MR monitoring of NaCl-enhanced radiofrequency ablations: observations on low- and high-field-strength MR images with pathologic correlation.

PURPOSE: To test the hypothesis that magnetic resonance (MR) imaging can be used to monitor both intraparenchymal injection of NaCl solution and subsequent radiofrequency ablation (RFA) within tissues pretreated with NaCl, report the low- and high-field-strength MR appearance of NaCl-enhanced RFAs, and compare MR findings with pathologic findings. MATERIALS AND METHODS: Ten ex vivo calf liver specimens were injected with saturated NaCl (seven were mixed with methylene blue during MR fluoroscopic monitoring) and reexamined with fast imaging with steady-state progression (FISP), true FISP, reversed FISP (PSIF), and fast spin-echo T2-weighted MR sequences. The NaCl-to-liver contrast-to-noise ratio (CNR) was calculated for various sequences, and CNRs were compared with the Student t test. Distribution on MR images was compared with the results of pathologic analysis. Forty additional in vivo monopolar RFAs were performed in paraspinal muscles of seven minipigs after animal care committee approval (10 standard control ablations, 30 were preceded by direct injection of saturated NaCl at various volumes [3-9 mL] and rates [1 or 6mL/min]). Postablation low-field-strength (n = 20) and high-field-strength (n = 20) MR examinations consisted of T2-weighted imaging, short inversion time inversion-recovery (STIR) imaging, and contrast material-enhanced T1-weighted imaging. Ablation shape, conspicuity, volume, and signal intensity were compared between the two groups and with the results of pathologic analysis. The difference in volumes with and without NaCl injection was evaluated by using two-way analysis of variance. RESULTS: Mean CNR was highest on fast spin-echo T2-weighted images and was significantly higher for PSIF than for FISP (P < .0001) or true FISP (P = .003). NaCl distribution on MR images corresponded with the results of pathologic analysis in ex vivo livers. Interactive in vivo monitoring of NaCl injection and electrode placement was feasible. NaCl-enhanced ablations had irregular shapes, a higher CNR, and significantly larger volumes (F = 22.0; df = 1, 90; P < .00001). All ablations had intermediate or low signal intensity with high-signal-intensity rims on all images. Fluid signals overlaid NaCl-enhanced ablations on fast spin-echo T2-weighted and STIR images, particularly on high-field-strength MR images. CONCLUSION: MR imaging can be used to reliably monitor the distribution of injected NaCl solution in tissues. Interventional MR imaging techniques can be used to guide and monitor RFAs within NaCl pretreated tissues, with good correlation with pathologic results.

A technique for MRI-guided transrectal deep pelvic abscess drainage.

OBJECTIVE: The purpose of this article is to introduce a technique for transrectal drainage of deep pelvic abscesses performed under interactive MRI guidance. CONCLUSION: A new method for triorthogonal image plane MRI guidance was developed and used to interactively monitor the puncture needle on continuously updated sets of adjustable three-plane images. The merits and limitations of the technique are highlighted and the patient population that is likely to benefit from this approach is suggested.

MR-guided biopsy: a review of current techniques and applications.

Biopsy has become a cornerstone of modern medicine and most modern biopsies are performed percutaneously using image guidance, typically computed tomography or ultrasound. MR-guided biopsy offers many advantages over these more traditional modalities, and the recent development of interventional MR imaging techniques has made MR-guided percutaneous biopsies and aspirations a clinical reality. As the field of MR-guided procedures continues to expand and to attract more attention from radiologists, it is important to understand the concepts, techniques, applications, advantages, and limitations of MR-guided biopsy/percutaneous procedures. Radiologists should also recognize the need for their significant involvement in the technical aspects of MR-guided procedures, since several user-defined parameters can alter device visualization in the MR imaging environment and affect procedure safety. This article reviews the prerequisites, systems, and applications of MR-guided biopsy.

Intraoperative MRI with a rotating, tiltable surgical table: a time use study and clinical results in 122 patients.

