive Sequences for Musculoskeletal Interventions Using an Open High-Field Magnetic Resonance Imaging Scanner

In this article, we study in vitro evaluation of needle artefacts and image quality for musculoskeletal laser-interventions in an open high-field magnetic resonance imaging (MRI) scanner at 1.0T with vertical field orientation. Five commercially available MRI-compatible puncture needles were assessed based on artefact characteristics in a CuSO4 phantom (0.1%) and in human cadaveric lumbar spines. First, six different interventional sequences were evaluated with varying needle orientation to the main magnetic field B0 (0 degrees to 90 degrees ) in a sequence test. Artefact width, needle-tip error, and contrast-to-noise ratio (CNR) were calculated. Second, a gradient-echo sequence used for thermometric monitoring was assessed and in varying echo times, artefact width, tip error, and signal-to-noise ratio (SNR) were measured. Artefact width and needle-tip error correlated with needle material, instrument orientation to B0, and sequence type. Fast spin-echo sequences produced the smallest needle artefacts for all needles, except for the carbon fibre needle (width <3.5 mm, tip error <2 mm) at 45 degrees to B0. Overall, the proton density-weighted spin-echo sequences had the best CNR (CNR(Muscle/Needle) >16.8). Concerning the thermometric gradient echo sequence, artefacts remained <5 mm, and the SNR reached its maximum at an echo time of 15 ms. If needle materials and sequences are accordingly combined, guidance and monitoring of musculoskeletal laser interventions may be feasible in a vertical magnetic field at 1.0T.

Ultrasound-guided interventional procedures for chronic pelvic pain

Chronic pelvic pain refers to noncyclical pain localized to the pelvic region for longer than 6 months. There can be numerous causes for pelvic pain excluding visceral pathology. Familiarity with disorders of peripheral nerve and musculoskeletal structures may assist in diagnosis and therapy. This review focuses on three pelvic pain syndromes mediated by somatic nerves: the “border nerve syndrome” (consisting of the ilioinguinal, iliohypogastric, and genitofemoral nerves), the piriformis syndrome, and pudendal neuralgia. Various approaches have been described to block these nerves, ranging from simple blind techniques to the use of open magnetic resonance imaging facilities. The degree of accuracy and safety of the blocks seem to correlate well with the sophistication of the technology employed. The use of ultrasound (US) can consolidate the advantages of many of these methods. US is portable, readily available to pain specialists, and avoids the use of ionizing radiation. It allows the visualization of a wide range of tissues and possibly improves accuracy of needle placement. The purpose of this review is to describe the anatomy and sonoanatomy of the aforementioned nerves. This review will also report the US-guided techniques used to block them for the diagnosis and therapy of chronic pelvic pain.

In Vivo Magnetic Resonance Imaging Measurement of Tibiofemoral Relation With Different Knee Flexion Angles After Single- and Double-Bundle Anterior Cruciate Ligament Reconstructions

Double-bundle anterior cruciate ligament (ACL) reconstruction has been shown to restore better kinematics in vitro, but it is uncertain whether this technique can achieve this in vivo. We investigated whether anatomic double-bundle ACL reconstruction can restore a better tibiofemoral relation in the sagittal plane under static unloading conditions. The tibiofemoral relation was assessed with an open magnetic resonance imaging scanner (0.5 T) in 15 patients with anatomic double-bundle reconstruction (double-bundle group) and 14 patients with single-bundle reconstruction (single-bundle group) by use of hamstring tendons. T1-weighted magnetic resonance imaging for both knees was obtained at 0 degrees, 45 degrees, 90 degrees, and 120 degrees of flexion without external force and muscle contraction 6 months after surgery. The position of the posterior femoral condyles relative to the tibia was measured in the midmedial and midlateral sagittal sections of the knee. Clinical evaluation (range of motion, KT-2000 measurement [MEDmetric, San Diego, CA], and pivot-shift test) was performed at the same time. KT-2000 testing showed that the mean side-to-side difference in the double-bundle group (0.7 mm) was significantly smaller than that in the single-bundle group (1.7 mm). In the double-bundle group, the tibiofemoral relation in operated knees was not significantly different from that in the contralateral knees at 0 degrees, 45 degrees, 90 degrees, and 120 degrees of flexion, although at 0 degrees of flexion, the femoral condyles were positioned anteriorly relative to the tibia compared with that in the contralateral knees. Similar results were obtained in the single-bundle group. The difference in the tibiofemoral relation between reconstructed and contralateral knees was not significantly different between the 2 groups. According to KT-2000 measurement, this study found that anteroposterior stability was better with anatomic double-bundle ACL reconstruction than with single-bundle ACL reconstruction. However, under static conditions without an anterior drawer force, anatomic double-bundle ACL reconstruction did not show superiority in terms of restoring a better tibiofemoral relation compared with single-bundle ACL reconstruction. Level III, comparative study.

