1. Discuss the current guidelines which are relevant to the safe practice of Magnetic Resonance Imaging (MRI) in the United Kingdom.
Since its arrival in the UK in the mid 1980s, magnetic resonance imaging (MRI) has been hailed as a major advance in diagnostic imaging technology. MRI uses powerful magnets and radio frequencies, together with sophisticated viewing software, to produce high quality, cross sectional images of the patient’s body ( 1999). Unlike x-ray exams, which use radiation, magnetic resonance (MR) images are created with magnetic fields and radio waves. MR scanning is a very safe and effective technique for examining the body’s soft tissues, such as organs, muscles, ligaments and tendons ( 2002). One of the advantages of MRI is its apparent absence of adverse effects on the patient. Unlike the ionizing radiation used in ordinary radiographic studies, this technique does not pose any risk of genetic damage (1990). However, because MR scanning uses a powerful magnet, patients and healthcare professionals alike need to know about some special precautions and check-in procedures in order to provide safe practice.
A critical component of the MRI system is the magnet, which has undoubtedly tremendous strength. Performing any healthcare practice like surgery near a high magnetic field posed an interesting challenge for healthcare team members. The invisible magnetic force of the MRI system is strong enough to pull mop buckets, stretchers, and heavy-duty floor buffers into the bore of the magnet–it can even turn oxygen tanks into flying projectiles. Serious injury or death can occur from being struck with such objects. Accordingly, personal items such as scissors, loose change, pagers, pins, and identification badges are strictly forbidden in areas with MRI ( 1999).
In addition to creating projectile hazards, magnetic fields also can affect biomedical implants. MR scanning is contraindicated for any patient with a pacemaker, especially if even temporary malfunction of the device would endanger the patient’s health. The magnetic field can interfere with electromagnetic devices and cause serious damage to the internal components of pacemakers, inner ear implants, and neurostimulators (1999).
Magnetic fields can cause pacemakers to malfunction at fields as low as 17 G, and some pacemakers may be affected by fields as low as 5 G. It is recommended, therefore, that anyone equipped with a pacemaker stay outside the 5 G line. In high field MR units, the 5 G line will extend farther from the scanner than it does with conventional units (1996). Patients who have had previous surgeries, have had metal in their eyes, or have implants are at risk and must be thoroughly screened before undergoing MRI (1999).
The incorporation into UK law of the 2004 EU Physical Agents Directive, to impose legal limits on electromagnetic field (EMF) exposure, will result in significant restrictions on the use of MRI, for which there is no evidence of harm; it will obstruct plans to reduce X-ray exposure, for which there is a longstanding and clear evidence of harm, notably from increased risk of cancers.
The EU Physical Agents Directive seeks to define safe levels for equipment operators’ exposure to electromagnetic fields. The directive puts limits on the exposure of operating staff from zero frequency up to 300GHz. Cautionary use of MR equipment is already standard. The very first set of MRI clinical guidelines was produced by NRPB in Britain.
New European regulations threaten to reverse pioneering advances in the use of MRI to diagnose disease and treat patients. By 2008, many interventional MR procedures will be illegal, and it may be more difficult to use the most powerful, modern (high field) scanners ().
2. Explain how scan times can be kept to a minimum in MRI.
As the name implies, MRI uses a strong magnet to create a static magnetic field. Tesla is the unit used to measure the strength of the magnetic field. A magnet with 1.0 tesla field strength would have a stronger magnetic field and be more forceful than a magnet with a 0.5 tesla magnetic field (2003). In a conventional 1.5 T machine, the scanner has an operational field strength approximately 30,000 times as strong as the earth’s magnetic field. In the new machines, field strength can be increased by a factor of 3 or 4. This situation amplifies the known hazards of operating a powerful magnet. The most obvious of these dangers is ferromagnetic attraction (1996).
The strength of this magnet is usually tremendous — about 30,000 times stronger than the magnetic held of the earth. To contain its magnetic field and eliminate the need for steel shielding in the walls of the OR, the magnet is actively shielded. This means that, within the magnet, a secondary magnetic coil of opposite polarity is outside of the main magnetic coil. The secondary coil greatly reduces the magnetic fringe field that extends into the area surrounding the magnet (1999).
The MRI machine is a long cylinder, much like a standard MRI, but it contains a vertical gap in the center of the magnet that is approximately 58-cm wide. The surgical area is located in this vertical gap between the two halves of the magnet. This allows surgeons to perform surgical interventions on the patient while the patient remains in a stationary position throughout the procedure (2003).
Patients can enter the magnet in one of two ways depending on tumor location and surgeon preference. In the standard position, the patient enters the magnet from the end, much like a bullet goes through a gun barrel. In this position, surgeons stand opposite each other during the surgery. This allows two surgeons to work at the same time. This docking position typically is used for brain tumor resections and brain biopsies (2003).
Scan times of several minutes often are required to produce a diagnostic image with MRI. One way to reduce scan times is to increase the field strength, thereby boosting the signal available to produce the image. Another method to reduce scan times is to increase the rate of variations of the magnetic field gradients, as with the echoplanar imaging method. Rapid changing (pulsing) of the gradient magnetic field may induce voltages in any electrical conductor within the field. Possible conductors include the human body and biomedical implants, as well as devices used to monitor the patient during scanning. Burns have resulted when a loop is formed between the patient and the conductive leads to monitoring equipment within the gradient magnetic field ( 1996). Therefore, it is important to check all equipment to ensure that no loops are formed with the patient.
3. Explain how the adjustable parameters you have available on the MRI console generally affect the quality of your image.
Two populations of hydrogen protons exist in magnetic resonance imaging: free protons and bound protons. Bound protons do not contribute to normal MR signal because they resonate somewhat off the center frequency of water. A special pulse sequence called magnetization transfer contrast (MTC) was developed to compensate for this off-resonance limitation. A saturation transfer of bound protons occurs during MTC, enabling a transfer of energy to the free proton pool and thereby contributing to the overall MR signal (1996).
Bound hydrogen protons are not visualized in MR because of their broad bandwidth, which is slightly offset from the center frequency of the free proton pool, or water peak, in MR imaging. The broad bandwidth, combined with cross-relaxation and chemical exchange processes, contributes to the lack of visualization (1996).
The broad bandwidth of bound protons can be forced to contribute to the free proton pool signal by applying a presaturation pulse a few hundred hertz off the center frequency of the free proton pool or water peak. This off-resonance pulse is known as the MTC pulse. An off-resonance pulse implies that a frequency different from free hydrogen protons is utilized. Typical offset values of pulses range from 1000 Hz to 2500 Hz (1996).
Magnetization transfer contrast sequences usually require no additional hardware; typically, all that is required is a computer software package that allows the use of the special presaturation pulse. This pulse allows the magnetization induced in the bound proton pool to be transferred to that of the free proton pool. The MTC effect is best visualized in tissues with a lower water content, such as cartilage, muscle and brain parenchyma. Tissues that have a high water content demonstrate very little MTC effect ( 1996).
Advances in computer hardware and software and the expansion of MRI have increased the use of 3-D imaging to display sectional anatomy ( 1993). A computer is used to control and orchestrate all scanner electronics in MRI such as transmission, signal reception, pulse timing, and signal delay time. The software written to control these channels is called a pulse sequence. The timing parameters inside the pulse sequence can be adjusted to produce various types of tissue contrast ( 2001), thus affecting the overall MRI image quality.
The better the magnetic resonance imaging quality of the image, the better and bigger is the chance for detection of diseases or the accuracy of screening certain diseases. The advancement in technology which has moved imaging from beyond simple anatomical images to computer-generated 3-D images has helped a lot.
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