Magnetic Resonance Imaging (MRI), Low-Field

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guidance in minimally invasive neurosurgery. Intraoperative MRI is an imaging tool that can be usedeffectively for navigation with an attached instrument tracking device because a low-field scanner can beturned off during surgery. MRI is increasingly used for guiding, monitoring and controlling percutaneousprocedures and surgery. Open configuration MRI scanners on which one side is usually open for patientaccess are obviously more suited to interventional procedures than closed systems. Although there istrade-off in image quality towards less resolution due to open structure of these systems, the imagequality of low-field scanners is, however, sufficient for interventional use (Blanco, et. al., 2005).In a review article, Martin et al. (2005) stated that recent development of open-configuration MRIscanners—which have allowed improved patient access, near real-time imaging, and more available MRIcompatibleequipment—has opened up an entirely new area of image-guided surgical and interventionalprocedures. The use of intraoperative MRI-guided therapy has continued to grow since initial reportspublished in the 1990s. Intraoperative MRI has been reportedly used in hepatic tumor ablations,endometrial treatment, sarcoma resection, perirectal disease, and most commonly in neurosurgicalprocedures.Samdani et al. (2005) used a 0.12 T open-design MR scanner in performing pediatric neurosurgery totreat 20 patients with astrocytoma, craniopharyngioma, cortical dysplasia, and hydrocephalus. Theauthors concluded that in addition to neuronavigation, the MR system provided information on extent ofresection, real-time guided catheter placement, and avoided complications. Nimsky et al. (2002) used a0.2 T open-design MR scanner to perform neurosurgery in 310 patients. There were no adverse effectsdue to intraoperative MRI. Image quality was sufficient to evaluate the extent of the tumor resection in themajority of cases. The main indications for intraoperative MRI were the evaluation of the extent of aresection in glioma, ventricular tumor, pituitary tumor, and in epilepsy surgery. The authors stated thatintraoperative MRI offers the possibility of further tumor removal during the same surgical procedure incase of tumor remnants, increasing the rate of complete tumor removal. Schwartz et al. (1999) used a0.5 T open-design MR scanner to perform 200 intracranial surgical procedures. The authors concludedthat intraoperative MRI was successfully implemented for a variety of intracranial procedures andprovided continuous visual feedback, which can be helpful in all stages of neurosurgical interventionwithout affecting the duration of the procedure or the incidence of complications. They noted their systemhas potential advantages over conventional frame-based and frameless stereotactic procedures withrespect to the safety and effectiveness of neurosurgical interventions.Low-field MR scanners were used to guide or perform the following: hepatic tumor ablation (Martin, et al.,2005), transgluteal-approach prostate biopsies (Zangos, et al., 2005), microwave thermocoagulationtherapy to treat metastatic liver tumors from breast cancer (Abe, et al., 2005), abdominal biopsy onpatients in whom ultrasound (US)-guided abdominal biopsy was not possible because the lesion was notvisualized in US or an US-guided procedure was not considered safe (Kariniemi, et al., 2005),percutaneous cryosurgery for invasive breast cancer (Morin, et al., 2004), percutaneous transthoracicfine-needle aspiration biopsy of lung masses (Sakarya, et al., 2003), discography (Sequeiros, et al.,(2006), transvaginal cryotherapy for treating intramural, submucosal, and sub-serosal uterine fibroids(Dohi, et al., 2004), and sacro-iliac joint arthrography in patients with low-back pain (Ojala, et al., 2001).Musculoskeletal: MRI of the musculoskeletal system is performed most commonly using 1.5 T scanners.In the previous decade much vendor research and development went in to the optimization of low-fieldand open scanners. In the last 5 years, as the focus of MRI became less anatomic there has been apronounced trend to high-field strengths; however, higher-field systems of 3 T are becoming more widelyavailable in clinical practice, and developments with 7 T and high-fields are ongoing (Cunningham, et al.,2006).Knee: In a randomized trial of 189 patients with recent knee injury, Oei et al. (2005) utilized a 0.2 Textremity-dedicated MR scanner to assess the predictive value of using the low-field MRI in a