Quantitative Assessment of Rotatory Instability after Anterior Cruciate Ligament Reconstruction

Background Anterior cruciate ligament reconstruction successfully reduces anterior knee instability, but its effect on rotatory stability is not fully understood. In addition, a definitive method for the quantitative evaluation of rotatory instability remains to be established. Hypothesis Measurement of anterolateral tibial translation by open magnetic resonance imaging could positively correlate with the clinical grading of the pivot-shift test and would clarify residual rotatory abnormalities not shown by conventional methods for measurement of anterior stability. Study Design Controlled laboratory study. Methods An anterior cruciate ligament—reconstructed group (n = 21) and an anterior cruciate ligament—deficient group (n = 20) were examined using a Slocum anterolateral rotatory instability test in open magnetic resonance imaging. Anterior tibial translation was measured at the medial and lateral compartments by evaluating sagittal images. Clinical knee stability was evaluated before the above measurement using the pivot-shift test, KT-2000 arthrometer, and stress radiography. A cutoff value for anterolateral tibial translation relating to pivot-shift was determined using a receiver operating characteristic curve. Results Side-to-side differences of anterolateral tibial translation correlated with clinical grade of the pivot-shift test and stress radiography but not with KT-2000 arthrometry in both groups. The cutoff value was established as 3.0 mm. Although the mean anterolateral translation showed no difference, 9 reconstructed knees revealed greater than 3 mm of anterolateral tibial translation, whereas only 3 uninjured knees did. Conclusion Measurement using an open magnetic resonance imaging successfully quantified the remaining rotatory instability in anterior cruciate ligament—reconstructed knees. Clinical Relevance This method is a useful means for quantifying anterior cruciate ligament function to stabilize tibial rotation.

Rectocele and intussusception: is there any coherence in symptoms or additional pelvic floor disorders?

Patients with a rectocele often suffer from such symptoms as obstructed defaecation, urine or stool incontinence and pain. The aim of this study was to assess other concomitant pelvic floor disorders and their influence on pelvic function. Included in the study were 37 female patients with a significant rectocele and defaecation disorder. Medical history and symptoms were analysed in terms of validated functional scores. All patients underwent open magnetic resonance defaecography (MRD) in a sitting position. Imaging was analysed for the presence and size of the rectocele, intussusception and other pelvic floor disorders. Patients with a higher body mass index tended to have a larger rectocele, whereas age and vaginal birth did not correlate with the size of the rectocele. In 67.5% of the patients with a previously diagnosed rectocele, an intussusception was diagnosed on MRD. This group suffered from significantly worse urine incontinence (p=0.023) and from accessory enteroceles 64%, compared with 17% (p=0.013) for those with a simple rectocele. Patients with higher grade intussusception suffered more frequently from incontinence than from constipation. Patients with a symptomatic rectocele frequently have other pelvic floor disorders that significantly influence the pattern of symptoms. Knowledge of all the afflictions is essential for determining the optimal treatment for each individual patient.

Hypofractionated Boost With High-Dose-Rate Brachytherapy and Open Magnetic Resonance Imaging–Guided Implants for Locally Aggressive Prostate Cancer: A Sequential Dose-Escalation Pilot Study

To evaluate the feasibility, tolerance, and preliminary outcome of an open MRI-guided prostate partial-volume high-dose-rate brachytherapy (HDR-BT) schedule in a group of selected patients with nonmetastatic, locally aggressive prostatic tumors. After conventional fractionated three-dimensional conformal external radiotherapy to 64-64.4 Gy, 77 patients with nonmetastatic, locally aggressive (e.g., perineural invasion and/or Gleason score 8-10) prostate cancer were treated from June 2000 to August 2004, with HDR-BT using temporary open MRI-guided (192)Ir implants, to escalate the dose in the boost region. Nineteen, 21, and 37 patients were sequentially treated with 2 fractions of 6 Gy, 7 Gy, and 8 Gy each, respectively. Neoadjuvant androgen deprivation was given to 62 patients for 6-24 months. Acute and late toxicity were scored according to the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer scoring system. All 77 patients completed treatment as planned. Only 2 patients presented with Grade > or =3 acute urinary toxicity. The 3-year probability of Grade > or =2 late urinary and low gastrointestinal toxicity-free survival was 91.4% +/- 3.4% and 94.4% +/- 2.7%, respectively. Rates of 3-year biochemical disease-free survival (bDFS) and disease-specific survival were 87.1% +/- 4.1% and 100%, respectively. Boosting a partial volume of the prostate with hypofractionated HDR-BT for aggressive prostate cancer was feasible and showed limited long-term toxicity, which compared favorably with other dose-escalation methods in the literature. Preliminary bDFS was encouraging if one considers the negatively selected population of high-risk patients in this study.

