Tour Of An Mri Facility

Published Date: 02 Nov 2017

11.1 Introduction

At the very minimum an MRI facility consists of the MRI suite, the console area, a computer and hardware room, a control room and the cryogen storage area. The filming area is usually part of the console area. In larger hospitals and clinical centers, there is often a separate room or area where an independent console or workstation physically exists that allows radiographers and radiologists to independently view, film, and process medical images.

The MRI suite is also known as the scanner room; it houses the MRI system for the clinical exams, the patient table, and a storage cabinet with all radiofrequency coils, and other accessories (ECG leads, phantoms, trigger or other auxiliary devices) necessary for conducting the MRI exams. In this room there are also penetration panels (metallic wall panels) that allow important electrical connections to be established with the electronics room (provides connections of preamplifiers to receivers, radiofrequency amplifier to radiofrequency (RF) coils, paths for DC bias voltages etc). The console area typically consists of the main computer that operates the scanner, and the processor for reconstruction. Often found in this area are archive devices (optical, magnetic, or digital) and media (tapes, optical or other re-writable media) for storing data. The universal image format utilized by most commercial scanners for medical images is the DICOM format. A film camera, a processor, and a view box complement this area. In some cases, networking hardware exists to allow computer networking, transfer of images, and to control shared cameras.

The other support area specific to the MRI suite is the cryogen storage area. This area is designed to meet specific construction and engineering criteria to allow adequate venting, thereby preventing excessive helium and nitrogen build up or to allow release of vaporised gases in the case of evaporation or emergency release (quench ventilation pipes). Other support areas that complete the center include a reading room, remote viewing areas, dressing rooms and a general waiting area for patients.

There are also stringent structural and environmental requirements that are considered carefully in the design and construction of an MRI suite to take into account magnet weight, vibrations, radiofrequency (RF) shielding, considerations for electrical or magnetic interference from external sources, electrical grounding, monitoring and control of temperature, humidity, and air quality. There is also advanced planning for water and sewer systems. Necessary for the proper functionality of the MRI system include systems that allow oxygen monitoring and alarm systems, quality of air, and adequate ventilation of the patient area within the magnet. An emergency quench mechanism is also available.

Communication systems are also carefully designed to allow intra-center communications, communications with the patient during scans, and to avoid interference of any kind with RF transmission and reception.

11.2 Hardware

The static magnetic field is oriented horizontally (Figure 11.3) and it defines the z-axis. The bore has a size of approximately 0.4 � 0.6 m to accommodate the patient comfortably. The magnet constitutes the most expensive aspect of the magnet.

The three sets of gradient magnetic fields must be positioned within the bore (Figure 11.2) and are driven by three separate drivers (these highly specialized audio amplifiers). The RF coils produce and receive the RF fields (RF transmitter and receiver). The bursts of RF energy are generated by a crystal oscillator and the pulse shaper and then amplified and delivered to the coils via the RF transmitter cabling. The weak RF signals are picked up by the same or other RF coils. After amplification and demodulation of the signal by the RF receiver, the NMR signal is sampled, digitized and entered into the main computer. The timing and other characteristics of the gradient fields and RF pulses are determined by the pulse programmer, a process that is under the computer control. When the computer is not involved with generating and acquiring resonance data, it performs the reconstruction, analysis, and processing of images for display.

11.2.1 Instrumentation-Magnets

Typically three types of magnets are used in clinical and research MRI scanners (as described analytically in a prior section):

a. Permanent magnets

b. Electromagnets

c. Superconducting magnets

11.2.2 Permanent Magnets

Large blocks of ferrous metal are used to generate the magnetic field. Unlike the other types, the main field of the permanent magnet is aligned vertically. Such magnets are largely maintenance-free and consume no electric power or cryogens (i.e. liquid helium and nitrogen), but their disadvantage is that they are capable of producing only relatively low fields (up to approximately 0.4T).