in the initial stage after knee trauma is limited. They noted that a short MRI, in addition to or instead ofradiography, improved the prediction of the need for additional treatment in patients with traumatic kneeinjury but did not significantly aid in the identification of patients who can be discharged without furtherfollow-up.Oei et al. (2003) conducted a literature review and performed a metaanalysis including 29 articles todetermine the diagnostic performance of all-strength MRI of the menisci and cruciate ligaments and toassess the effect of study design characteristics and magnetic field strength on diagnostic performance.Oei et al. stated that higher magnetic field strength improved the diagnostic performance for ACL tears.Cotton et al. (2000) used a 0.2 T scanner to compare the diagnostic efficacy of low- and high-fieldstrengthMR imagers in the diagnosis of anterior cruciate ligament tears and meniscus tears in 219patients with suspected internal derangement of the knee. Selection of patients for surgery wasperformed using only the data from the 1.5-T magnet. MRI scans were interpreted by musculoskeletalradiologists in a hospital setting. Therefore in 90 patients, using arthroscopy findings as the standardreference, the authors found no significant difference in diagnostic performance between low and highfield-strengthMR imaging of the knee. Several points should be noted: Gradient-echo vs. spin-echosequences were used with both MRI. Results might have been different if spin-echo T2-weightedsequences had been used for both. Also, the low-field scan took 15 minutes longer than 1.5 scan. Inaddition, the two reviewers were musculoskeletal radiologists.Rand et al. (1999) noted that qualitative evaluation of the level of confidence in decision making wassignificantly superior on high-field strength images. Boxheimer et al. (2006) prospectively evaluated ifkinematic MRI of the knee can demonstrate displacement of menisci with tears and characterizedisplaceable and non-displaceable meniscal tears in conjunction with the patient’s level of pain. Thisstudy did not address any value of positional low-field MRI compared to conventional MRI or impact topatient outcomes.Shoulder: Ghazinoor and Crues (2006) state that a handful of studies have been performed comparinghigh-field versus low-field MRI of the shoulder. Most have demonstrated no significant difference insensitivity and specificity in detection of rotator cuff and labral lesions. They note their subjectiveexperience is in concordance with many studies demonstrating the high diagnostic value in the use oflow-field scanners in musculoskeletal pathology. In general, the experience and training of the reader islikely to impact the interpretation of these images. MRI performed on extremity scanners would beperformed best by musculoskeletal-trained radiologists with experience reading on low-field systems,working closely with referring orthopedic surgeons (Ghazinoor and Crues, 2006).Zlatkin et al. (2004) retrospectively assessed a 0.2 T extremity-dedicated MR scanner in 160 patients withsuspected tears of the rotator cuff and glenoid labrum, compared with surgical findings. Surgical findingsdemonstrated rotator cuff tears in 131 patients and labral tears in 60 patients. For the rotator cuff, thesensitivity, specificity, positive predictive value, and negative predictive value were 90%, 93%, 98%, and68% (two false positives and 13 false negatives), respectively. For the labrum, the sensitivity, specificity,positive predictive value, and negative predictive value were 55%, 100%, 100%, and 82% (24 falsenegatives), respectively. This study was small and retrospective in design and did not include acomparison to conventional MRI.In a review article, Kreitner et al. (2003) summarized experiences with low-field MR arthrography of theglenohumeral joint with respect to image quality and diagnostic accuracy in detecting labral and rotatorcuff lesions. “Up to now, there has been only a limited number of studies available dealing with low-fieldMR arthrography of the glenohumeral joint. They reveal that, despite a minor image quality in comparisonwith high-field imaging, low-field MR arthrography of the shoulder allows for sufficient evaluation of intraandextra-articular structures in the detection of major abnormalities such as glenohumeral instability orrotator cuff disease. Furthermore, open-configured MR scanners enable kinematic studies: besides theanalysis of normal motion, pathological findings in patients with instabilities and impingement syndromecan be delineated. They further offer the possibility for performing MRI-guided arthrography of theshoulder” (Kreitner, et al., 2003).Page 5 of 17Coverage Position Number: 0444
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