Contrast-enhanced breast MR imaging of claustrophobic or oversized patients using an open low-field magnet

A number of women who should undergo magnetic resonance (MR) imaging of the breast cannot use this diagnostic tool due to claustrophobia or excessive body size for the restricted confines of standard closed MR systems. Our aim was to evaluate the performance of open low-field magnet breast MR imaging in such patients using a high-relaxivity contrast agent. Of 397 consecutive patients undergoing breast MR imaging, 379 (95.5%) were studied at 1.5 T. Due to claustrophobia (n=15) or large body size (n=3), 18 patients (4.5%) were studied on a 0.2-T open magnet using a body coil. A 3D dynamic T1-weighted gradient-echo 94-s sequence was acquired with intravenous injection of gadobenate dimeglumine (0.1 mmol/kg). The standard of reference was pathological examination for 16 lesions classified with a maximal Breast Imaging Reporting and Data System (BI-RADS) score from 3 to 5, fine-needle aspiration cytology and >or=2-year follow-up for two lesions classified as BI-RADS 3, and >or=2-years follow-up for five lesions classified as BI-RADS 2. Diagnostic MR image quality was achieved for 20/23 lesions in 15/18 patients. Three lesions (two invasive cancers and a cyst) were not assessed due to patient movement and considered as two false negatives and one false positive. Thus, an 86% sensitivity [13/15; 95% confidence interval (CI): 70%-100%], an 87% specificity (7/8; 95% CI: 65%-100%) and an 87% accuracy (20/23; 95% CI: 73%-100%) were obtained. The intraclass correlation coefficient between MR and pathologic lesion size was 0.845. In claustrophobic or oversized patients, open low-field breast MR with gadobenate dimeglumine yields good diagnostic performance.

MR Imaging-guided Percutaneous Cryoablation of the Prostate in an Animal Model: In Vivo Imaging of Cryoablation-induced Tissue Necrosis with Immediate Histopathologic Correlation

To evaluate the feasibility of magnetic resonance (MR) imaging-guided percutaneous cryoablation of normal canine prostates and to identify MR imaging features that accurately predict the area of tissue damage at a microscopic level.Six adult male mixed-breed dogs were anesthetized, intubated, and placed in a 0.5-T open MR imaging system. A receive-only endorectal coil was placed, and prostate location and depth were determined on T1-weighted fast spin-echo (FSE) MR imaging. After placement of cryoprobes and temperature sensors, three freezing protocols were used to ablate prostate tissue. Ice ball formation was monitored with T1-weighted FSE imaging. Tissue necrosis area was assessed with contrast-enhanced weighted MR imaging and compared with histopathologic findings.A total of 12 cryolesions (mean size, 1.2 cm) were bilaterally created in six prostates. Ice ball formation was oval and signal-free on T1-weighted FSE sequences in all cases. Postprocedural contrast-enhanced MR imaging typically showed a nonenhancing area of low signal intensity centrally located within the frozen area, surrounded by a bright enhancing rim in all cases. On histopathologic examination, two distinct zones were identified within cryolesions. Centrally, a necrotic zone with complete cellular destruction and hemorrhage was found. Between this necrotic zone and normal glandular tissue, a zone of fragmented and intact glands, interstitial edema, and rare acute inflammatory cells was seen. Correlation between nonenhancement on contrast-enhanced weighted MR images and tissue necrosis on pathologic examination was consistent within all six dogs.MR imaging-guided cryoablation of the prostate is technically feasible. The nonenhancing area on postablation contrast-enhanced weighted MR imaging accurately predicts the area of cryoablation-induced tissue necrosis on pathologic analysis.