11.2.3 Electromagnets

Most systems are built around a superconducting magnet or an electromagnet. Both types use electrical current to generate a uniform field.

An adequate field is produced by several pairs of large-diameter coils of wire known as Helmholtz coils. The primary field is made as homogeneous as possible by moving about shim plates of steel (passive shimming) and by adjusting the small currents in various additional shim coils which produce slight field corrections (active shimming).

So for an electromagnet, copper or aluminium conductors are wound on an aluminium frame with a bore diameter of about 1 meter within which the gradient field coils and the RF coils are fitted (Figure 11.2). An inner bore of about 40-60 cm is left for the patient. Conductor Joule heating occurs so the conductors are hollow to allow circulating cooling water to cool them down.

11.2.4 Superconducting Magnets

Most modern machines use a superconductor. A superconducting magnet exploits the significant reduction of electrical resistance in some materials at very low temperatures (e.g. Niobium-Titanium alloy at -253oC). Once the current is initiated, there is no further need for power input, nor there are any conducting losses.

To maintain the superconducting condition, the entire coil must be immersed in liquid helium (approximately -269oC) contained within a cryostat (helium dewar vessel), i.e. a stainless steel vacuum insulated cryostat which is filled with liquid helium. A surrounding refrigerator prevents warming of helium from the outside sources. Other systems use both liquid nitrogen (boiling point of approximately -196oC) to prevent heat inflow to the helium container. Helium boils off slowly so it has to be replenished every few months and so does liquid nitrogen (even more frequently). The high cost associated with the MRI unit is mostly due to the manufacture of the magnet/cryogen assembly and its power consumption.

Other critical issues for the performance of the system are the magnet stability (which needs to be better than 0.1 parts per million per hour) and the field homogeneity. Filed homogeneity is often defined as:

where ,represent the maximum and minimum field strengths of the field (determined accurately through field mapping) and the nominal field strength of the scanner. The homogeneity in a typical MRI unit is approximately 10 ppm over the central 50 cm bore region. Presence of any inhomogeneities in the field of view (FOV) often leads to image distortion and diminution of image spatial resolution.

11.2.5 Gradient Coils

Gradient coils are also situated within the bore of the magnet and they generate three independent sets of fields superimposed on the main static field. Such fields are spatially varying and much weaker than the main static field, ranging typically from about 1-20 mT/m for clinical imaging. The gradient sets are usually magnetically decoupled from the main magnet through the use of a shield. Critical to the performance of the gradient sets for some imaging exams are also their slew rates (rate of change of gradient strength per unit time), rise and fall times (times to attain a certain percentage of the maximum or minimum gradient strength).

NMR microscopy employs fast and much more powerful gradient sets than those of a typical system to achieve smaller field of views (FOVs) and finer spatial resolution (the voxel dimensions decrease by a factor of 100 and a resolution of about 10 um can be achieved).

Remarkably, patients and observers are often surprised from the pulsation of the gradients and relevant noise. Such effects are the result of increased Lorentzian forces exerted on the conducting wires of the gradient coils. Ultra-fast imaging techniques, such as Echo Planar Imaging (EPI) utilize gradient sets that can generate very high slew rates, and ultrasmall rise and fall times.

