MRI Basics
The aim of this overview is to give you an introduction to magnetic resonance imaging (MRI) that we hope will help you better understand the research we are doing in our project, “Instrument Development for Open MRI. This is a cooperative project of Charité - Universitätsmedizin Berlin and several medical technology companies of the Berlin/Brandenburg region.
Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging (MRI, referred to as nuclear magnetic resonance imaging in its early years) is a widely used diagnostic technique in radiology that can visualize the internal structures of the body with excellent image quality. MRI offers unrivaled soft tissue contrast that enables detailed diagnostic evaluation of almost every organ and body region.
Fig. 1: Whole body MR image of a human.
All biological tissues with a high water and/or fat content are depicted on MR images with exquisite detail and unmatched contrast, even without using an intravenous contrast agent. MRI is less well suited for imaging organs that contain air, such as the lungs, but does provide diagnostic images of the bones despite their comparatively low water content.
Image contrast in MRI results from the concentration of protons (hydrogen ions) in tissues, the ability to magnetize these particles by external magnetic fields, and their demagnetization (relaxation) following excitation by radiofrequency pulses. The latter are low-energy radio waves similar to those used in radio broadcast (see Table 1).
Table 1: The electromagnetic pulses used to generate
MR images have frequencies in the range of radio broadcasts
The differences in magnetic susceptibility of biological tissues are due to differences in the molecular environments of the protons. MRI uses strong magnetic fields: a main magnetic field, which is static, and variable fields generated by rapidly switched gradient systems. The main field is generated by a superconducting electromagnet housed in the MRI scanning tube (Fig. 2).
Fig. 2 Coils
Three types of clinical MR scanners are distinguished on the basis of the magnetic field strength at which they operate: low-field scanners (up to 0.5 Tesla), intermediate-field scanners (0.5 to 1.0 Tesla), and high-field scanners (over 1.0 Tesla). The most widely used medical MR scanners are so-called tunnel systems (Fig. 3), which can be used for whole-body imaging. Depending on the examination performed and type of scanner used, the relatively long tunnel (also known as a bore) can fully accommodate the patient.
Tunnel Systems
Fig. 3: Classic tunnel system with a long bore
and Fig. 4: Tunnel system with a short bore
Some patients do not tolerate being examined in the narrow tube of an MR imager. If there is no alternative to an MRI examination, these patients may need to be examined under general anesthesia. While most MR imagers in clinical use today have a field strength of 1.5 Tesla, scanners with 3.0 Tesla are being used more and more. Clinical MR imagers are operated with superconducting electromagnets. In the illustrations, ellipses represent the field lines of the main magnetic field, and the arrow indicates the patients longitudinal axis. Recent technical advances in MRI have led to the development of shorter magnets with shorter bores for use in medical MR systems. In conjunction with greater tunnel diameters and wider openings, these innovations considerably improve patient comfort. The main advantage of such a system is that it will increase patient acceptance of MRI scanning.
Minimally Invasive Surgery
A minimally invasive operation (keyhole surgery) is a form of surgical treatment that uses very small incisions to reach the target site and can therefore be performed with little damage to surrounding tissue. It is a very gentle method. Many operations that used to require large cuts, such as the removal of a gallbladder, can now be performed in a minimally invasive fashion. Patients who undergo keyhole surgery recover much faster and can leave the hospital earlier. Minimally invasive operations and interventions can be performed using so-called endoscopes, which allow the physician to directly watch the procedure. Alternatively, an operation can be guided using a noninvasive radiological imaging modality. Classic areas of application for such radiological interventions are the blood vessels throughout the body. Vascular interventions are monitored by digital subtraction angiography (DSA), a diagnostic test that uses X-rays. To remove tissue samples or fluids from the body or to insert drains, a cross-sectional imaging modality is used. These are MRI, computed tomography (CT or CAT scan), and ultrasonography (US), see Table 2.
Table 2: Overview of minimally invasive interventions
and imaging modalities used for monitoring purposes
Unlike DSA or endoscopy, a cross-sectional imaging technique allows precise spatial orientation at the site of the intervention and thus enables the accurate determination of the spatial relationship of a tumor to its surrounding structures. There are tendencies to establish intraoperative imaging as an integral component of surgery, which offers advantages primarily in tumor surgery. It has been shown that intraoperative MRI improves the surgical removal of brain tumors because the additional image information helps the surgeon to remove small tumor residues as well. Liver tumor surgery might also benefit from intraoperative imaging. Furthermore, intraoperative use of imaging might reduce complications because it provides information on nearby sensitive structures, such as vessels, that must not be damaged.
Since MRI uses strong magnetic fields, special instruments are required for performing operations and interventions under MR guidance. Typically, the operation is briefly interrupted to acquire MR images because the patient is not inside the scanner while being operated on. To enable the acquisition of MR images without having to interrupt the operation, special configurations of MR scanners are needed, that provide access to the site of surgery during MR imaging. Open MR scanners have been developed with this in mind.
Open MRI
Various types of open-configuration MR scanners are available from different manufacturers. In the course of development of open MR scanners, a variety of physical, technical, and constructional problems had to be resolved. The open-configuration MR scanners of the first generation are low-field systems that typically operate with permanent magnets, but less commonly with resistive electromagnets. Open scanners in the high-field range became available recently and are similar to conventional tunnel systems in terms of image quality. Open scanners improve patient comfort and access to the patient, which are prerequisites for performing MRI-guided operations. The types of open scanners are presented below.
Fig. 5: Classic design based on the configuration
of an X-ray C-arm (X-ray radiography)
The classic design presented in Fig. 5 is used in MR systems from different vendors that typically operate at 0.23 Tesla (mostly using solid state magnets).
Fig. 6: Configuration of an open MR scanner
The open configuration presented in Fig. 6 is based on a tunnel system with the center of the magnetic field being accessible from the outside (opening with a width of 56 cm; used by GE). The superconducting electromagnet of this system can generate a magnetic field of 0.5 Tesla. Patients can stand or sit during the examination.
Fig. 7: Two horizontally opposed magnetic pole shoes
The high-field panorama design developed by Philips consists of two horizontally opposed magnetic pole shoes and is based on a 1.0-Tesla superconducting electromagnet.
Fig. 8: Two horizontally opposed magnetic pole shoes
Open high-field MR scanners are not only fast but also yield images with high spatial resolution. These new scanners can be used to perform operations and interventions under real-time MRI guidance. Moreover, an open scanner with a 1.0-Tesla magnetic field also enables evaluation of biochemical compounds, temperature, and the direction of particle motion in the human body. Imaging of these processes requires special MR techniques which may also have a role in intraoperative monitoring. This is another research focus to be pursued by our study group at CC6 of Charité - Universitätsmedizin Berlin.
Another advantage of open MR scanners is that claustrophobic patients who panic in a confined space will be able to cope much better with the examination. This is another area of scientific investigation our group pursues in an interdisciplinary research project. Children are also more comfortable when they are examined in an open MR scanner.
Note that claustrophobia is often confused with agoraphobia, which is the fear of open spaces and quite the opposite of what claustrophobic patients may experience in the narrow confines of an MR scanner tube.
Claustrophobia – the fear of being in narrow, confined spaces (such as an MR tube)
Agoraphobia – the fear of open spaces (e.g., Alexanderplatz, Berlin)
Jens Pinkernelle
Felix Güttler (Ed.)
Ulf Teichgräber
Jens Rump
Thula Walter (Trans.)
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