Measuring three-dimensional knee kinematics under torsional loading

Excessive knee joint laxity is often used as an indicator of joint disease or injury. Clinical assessment devices are currently limited to anterior–posterior drawer measurements, while tools used to measure movement in the remaining degrees of freedom are either invasive or prone to soft tissue artefact. The objective of this work was, therefore, to develop a methodology whereby in vivo knee joint kinematics could be measured in three dimensions under torsional loading while still maintaining a non-invasive procedure. A device designed to administer a subject-normalized torque in the transverse plane of the knee was securely fastened to the outer frame of an open magnetic resonance imaging (MRI) magnet. Low resolution 3D T1-weighted images (6.25 mm slice thickness) were generated by the 0.2 Tesla MRI scanner in less than 3 min while the joint was under load. The 3D image volume was then shape-matched to a high resolution image volume (1.56 mm slice thickness) scanned in a no-load position. Three-dimensional rotations and translations of the tibia with respect to the femur were calculated by comparing the transformation matrices before and after torque was applied. Results from six subjects showed that this technique was repeatable over five trials with the knee in extended and flexed positions. Differences in range of rotation were shown between subjects and between knee positions, suggesting that this methodology has sufficient utility for further application in clinical studies.

MR-Guided Laser Ablation of Osteoid Osteoma in an Open High-Field System (1.0 T)

Computed tomography is the standard imaging modality to minimize the extent of surgical or ablative treatment in osteoid osteomas. In the last 15 years, since a description of thermal ablation of osteoid osteomas was first published, this technique has become a treatment of choice for this tumor. We report the case of a 20-year-old man with an osteoid osteoma treated with laser ablation in an open high-field magnetic resonance imaging scanner (1.0 T). The tumor, located in the right fibula, was safely and effectively ablated under online monitoring. We describe the steps of this interventional procedure and discuss related innovative guidance and monitoring features and potential benefits compared with computed tomographic guidance.

All-in-one magnetic resonance arthrography of the shoulder in a vertically open magnetic resonance unit

Magnetic resonance (MR) arthrography frequently involves joint injection under imaging guidance followed by MR imaging in static positions. To evaluate if MR arthrography of the shoulder joint can be performed in a comprehensive fashion combining the MR-guided injection procedure, static MR imaging, and dynamic motion MR imaging in a single test. Twenty-three shoulder joints were injected with Gd-DTPA2- under MR guidance. Static MR imaging was performed and included a three-point Dixon method to achieve water-selective images. Dynamic motion MR imaging with and without applying pressure to the upper arm was used to evaluate glenohumeral joint instability. In 10 cases, surgical correlation was available. The all-in-one MR arthrography technique was successful in all patients, and took an average time of 65 min. All but one glenohumeral injection procedure were performed with a single needle pass, and no complications were observed. Out of eight labrum tears seen with static MR imaging, seven were confirmed at surgery. In 10 cases, dynamic motion MR imaging correlated well with the surgeon's intraoperative evaluation for presence and direction of instability. MR arthrography of the shoulder joint using a vertically open magnet can be performed as a single comprehensive test, including the injection and the static and dynamic motion MR imaging. Good diagnostic accuracy for intraarticular lesions and glenohumeral instability was found in a small sample.

INTRAOPERATIVE NEUROPHYSIOLOGICAL MONITORING IN AN OPEN LOW-FIELD MAGNETIC RESONANCE IMAGING SYSTEM

The intraoperative combination of an open magnetic resonance imaging (MRI) system with neurophysiological localization and continuous monitoring techniques allows for the best available anatomic and physiological orientation as well as real-time functional monitoring. Methodological aspects and technical adaptations for this combination of methods and the experience in 29 patients with tumors in the central region are reported. MRI-compatible platinum/iridium electrodes for intraoperative neuromonitoring were attached to the patient's head. All other electrodes located outside the magnet were stainless steel needle-electrodes for recording of motor evoked potentials and for stimulating somatosensory evoked potentials. Intraoperative MRI was performed using a 0.15-T intraoperative magnetic resonance scanner (PoleStar N20; Medtronic Surgical Navigation Technologies, Louisville, KY). The calculated and measured values of the maximum induced magnetic field (2 x 10(-6) T), induced voltage (0.1 V), and force (0.01 N) by the static or changing magnetic field within all attached electrodes were negligible and proved the method's safety. In 29 patients, platinum/iridium electrodes with low susceptibility showed no interference with the imaging quality. Furthermore, neurophysiological monitoring could be performed with unaffected recording quality. Side effects (e.g., thermal induction) were not observed. Neurophysiological monitoring for evoked potentials and direct cortical stimulation can be performed with standard quality within a low-field intraoperative MRI system. Electrodes fixed to the head should be of low magnetic susceptibility to guarantee optimal imaging quality. The combined use of an open ultra low-field MRI system and intraoperative monitoring allows for resection control and continuous functional monitoring.