OBJECTIVE: The objective of our study was to evaluate intraoperative low-field MRI for the frequency and duration of imaging sessions needed during surgery, the direct additional procedure time attributable to imaging, and the proportion of cases in which information provided by intraoperative MRI led to a change in the procedure or otherwise was deemed valuable by operating surgeons. MATERIALS AND METHODS: One hundred twenty-two patients (65 males, 57 females; age range, 6-77 years; mean age, 43.8 years) underwent 130 neurosurgical and ENT procedures (106 craniotomies, 17 transsphenoidal pituitary resections, three biopsies, three intracranial cyst aspirations or injections, and one skull base resection) in a specially designed surgical MRI suite equipped with a 0.2-T imager and a prototype rotating, tiltable surgical table. The intraoperative MR sequences included free induction with steady-state precession (fast imaging with steady-state precession [FISP]), steady-state free precession T2-weighted, reverse fast imaging with steady-state free precession (PSIF), FLASH, spin-echo T1-weighted, turbo spin-echo (TSE) T2-weighted, and TSE FLAIR. Each case was analyzed for the number of imaging sessions, duration of each session, total imaging time during surgery, and impact of imaging information on procedure. RESULTS: Each patient underwent between one and five intraor postoperative imaging sessions. Imaging times were 1.7 seconds-8 minutes 31 seconds per sequence. The mean total imaging time was 35 minutes 17 seconds per surgical procedure. Imaging was continuous during biopsy and cyst aspiration procedures and averaged 200.67 and 54.66 minutes, respectively. Additional surgical resection based on intraoperative imaging findings was performed in 72.8% of the cases. CONCLUSION: Intraoperative low-field MRI provides valuable information for surgical decision making that is predominantly related to detection of residual tumor and the exclusion of complications. The benefits of this technology surpass the time cost associated with its implementation when using proper imaging strategies.

Percutaneous biopsy from blinded to MR guided: an update on current techniques and applications.

The advent of interventional MR imaging techniques as well as their adoption to guide percutaneous biopsies and aspirations has served as a further step along a series of technical refinements that commenced with the implementation of image-guided approaches for tissue sampling. Nowadays, the practice of and the expectations from these procedures are quite different from those of the blind percutaneous thrusts performed in the late nineteenth and early twentieth centuries. As the field of interventional MR imaging continues to flourish and to attract more radiologists who realize the many opportunities that this technology can offer to their patients, there is a need for a full comprehension of the concepts, techniques, limitations, and cost-effectiveness of MR imaging guidance to present this service to clinical partners in the appropriate setting. Radiologists should also recognize the need for their significant involvement in the technical aspects of MR-guided procedures, because several user-defined parameters and trajectory decisions can alter device visualization in the MR imaging environment and hence affect procedure safety.

Radiofrequency thermal ablation: the role of MR imaging in guiding and monitoring tumor therapy.

Performing RFA procedures under MR imaging involves two distinct processes: interactive guidance of the RF electrode into the targeted tumor and monitoring the effect of therapy. The justification for using MR imaging for electrode guidance is quite similar to its use to guide biopsy and aspiration procedures, where MR imaging offers advantages related to superior soft tissue contrast, multiplanar capabilities, and high vascular conspicuity that facilitate safe and accurate guidance in selected lesions. The major contribution of MR imaging to thermal ablation procedures is its ability to monitor tissue changes associated with the heating process instantaneously, an attribute that is not paralleled by any other currently available imaging modality. Such ability facilitates a controlled approach to ablation by helping to detect inadequately treated tumor foci for subsequent interactive repositioning of the RF electrode during therapy. As such, MR imaging guidance and monitoring enable treatment of the entire tumor on a single-visit basis while avoiding undue overtreatment and preserving often critically needed organ function. Although knowledge of interventional MR imaging concepts and familiarity with its technology and with the related safety issues are indispensable for interventional radiologists attempting thermal ablation procedures in the MR imaging environment, understanding the tissue basis of necrosis imaging is becoming an essential part of the knowledge base for the larger sector of general radiologists who are required to interpret the follow-up MR imaging scans of the increasing number of thermal ablation patients.

MR imaging follow-up after percutaneous radiofrequency ablation of renal cell carcinoma: findings in 18 patients during first 6 months.

