Teuvo Vaara

Nerve Root Infiltration of the First Sacral Root With MRI Guidance

The purpose of this clinical trial was to describe the methodology and evaluate the accuracy of optical tracking‐based magnetic resonance (MR)‐guided infiltration of the first sacral (S1) root. Thirty‐five infiltrations were performed on 34 patients with a 0.23‐T open C‐arm magnet installed in a fully equipped operation room with large‐screen (36 inches) display and optical navigator utilizing infrared passive tracking. T1 and T2 fast spin‐echo (FSE) images were used for localizing the target and fast field echo for monitoring the procedure. Saline as contrast agent in single‐shot (SS)FSE images gave sufficient contrast‐to‐noise ratio. Twenty‐four patients had unoperated L5/S1 disc herniation, and 10 had S1 root irritation after failed back surgery. Needle placement was successful in 97% of the cases, and no complications occurred. Outcome was evaluated 1–6 months (mean 2.2 months) after the procedure and was comparable to that of other studies using fluoroscopy or computed tomography guidance. MR‐guided placement of the needle is an accurate technique for first sacral root infiltration. J. Magn. Reson. Imaging 2000;12:556–561.

MR temperature measurement in liver tissue at 0.23 T with a steady-state free precession sequence.

MRI can be used for monitoring temperature during a thermocoagulation treatment of tumors. The aim of this study was to demonstrate the suitability of a 3D steady‐state free precession sequence (3D Fast Imaging with Steady‐State Precession, 3D TrueFISP) for MR temperature measurement at 0.23 T, and to compare it to the spin‐echo (SE) and spoiled 3D gradient‐echo (3D GRE) sequences. The optimal flip angle for the TrueFISP sequence was calculated for the best temperature sensitivity in the image signal from liver tissue, and verified from the images acquired during the thermocoagulation of excised pig liver. Factors influencing the accuracy of the measured temperatures are discussed. The TrueFISP results are compared to the calculated values of optimized SE and 3D GRE sequences. The accuracy of TrueFISP in the liver at 0.23 T, in imaging conditions used during thermocoagulation procedures, is estimated to be ±3.3°C for a voxel of 2.5 × 2.5 × 6 mm3 and acquisition time of 18 s. For the SE and GRE sequences, with similar resolution and somewhat longer imaging time, the uncertainty in the temperature is estimated to be larger by a factor of 2 and 1.2, respectively. Magn Reson Med 47:940–947, 2002.

Percutaneous MR‐guided discography in a low‐field system using optical instrument tracking: A feasibility study

To evaluate the feasibility of MRI‐guided discography with optical tracking.

Registration in Interventional Procedures With Optical Navigator

Performing interventional procedures in the close proximity to an MR scanner widens the range of operations available for an optical tracking system. In order to gain the full benefits from both unrestricted use of surgical instruments outside the magnet and intraoperative imaging, a method for transferring the registration data of the optical navigator between two locations is required. An optical tracking system, which provides such a transfer method and tracks patient position during a surgical procedure, has been developed, tested, and demonstrated with two patient cases. J. Magn. Reson. Imaging 2001;13:93–98.

Pulse repetition time and contrast enhancement: simulation study of Gd-BOPTA and conventional contrast agent at different field strengths.

Objectives:To investigate theoretically enhancement and optimal pulse repetition times for Gd-BOPTA and Gd-DTPA enhanced brain imaging at 0.23, 1.5, and 3.0 T. Methods:The theoretical relaxation times of unenhanced, conventional contrast agent (Gd-DTPA) and new generation contrast agent (Gd-BOPTA) enhanced glioma were calculated. Then, simulation of the signals and contrasts as a function of concentration and pulse repetition time (TR) in spin echo sequence was done at 0.23, 1.5, and 3.0 T. The effect of echo time (TE) on tumor-white matter contrast was also clarified. Three patient cases were imaged at 0.23 T as a test of principle. Results:Gd-BOPTA may give substantially better glioma-to-white matter contrast than Gd-DTPA but is more sensitive to the length of TR. These characteristics are accentuated at 0.23 T. Optimal TR lengths are shorter for Gd-BOPTA than for Gd-DTPA enhanced imaging at all field strengths. TR optimized for Gd-DTPA may thus give suboptimal contrast in Gd-BOPTA enhanced imaging. Higher enhancement with Gd-BOPTA is further accentuated by short TE. Conclusion:Appropriate TRs at 0.23 T appear to be approximately 300 to 400 milliseconds and 250 to 300 milliseconds, at 1.5 T 500 to 600 milliseconds and 400 to 450 milliseconds and at 3.0 T 550 to 650 milliseconds and 475 to 525 milliseconds using Gd-DTPA and Gd-BOPTA, respectively. For Gd-BOPTA enhanced imaging, it seems justified to optimize TR according to contrast and seek options like parallel excitation (Hadamard encoding) for increasing the number of slices and SNR.

Feasibility of Agar‐Silica Phantoms in Quality Assurance of MRgHIFU

Although many phantom types for magnetic resonance guided high intensity focused ultrasound (MRgHIFU) exist, the number of reusable phantoms for quality assurance (QA) purposes is limited. For reliability, the phantom should be structurally and compositionally uniform, and acoustically isotropic. It should also be cheap and easy to produce, and maintain its physical and chemical properties even in long‐term use. Various authors have used water, agar, and silicon‐dioxide (silica) to produce phantoms with ultrasound attenuation coefficient in a range typical of soft tissues. However, their applicability in MRgHIFU use has not been investigated systematically or verified in previous studies. In this study, agar‐gel‐based tissue‐mimicking heating phantom material is optimized and its MRgHIFU‐usability is tested and verified. Acoustic properties of the phantom material with different concentrations of silica were determined experimentally. The ultrasound attenuation coefficient was found to be linearly and pos...

Accuracy and precision of patient positioning for pelvic MR-only radiation therapy using digitally reconstructed radiographs

BACKGROUND AND PURPOSE Magnetic resonance imaging (MRI) has in recent years emerged as an imaging modality to drive precise contouring of targets and organs at risk in external beam radiation therapy. Moreover, recent advances in MRI enable treatment of cancer without computed tomography (CT) simulation. A commercially available MR-only solution, MRCAT, offers a single-modality approach that provides density information for dose calculation and generation of positioning reference images. We evaluated the accuracy of patient positioning based on MRCAT digitally reconstructed radiographs (DRRs) by comparing to standard CT based workflow. MATERIALS AND METHODS Twenty consecutive prostate cancer patients being treated with external beam radiation therapy were included in the study. DRRs were generated for each patient based on the planning CT and MRCAT. The accuracy assessment was performed by manually registering the DRR images to planar kV setup images using bony landmarks. A Bayesian linear mixed effects model was used to separate systematic and random components (inter- and intra-observer variation) in the assessment. In addition, method agreement was assessed using a Bland-Altman analysis. RESULTS The systematic difference between MRCAT and CT based patient positioning, averaged over the study population, were found to be (mean [95% CI])  -0.49 [-0.85 to  -0.13] mm, 0.11 [-0.33 to  +0.57] mm and  -0.05 [-0.23 to  +0.36] mm in vertical, longitudinal and lateral directions, respectively. The increases in total random uncertainty were estimated to be below 0.5 mm for all directions, when using MR-only workflow instead of CT. CONCLUSIONS The MRCAT pseudo-CT method provides clinically acceptable accuracy and precision for patient positioning for pelvic radiation therapy based on planar DRR images. Furthermore, due to the reduction of geometric uncertainty, compared to dual-modality workflow, the approach is likely to improve the total geometric accuracy of pelvic radiation therapy.