Tumor ablation therapy of liver cancers with an open magnetic resonance imaging-based navigation system

As minimally invasive treatments for liver cancers, percutaneous ablation therapies represent a valid alternative to liver resections, especially in patients with poor liver function. Recently, image-guided surgical and interventional procedures using open magnetic resonance imaging (MRI) have been introduced. We performed percutaneous ablation therapy for 51 nodules of liver cancer in 34 patients using a navigation system based on open MRI. During the ablation therapy, the ultrasonography (US) probe, needle, and tumor were displayed on the MR image. Immediately after the procedure, the therapeutic effect was evaluated by open MRI. In all cases, percutaneous puncture into the tumors was successful, even in the case of tumor undetectable by US. Mean fiducial registration error was approximately 3 mm. MR images captured after the procedure clearly demonstrated the ablated area. No mortality or major complications occurred, except for mild hemorrhage, pyrexia, and ascites. We developed a novel navigation system integrating US and MR images using open MRI for percutaneous ablation therapy of liver cancers. The presented system allows a safe and accurate approach to liver cancers, especially certain tumors that cannot be adequately visualized by US, and an evaluation of therapeutic results immediately after the procedures.

Deformation-Space Method for the Design of Biplanar Transverse Gradient Coils in Open MRI Systems

We propose an efficient real-space algorithm for the design of biplanar transverse gradient coils for use in open magnetic resonance imaging (MRI) systems. In our method, each wire arc is represented by a closed contour (Limacon). Using parametric equations, we deform/reshape an ensemble of closed contours in a simple manner, controllable by just a few parameters. These parameters are used to define system rearrangements in the design procedure. We use an iterative optimization procedure to adjust the control parameters in order to minimize cost functions such as gradient homogeneity and inductance. Here, we compare the coil pattern designed by our deformation-space method with a pattern designed by the conventional stream function approach, and we discuss the merit of the new method.

Functional malcentering of the humeral head and asymmetric long-term stress on the glenoid: Potential reasons for glenoid loosening in total shoulder arthroplasty

We tested the hypothesis that functional malcentering of the humeral head during arm elevation exists in patients with glenohumeral osteoarthritis and influences long-term glenoid loading. Twenty-eight healthy volunteers and 10 patients with primary osteoarthritis, 10 with cuff-arthropathy, and 1 with dysplastic glenoid were examined. Open magnetic resonance imaging and 3-dimensional (3D) digital postprocessing techniques were applied in various arm positions. Osteoabsorptiometry was used to determine 3D subchondral mineralization patterns of the glenoid as an indicator of integral long-term stress distribution. At 30 degrees of abduction, 5 patients demonstrated malcentering of the humeral head posteriorly; all patients with cuff arthropathy had malcentering superiorly. At 90 degrees, most patients displayed significant (P < .001) malcentering in the superior and posterior direction. The shoulders showed maximal subchondral mineralization patterns in the direction of malcentering. Most patients with glenohumeral osteoarthritis displayed functional malcentering, which might be responsible for postoperative glenoid loosening in shoulder arthroplasty if not corrected intraoperatively.

Whole-spine dynamic magnetic resonance study of contortionists: anatomy and pathology

Whole-spine magnetic resonance (MR) images were obtained using a cylindrical 3-T MR imaging system in 5 contortionists to assess the pathological changes possibly associated with the practice of contortion. Whole-spine dynamic MR images were obtained using a 1-T open MR imaging system in 2 of these contortionists with the purpose of defining the range of motion (ROM) achieved during extreme contortion. The range of spinal motion in this unique population was then quantified. The study included 5 female contortionists 20-49 years of age. Imaging was performed using open 1-T and cylindrical 3-T high-field MR imaging systems. Data were viewed and analyzed with DICOM-compliant tools. Real-time, dynamic, and standard MR imaging allowed for quantification of the contortionists' ROM. There was a difference of 238 degrees between full spinal extension and full flexion. Three of the 5 contortionists had 4 anterosuperior limbus vertebrae at T-11 and the upper lumbar levels. Whole-spine dynamic MR imaging is a valuable tool for the evaluation of the extreme ROM in contortionists, allowing for the quantification of extreme mobility. The limbus fractures present in 3 of the 5 contortionists is postulated to be due to avulsion on hyperextension. Future research may open the use of whole-spine dynamic MR imaging into such areas as pain management and traumatic spinal injuries.

