TwinTree Insert

03-07 The Right Choice

here are at least 50,000 MR imaging systems operating all over the world, most likely more. MR systems are available in the well-known tunnel shape (Figure 03-08) or as open sys­tems (Figure 03-15).

The traditional donut-shaped machines allow very high field strengths and thus are well suited for research applications and specific techniques. Many of these ma­­chi­nes are not very com­fortable for patients and an­noy­ingly, partly dangerously loud; the noise of gradient switching can sound like a jackhammer.

During the last two decades machines with larger bo­res have been developed to lower the rate of claus­trophobia and allow studies of obese pa­tients.

Figure 03-15:
Open MR systems. The magnet is se­pa­rat­ed into two sec­tions above and below or left and right of the pa­tient couch.
Top: Diagram. Center: 1.2 Tesla commercial system. Bottom: 0.5 Tesla commercial system with MgB2 coils.

Open systems reduce patient claustro­phobia and are better suited for interven­tional purposes because there is access to the patient from all sides. Open systems are avail­able between 0.3 T with permanent magnets to 1.5 T with su­per­con­duc­ting mag­nets. Recent developments focus on machines between 0.5 and 1.2 T. They in­­clu­de equipment with MgB2 coils.

Many open systems are quiet, offer more space for handicapped and stout pa­tients — and children.

Dedicated systems are aimed at, e.g., or­thopedic imaging (Figure 03-15 bottom). Exa­mi­na­tions of knees, elbows, hands, and shoulders are the main indications. These systems are smaller and easier to install.

spaceholder redThe choice of an MR system might be quite agonizing and depending on sub­ject­ive reasons, such as hear­say, fashions, money, ego (of both owners and cli­en­tèle), character strength, and politics. Field strength is one, but not the only and most im­por­tant, parameter; higher field strength does not necessarily guarantee a better quality system or better diagnostic outcome for the patient. Image quality can be worse; the intensity of arti­facts scales with field strength.

Beware of weasel words such as "increased diagnostic confidence score"; they are not objective measurements but sales jargon.

Needs must be carefully assessed in the choice of an MR machine. The quality of the component parts, both in terms of hard­ware and software, makes a con­si­der­able im­pact on the equip­ment. Service, mainte­nance, and know­led­ge of how to run the system are of pivotal importance for image quality and assessment. After soft­ware failures, gradient problems are the main source of downtime of MR systems.

When contemplating the purchase of an MR imaging machine thought­ful con­si­de­ra­tio­n must not only include price, service costs, and reimbursement struc­ture, but also other aspects that are involved in the cost versus benefit cal­cu­la­tion, for in­stance the most likely indications on the user’s side (Figure 21-01).

In the case of 1.5 Tesla versus 3.0 Tesla, one should weigh up the points in Table 03-03. It is unclear whether or not patients will receive different clinical ma­na­ge­ment or experience different health outcomes when examined on an ultra­high field machine [⇒ Wood 2012].

Never forget: Most clinical questions can be easily answered with a 0.5 Tesla ma­chine.

Table 03-03:
Some additional scientific and non-scien­tific aspects of 1.5 and 3.0 Tesla magnet systems [Modified from: Pattany 2004].