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Chapter 3

03-01
Essentials

The MR Machine
Field Strength
03-02
Magnet Types

Permanent
Resistive
Hybrid
Superconductive
03-03
Shimming

03-04
Magnetic Shielding

03-05
Gradient Coils

Eddy Currents
03-06
Transmitter and Receiver

Regular Coils
Surface Coils
03-07
Radiofrequency (Faraday) Shielding

03-08
Data Acquisition System and Computer

03-09
The Right Choice


03-09 The Right Choice

There are more than 36,000 MR imaging systems operating all over the world. MR systems are available in the well-known tunnel shape (Figure 03-07) or as open systems (Figure 03-14).


Figure 03-14:
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.


The traditional donut-shaped machines allow very high field strengths and thus are well suited for research applications and specific techniques such as BOLD imaging. 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.

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 large patients – and children.

Dedicated systems are aimed at, e.g., orthopedic imaging. Examinations of knees, elbows, hands, and shoulders are the main indications. These systems are smaller and easier to install.

The choice of an MR system might be quite agonizing and depending on hear­say, fashions, money, ego, 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.

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.

In the case of 1.5 Tesla versus 3.0 Tesla one should weigh the points in Table 03-04.




Table 03-04:

Some additional scientific and non-scientific aspects to be taken into account when deciding bet­ween 1.5 and 3.0 Tesla magnet systems. * [⇒ Pattany]


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Murphy's Law is the most reliable guideline:
How to buy an MR machine.

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