11.2.6 Radiofrequency Transmission and Reception

Apart from the computer, the other major hardware components are the RF coils and electronics. The latter generate and apply the pulses at the Larmor excitation frequency and then detect the NMR signal. The implementation of this system is almost identical to a basic communication system destined for amplitude modulation (AM) broadcasting. Instead of an audio modulator, however, a pulse shaper is used to form all kinds of pulses of precise and highly specialized shapes, typically in the range of 10-64MHz (for imaging up to clinical field strengths at 1.5T). Use of the gradients for spatial/frequency encoding imposes the use of a carrier signal which is not monochromatic but rather contains a band of frequency components, to cover the entire spread of the spin frequencies about the Larmor frequency. It is essential that the RF generator, amplifiers, and other electronic equipment have a bandwidth sufficiently broad to cover the entire range of useful frequencies but narrow enough to exclude most extraneous signals and most importantly, noise. Another added complexity associated with NMR signals (as compared to AM) is that the information on the amplitude, phase and frequency of the detected signal must be stored to be processed. Once again, RF excitation occurs via an RF transmitter which produces a weak field perpendicular to the main static field. The RF signal is then shaped (via computer control), amplified and then transmitted to the RF coil. The excitation-detection process can take place via the same RF coil through the use of either surface or volume or surface/volume excitation. Choice of an appropriate configuration mainly depends on the imaging protocol in hand, but a common choice involves the use of the build-in volume coil for excitation and of a surface coil for reception. The reason for such a choice follows from the fact that the volume coil is best at producing a highly homogeneous excitation field throughout the FOV and the fact that the surface coil possesses superior Signal-to-Noise (SNR) performance over other types of detection probes, particularly at superficial areas from its set location.

In summary, the system consists of a powerful superconducting magnet, three sets of gradients (shielded from the static field), driven by three gradient drivers, and RF transmitter and receiver attached to the appropriate RF probe(s). All of these pieces of equipment are controlled by the main computer. The entire magnet room is built within a Faraday cage (magnetic shield) to prevent interference from external signal sources and also prevent leakage of RF outside the room.

11.3 Imaging

The use of gradients allows imaging at different imaging slice orientations (axial, saggital, and coronal as well as oblique and double oblique). This feature in addition to the excellent soft tissue contrast it produces, renders MRI superior to Computer Tomography (CT) in many respects (which is confined to axial slices only). Typically, imaging at any plane is possible after positioning the patient and land-marking. This defines the isocenter point of the three-dimensional coordinate axes of the imaging setup. Prescription of a slice at any arbitrary orientation follows by defining a 3x3 rotation matrix that contains coefficients of the gradient components (Gx, Gy, Gz) to appropriately select (and subsequently excite) the prescribed plane-slice in three dimensions.

11.4 Generation of MRI Images

Scanners have a built-in birdcage coil for RF excitation. Typically such a coil is also used for signal reception, unless a surface coil is used separately. Instructive is also the placement of the shimming plates (passive) and active shimming coils for attaining increased homogeneity within the field of view and prescribed slice. Shimming is nowadays computer controlled and is often carried out during a pre-scan phase of the data acquisition. Water or other phantoms are also used for calibration, quality control, testing and debugging pulse sequences or electronics, as well as for coil tuning and matching. Important also are peripheral devices that include the cardiac pacing leads and pulse oximeter to monitor patient vital signs during positioning and provide trigger signals to the scanner for dynamic studies such as cardiac imaging.

Once at the operator�s console, the procedural steps to generate an image include selection of:

1. The different imaging parameters (geometry, image dimensions, encoding etc.)

2. Pulse sequence (acquisition scheme)

3. Reconstruction parameters

The data acquired from the RF coil is amplified via low noise preamplifiers and boosted with the use of gain blocks before transmitted to the receivers in the electronics room. The signal is demodulated to baseband, filtered, and then digitized. Typically, Discrete Fourier transformation (DFT) is used to allow image reconstruction. Computer control (or built-in algorithms) allow visualization of the reconstructed images on the console window.

Most commercial clinical scanners are equipped with a database for storage and retrieval of images and archiving devices (magnetic, optical or magneto-optical drivers) and media for archiving.

11.5 Safety

There are numerous precautionary measures that International Committees and other organizations (FDA, IEC) require for safety. These cover all aspects and areas of the MRI center that include the magnet, implanted devices, the use of metallic or other materials and auxiliary devices, RF power deposition, temperature changes during scans, use and operational limits of hardware, specific absorption rates, safety of RF coils, and others. Such safety measures are summarized in guidelines or directives published by such committees or organisations. Clinical centers are required to post signs with contraindications and warning signs to ensure safety. For patient scanning, screening and consent forms from patients are also strictly required before MRI exams.