Diagnosis and Management of Pudendal Nerve Entrapment Syndromes: Impact of MR Neurography and Open MR-guided Injections

Nerve entrapment syndromes involving the pudendal nerve have been challenging for the neurologist and neurosurgeon relative to other nerve entrapment syndromes because of the nature of the presenting symptoms and the technically demanding aspects of specific diagnosis. Further, effective treatment options have been few. Recently however, there have been a series of advances in diagnosis and treatment. Application of magnetic resonance neurography imaging and the use of open magnetic resonance guided injection techniques have greatly improved diagnostic efficacy. New minimal access surgical techniques targeted at various subtypes of pudendal entrapment—greater sciatic notch, ischial spine, obturator internus/Alcock canal, and distal branch have resulted in 87% good to excellent treatment outcomes in a prospective study of 55 patients.

Dosimetric and geometric evaluation of an open low-field magnetic resonance simulator for radiotherapy treatment planning of brain tumours

Background and purpose Magnetic resonance (MR) imaging is superior to computed tomography (CT) in radiotherapy of brain tumours. In this study an open low-field MR-simulator is evaluated in order to eliminate the cost of and time spent on additional CT scanning. Materials and methods Eleven patients with brain tumours are both CT and MR scanned and the defined tumour volumes are compared. Image distortions and dose calculations based on CT density correction, MR unit density and MR bulk density, bone segmentation are performed. Monte Carlo simulations using 4 and 8 MV beams on homogeneous and bone segmented mediums are performed. Results Mean MR and CT tumour volumes of approximately the same size ( V MR ¯ = 55 ± 34 cm 3 and V CT ¯ = 51 ± 32 cm 3 ) are observed, but for individual patients, small intersection volumes are observed. The MR images show negligible distortion within radial distances below 12 cm (<1.5 mm). On unit density mediums, dose errors above 2% are observed in low dose areas. Monte Carlo simulations with 4 MV photons show large deviations in dose (>2%) just behind the skull if bone is not segmented. Conclusions It is feasible to use an MR-simulator for radiotherapy planning of brain tumours if bone is segmented or a careful choice of beam energy (>4 MV) is selected.

Computing Lorentz forces generated by gradient coils in an open MRI system

Magnetic Resonance Imaging requires the use of gradient coils to perturb the main homogeneous static magnetic field. Sequential perturbations in each of the orthogonal cartesian directions results in the magnetic field becoming spatially encoded. The fast switching of the gradient coils produces large Lorentz forces which act on the coils. These unwanted forces dissipate as acoustic noise at sound pressure levels upward of 130dB. We present a method for designing open magnetic resonance imaging coils that produce reduced Lorentz forces whilst still producing highly accurate gradient fields. We show that when gradient fields of approximately 1% are considered, a linearised regularisation solves for the Fourier series coefficients. References R. Turner. A target field approach to optimal coil design. J. Phys. D: Appl. Phys., 19, 1986, pages L147--L151. doi:10.1088/0022-3727/19/8/001 M. A. Brideson, L. K. Forbes and S. Crozier. Determining complicated winding patterns for shim coils using stream functions and the target-field method. Concepts in Magnetic Resonance, 14(1), 2002, pages 9--18. doi:10.1002/cmr.10000 P. Mansfield, P. M. Glover and J. Beaumont. Sound generation in gradient coil structures for mri. Magnetic Resonance in Medicine, 39(4), 1998, 539--550. doi:10.1002/mrm.1910390406 P. Mansfield, B. Haywood and R. Coxon. Active Acoustic Control in Gradient Coils for mri. Magnetic Resonance in Medicine, 46(4), 2001, 807--818. doi:10.1002/mrm.1261 M. A. Brideson, L. K. Forbes and S. Crozier. Winding patterns for actively shielded shim coils with asymmetric target-fields. Measurement Science and Technlogy, 14, 2003, pages 484--493. doi:10.1088/0957-0233/14/4/312 L. K. Forbes, M. A. Brideson, S. Crozier and P. T. While, An analytical approach to the design of quiet cylindrical asymmetric gradient coils in mri. Concepts in Magnetic Resonance Part B. 31B(4), 2007, 218--236. doi:10.1002/cmr.b.20095.