PURPOSE: To prospectively evaluate the magnetic resonance (MR) imaging findings seen within the first 6 months after radiofrequency (RF) thermal ablation of renal cell carcinoma (RCC). MATERIALS AND METHODS: After providing written informed consent, 18 patients (17 men, one woman; mean age, 71.2 years) with RCC underwent MR imaging-guided percutaneous RF thermal ablation, which was performed by using protocols approved by a comprehensive cancer center protocol committee and the institutional review board for human investigation. The study was Health Insurance Portability and Accountability Act compliant. Follow-up unenhanced T2-weighted MR images and unenhanced and gadolinium-enhanced T1-weighted MR images were acquired immediately, 2 weeks, 3 months, and 6 months after ablation. Thermal ablation zone size was analyzed, and contrast-to-noise ratios (CNRs) were calculated from the signal amplitudes of the thermal ablation zone, perirenal fat, and normal renal cortex on the MR images. Statistical analyses were performed by using the paired Student t test. P < .05 was considered to indicate statistical significance. RESULTS: The mean follow-up time was 16.1 months (range, 6.0-41.2 months). The mean sizes of the thermal ablation zones were 6.8, 7.0, 6.1, and 4.7 cm2, respectively, at immediate, 2-week, 3-month, and 6-month follow-up MR imaging examinations. Thermal ablation zones were uniformly hypointense and had a surrounding bright rim on T2-weighted images and were predominantly hyperintense on T1-weighted images. Thin rim enhancement with central hypointensity was noted on the gadolinium-enhanced images. Gadolinium-enhanced T1-weighted and unenhanced T2-weighted MR images showed significantly higher CNRs than unenhanced T1-weighted MR images. Residual tumor was detected after RF thermal ablation in two cases and was best seen on unenhanced T2-weighted and gadolinium-enhanced T1-weighted MR images. CONCLUSION: After initially increasing in size within the first 2 weeks, renal RF thermal ablation zones involuted during the remainder of the MR imaging follow-up period.

Phase II clinical trial of interactive MR imaging-guided interstitial radiofrequency thermal ablation of primary kidney tumors: initial experience.

PURPOSE: To perform a phase II clinical trial to evaluate efficacy and safety of interactive magnetic resonance (MR) imaging-guided radiofrequency (RF) interstitial thermal ablation (ITA) of primary renal tumors. MATERIALS AND METHODS: Ten male patients (age range, 25-83 years) with peripheral renal cell carcinoma and contraindications to surgery were treated with percutaneous RF ITA entirely guided and monitored with a 0.2-T MR imaging unit. By using a 200-W RF ablation system and custom-fabricated MR imaging-compatible cool-tip electrodes, pulsed RF current was applied for single or multiple ablation cycle(s) of 12-15 minutes until the entire tumor was replaced by an enlarging zone of low signal intensity on T2-weighted and/or short inversion time inversion-recovery images acquired intermittently during the procedure. Kidney MR images were acquired before, immediately after, and 2 weeks after ablation and then every 3 months for 1 year and every 6 months thereafter. Intra- and postprocedural complications were assessed with clinical evaluation of patients for pain and hemodynamic instability and evaluation of MR images for evidence of hemorrhage or other unexpected findings. Follow-up images were assessed for delayed complications such as renal ischemia, infarct, urinoma, or tumor recurrence. RESULTS: Treated tumors ranged between 0.63 and 16.90 mL in volume and 1.0 and 3.6 cm in maximum diameter. Successful RF electrode insertion and/or repositioning into the renal mass was achieved in all cases with direct MR "fluoroscopic" guidance. Thirty ablation cycles were conducted at 21 electrode positions in the 10 procedures, and complete ablation, as defined with MR imaging, was achieved in all cases by the end of the procedure. Apart from two small self-limited perirenal hematomas, no intra- or postprocedural complications were observed. No delayed complications or tumor recurrence occurred during a mean follow-up period of 25 months +/- 9.4 (standard deviation). CONCLUSION: Although these results are preliminary, interactive MR imaging-guided RF ITA for treatment of primary renal tumors has a high success rate.

Sonographic evaluation of first-trimester bleeding.

Vaginal bleeding is a leading cause of presentation for emergency care during the first trimester of the pregnancy. Clinical assessment of the pregnancy outcome at this stage is less reliable. US examination is crucial in establishing IUP and early pregnancy failure and to exclude other causes of bleeding, such as ectopic pregnancy and molar pregnancy. Diagnosis of a normal IUP at this stage not only assists the physician in an expectant management, but also gives a psychologic boost to the patient. With recent advances in US technology and the availability of high-frequency transvaginal transducers, reliable diagnosis of early pregnancy failure can be made even before the embryo is visible.

