System Requirements

MR imaging offers advantages when compared with other imaging modalities utilized for musculoskeletal interventions. It has a superior soft-tissue contrast, the possibility to choose nonstandard imaging planes, and the lack of ionizing radiation. MR also has certain disadvantages—limited workspace and patient access (in a closed bore magnet system) and limited temporal or spatial resolution (in an open MR system). Principally, there are three basic types of MR systems commercially available which may be used for interventional procedures.

Vertical, open, low-field scanners (open C-arm configuration) (Fig. 1A) are operated between 0.064 and 0.7 T and allow a lateral approach to the patient (19). Because the signal-to-noise ratio (SNR) depends directly on the field strength, acquisition time is longer for many diagnostic imaging sequences as compared to higher field strength magnets. However, specific fast imaging techniques such as steady-state free precision, for example, True FISP, have been implemented on these scanners.

Open midfield scanners, such as that manufactured by General Electric (Fig. 1B), consist of two vertically positioned super-conducting magnets, allowing a workspace of about 60 cm between the magnets (20). Still, the SNR is inferior as compared to closed-bore, super-conducting, high-field systems. In addition, the homogeneity of the magnetic field is also inferior as compared to closed systems (e.g., ± 7.5 ppm in a 30 cm sphere for the Signa SP ). Moreover, these large scan rooms are difficult to shield, and because the magnets weigh eight tons, sitting is quite difficult.

Short-bore, high-field system combined with X-ray fluoroscopy (hybrid system) is also utilized. These types of magnets are those typically used for "conventional" MR imaging. Modern systems offer a gantry space of about 60 cm. If the construction of the patient table is modified so that it can be moved and repositioned exactly during a procedure, these scanners

FIGURE 1 (A) Open low-field system, with in-room console, monitors, and in-room chair (Siemens Magnetom Open). (B) Open mid-field system (gradient echo Signa SP). (C) Hybrid system, with installation of a 1.5 T system, together with a fluoroscopy unit in one room (Philips ACS NT and BC 212 fluoroscopy unit).

FIGURE 1 (A) Open low-field system, with in-room console, monitors, and in-room chair (Siemens Magnetom Open). (B) Open mid-field system (gradient echo Signa SP). (C) Hybrid system, with installation of a 1.5 T system, together with a fluoroscopy unit in one room (Philips ACS NT and BC 212 fluoroscopy unit).

may be used for interventional procedures (21-23). The only restriction for interventions is the limited space within the gantry, which may make an intervention in obese patients impossible. These magnets offer advantages of high-field MR, including various contrast preparation sequence techniques and ultrafast (real-time) imaging capabilities. The fluoroscopy system needs dedicated shielding to prevent image distortion by the residual magnetic field when the patient is moved to the fluoroscopy system. The center of the fluoroscopy system has to be outside the 5 Gauss line. Usually the electronics of the fluoroscopy system are switched off when the patient is moved via the floating table top into the magnet.

More recently, a "true hybrid" interventional system has been released (24). In this system a flat panel radiographic detector has been integrated in the patient table of an open Signa SP system, allowing MR acquisition and generation of X-ray images without patient repositioning.

The appropriate choice of the magnet site depends primarily on the frequency in which interventional procedures are performed. A closed-bore, high-field whole-body scanner combined with a fluoroscopy system may be a good overall compromise, because it can be used not only for interventions, but also for high quality whole-body applications. In many circumstances, a conventional short-bore, high-field strength scanner without fluoroscopy can be utilized. For sites with larger volumes of procedures, an open, lower field strength scanner is adequate. For the highest volume sites, where the scanner is combined with an operating room, the double doughnut gradient echo (GE) is a good system.

Some modifications of the MR suite are required for interventional procedures. Firstly, a communication system between the magnet room and the main console of the scanners, which is usually outside the scan room, is required. In addition, it is helpful if some of the basic functions are controllable from the scan room, either using a foot pedal or a button on the magnet casing, in order to view a previously performed sequence. Furthermore, in-room monitors are essential to allow direct viewing after acquiring an image set or a single image. These are often ceiling mounted. As mentioned above, the MR table may be modified so that smooth and exact positioning of the patient is possible. The exact table position should be indicated on both the room console and directly on the magnet bore casing. For interventional purposes, either the body coil or dedicated surface coils may be used.

Because the majority of musculoskeletal interventions can be performed under local anesthesia, general anesthesia is rarely required. However, because it may be necessary in some patients (e.g., in pediatric interventions), outlets through the shielding for medical gases are required. In addition, MR-compatible anesthesia monitoring equipment has to be available for anesthetized patients.

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