The interventional magnetic resonance unit--the minimal access operating theatre of the future?

Temperature mapping of thermal ablation using MRI

MRI is a unique tool for minimally invasive thermal ablation in that it can provide both targeting, monitoring and control during the procedure. Monitoring is achieved by using MRI temperature mapping. In this review the relevant physics is explained as a background to the state-of-the-art methods for computing temperature maps as well as the more cutting edge methods. The review covers both methods to monitor heating and cooling of tissue and explains temperature mapping using Proton Resonance Frequency shift, T1 mapping, diffusion mapping, R2* mapping and thermal models.

Miniature ultrasonic probe construction for minimal access surgery

The increasing use of minimal access techniques for surgery has produced a need for imaging technologies that can be used during such interventions. Ultrasound imaging has the advantage that the probe itself can be interventional. Interventional ultrasound probes must be sufficiently small to gain access to the surgical site, and any rigid portion must be limited in length to permit adequate flexibility. In practice this means the ultrasound probes have to operate at high frequencies, and a set of design curves have been produced which relate the number of elements and the ultrasound frequency to the probe dimensions for both linear and cylindrical array configurations. Constructing high-frequency sub-miniature probes presents a number of technical challenges, in particular relating to interconnects and packaging. Solutions to these challenges are discussed using the fabrication of a 1 mm diameter intravascular probe as an example.

Image-guided surgery for anal fistula in a 0.5T interventional MRI unit

PURPOSE

To determine whether MR-guided anorectal surgery is feasible, and to develop techniques for MR-guided anal fistula surgery.

MATERIALS AND METHODS

Six patients with pilonidal sinus (PNS), and 21 with suspected anal fistulae were operated on in the GE Signa SPIO 0.5T interventional MRI unit. Procedures were performed with magnet-safe Lockhart-Mummery fistula probes. Preprocedural and intra-operative MRI (IOMRI) techniques were used to identify the extent of the fistula tracts and septic foci, and to ensure the adequacy of the surgical procedure.

RESULTS

IOMRI demonstrated the PNS lesions and the adequacy of excision. Imaging failed to demonstrate a fistula in two patients, as confirmed by surgical examination. No images were obtained in one patient due to his size (weight in excess of 100 kg). In 18 patients a fistula tract or abscess was demonstrated and IOMRI was used to assess the adequacy of the surgical procedure. In three patients this demonstrated incomplete drainage of septic foci, which was not obvious on inspection of the surgical field. We believe that in these patients IOMRI prevented an incomplete procedure and the potential requirement for a second operation. Further surgery was performed to rectify this situation. The fistula tract was laid open in 13 patients, and a Seton drain was inserted in five.

CONCLUSION

MRI-guided surgery for anal fistula is feasible. IOMRI demonstrates the exact anatomy of the tracts and abscesses, and confirms that all have been adequately treated. We believe it may become particularly useful in surgery for recurrent and complex anal fistulae, and may lead to fewer recurrences.

Resection of soft tissue sarcomas with intra-operative magnetic resonance guidance

PURPOSE

To report on a preliminary series of magnetic resonance (MR)-guided sarcoma resections.

MATERIALS AND METHODS

Three patients with the skin sarcoma dermatofibrosarcoma protuberans underwent MR-guided resection.

RESULTS

The extent of the tumor at MR imaging was greater in each case than suggested by clinical examination. Adequate resection margins were planned using short Tau inversion recovery (STIR) sequences and intra-operative imaging was used to confirm complete tumor excision. Histological examination confirmed clear surgical margins of at least 1 cm in each case.

CONCLUSION

We believe this technique may reduce the incidence of specimen margins positive for tumor following resection, and consequently reduce the need for further excisional surgery.

MR imaging-guided biopsy and therapeutic intervention in a closed-configuration magnet: single-center series of 361 punctures

OBJECTIVE

The purpose of the study was to evaluate practical experience with interventional MR imaging in a closed-bore 1.5-T imaging system. A total of 361 MR-guided biopsies were performed in 250 patients and analyzed retrospectively. The technique comprised four steps: localization; planning; action (cutting or aspiration biopsy, or instillation of a therapeutic agent), with verification in two perpendicular planes; and obtaining control scans.

CONCLUSION

The mean duration of a biopsy was 21 min; there were no major complications. Image contrast, signal, matrix options, and visibility of needle track and tip position permitted uncomplicated orientation. The interventional MR technique could be applied in any puncture setting.

Validation of estimated 3D temperature maps during hepatic cryo surgery

A simple model for estimating temperature distribution within the frozen region during cryo ablation was tested for accuracy. Freezing experiments were conducted in both ex vivo and in vivo porcine livers. Temperature was measured during freezing using a fiber-optic temperature sensor. Three-dimensional MR images were obtained at the end of each freezing cycle. From the MR image volumes, three-dimensional temperature maps were calculated numerically using a simplified bio-heat model. Estimated temperatures were compared to measured temperatures. The median difference between measured and estimated temperature was 3.03 degrees C. The median distance from a sensor element to the closest point on a isotherm surface with the corresponding estimated temperature was 0.70 mm. The accuracy of this model is acceptable. Temperature maps as outlined here may be used for monitoring of cryotherapy in order to increase clinical effectiveness.

Interventional magnetic resonance imaging

The development of minimally invasive surgical and interventional techniques has created a need for more accurate and sensitive image guidance and monitoring. Magnetic resonance imaging, with its superior soft tissue discrimination and multiplanar facilities, seems the obvious choice for an ideal image-guidance tool. Until recently, the employment of MRI in this role has been prevented by the physical constraints of conventional, closed-configuration machines. The problem has now been overcome by the development of an open design allowing both horizontal and vertical access to the patient in the scanner so that procedures can be performed concurrent with image acquisition. This configuration, together with the use of fast gradient echo sequences which can scan at speeds close to real time, means that a wide range of interventional procedures can be performed with on-line image guidance and monitoring. In addition, the versatility of the open design means that patients can assume physiological positions to allow dynamic joint imaging to be performed. This opens up a whole new field in the understanding of joint pathophysiology. This review article discusses these recent technological developments and their clinical applications. In particular, the potential role in guidance of biopsies, monitoring of thermal ablation techniques and applications in endoscopic surgery is outlined.