Percutaneous MR imaging-guided radiofrequency interstitial thermal ablation of tongue base in porcine models: implications for obstructive sleep apnea syndrome.

PURPOSE: To test the feasibility and safety of a percutaneous magnetic resonance (MR) imaging-guided technique for radiofrequency (RF) interstitial thermal ablation of the tongue base and to correlate MR appearance of induced thermal lesions with histopathologic findings in pigs in acute and chronic porcine models. MATERIALS AND METHODS: A 1-cm-tip RF electrode was inserted percutaneously into the tongue in 10 pigs with 0.2-T real-time MR guidance. The RF electrode was advanced up the midline between lingual arteries and stopped short of tongue mucosa. RF interstitial thermal ablation was performed at 90 degrees C +/- 2 and lasted 10 minutes. Postablation images were obtained with a 1.5-T MR imager. Five pigs were sacrificed immediately (acute model), while five were followed up for 1 month (chronic model) before they were sacrificed. MR-compatible fiducial coils were inserted into tongues with MR imaging guidance prior to RF ablation in the chronic group. Tongues were harvested for histopathologic analysis. Mean thermal lesion volume was compared with the Student t test on images obtained immediately, 2 weeks, and 1 month after RF ablation. Interclass correlation coefficients of lesion diameters at gross pathologic analysis and corresponding diameters with each pulse sequence were calculated. RESULTS: Successful MR imaging-guided electrode positioning was achieved in all procedures without intra- or postprocedure complications because there was high vascular conspicuity and tissue contrast. Thermal lesions appeared hypointense with hyperintense surrounding rims with all sequences in both groups. At pathologic analysis, acute lesions appeared as pale necrotic areas surrounded by hyperemic rims, while chronic lesions demonstrated progressive circumferential fibrosis and significant volume shrinkage (P <.01). Thermal lesion diameters measured at gross pathologic analysis best agreed with corresponding diameters measured on short inversion time inversion-recovery images (interclass correlation coefficient = 0.85). CONCLUSION: The results of this investigation demonstrate MR imaging-guided RF interstitial thermal ablation of the tongue base is feasible and safe and illustrate imaging and pathologic phenomena associated with creation and evolution of the induced thermal lesions.

MR imaging-guided radio-frequency thermal ablation of the lumbar vertebrae in porcine models.

PURPOSE: To test the hypotheses that (a) magnetic resonance (MR) imaging-guided radio-frequency (RF) thermal ablation of the vertebrae is feasible in porcine models, (b) procedure safety depends on the location of ablation within the vertebra, and (c) MR imaging allows accurate monitoring of induced thermal lesion size and shape. MATERIALS AND METHODS: Ten percutaneous MR imaging-guided RF thermal ablations were randomized over various lumbar vertebral levels and locations in seven pigs. Animals were followed up for 2, 7, or 14 days before sacrifice. Thermal lesion size and shape as measured on MR images obtained immediately after ablation and at follow-up were compared with gross pathologic findings. Mean absolute differences between lesion diameters at pathologic examination and MR imaging were evaluated by using a paired t test, as were differences between lesion-to-vertebra contrast-to-noise ratios obtained for each sequence. Clinical and imaging data were correlated with histologic findings. RESULTS: Successful RF electrode placement in the targeted part of the vertebra was achieved in all procedures. Ablations performed away from neural elements were safe to perform. Pedicular ablations resulted in radiculopathy, whereas ablations performed directly over the posterior cortex resulted in paraplegia. Lesion sizes measured on T2-weighted images were closest to those measured at gross pathologic examination (mean absolute difference, 0.72 mm +/- 0.83 [SD]), followed by those measured on contrast material-enhanced T1-weighted (1.27 mm +/- 0.83) and short inversion time inversion-recovery (STIR) (1.5 mm +/- 1.84) images. Size measurements obtained on T2-weighted images were significantly closer to gross pathologic measurements than were those obtained on contrast-enhanced T1-weighted images (P =.013) but were not different from those obtained on STIR (P =.27) images. The contrast-to-noise ratio was significantly higher for contrast-enhanced T1-weighted images than for T2-weighted (P <.001) or STIR (P <.001) images. CONCLUSION: MR imaging-guided RF thermal ablation of the vertebrae is feasible in porcine models, but the safety of the procedure depends on the location of ablation within the vertebra. MR imaging allows accurate monitoring of thermal lesion size and shape.