This or similar accidents can also occur on your MRI system if you fail to observe the safety regulations. Safety training for all personnel entering the MR environment is a requirement. Such training ensures the safety of personnel and the patients undergoing MRI examinations. This video provides medical personnel with an overview.
of the basic safety precautions to be observed when using a magnetic resonance imaging system. You will be introduced to different magnets and learn the effect of those various magnets. Examples will be used to demonstrate the strength of the magnetic field and the dangers faced when handling magnetic objects in the examination room. This video also addresses proper patient preparation, examination, as well as how to correctly handle the surface coils.
Finally, We will show examples of emergencies or accidents and how to avoid them, and how to respond appropriately should they occur. This video serves only as a supplement to a more comprehensive safety training program. Specific MR safety-related information is provided in the Siemens MR System and Operating Instruction Manuals.
Magnetic resonance imaging, abbreviated as MRI, is a diagnostic method for generating cross-sectional images of the human body. This method is based on the interaction between the human body and externally generated magnetic fields. In contrast to computed tomography, CT, MRI does not use ionizing x-rays, but uses static as well as slowly and quickly varying magnetic fields. Physical Principles The patient is positioned in a powerful, homogenous static magnetic field, generated here by a cylindrical magnet. The strength of the magnetic field is indicated in units of Tesla.
The Tesla strength for human imaging ranges between 0.2T and 3T. In comparison, the Earth's magnetic field is approximately 0.00005T. The static main magnetic field of a 1.5T magnet is approximately 30,000 times stronger than the Earth's magnetic field.
The MRI is based on the magnetic field of a 1.5T magnet. properties of atomic nuclei. Because the human body is approximately 75% water, the hydrogen nucleus is the most abundant nucleus in the human body.
For this reason, the hydrogen nucleus, called the proton, is the most commonly used nucleus in MRI. The protons found in the body's water and fat molecules align themselves to this main magnetic field. The alignment of the proton is then systematically disrupted by an alternating magnetic field of suitable frequency.
For a magnetic field of 1.5 T, the frequency required to affect the proton is about 63 MHz, which is in the frequency range of radio waves. A transmitting coil is a component of the MR scanner and generates the alternating magnetic field in the area under examination. After being excited by high frequency energy, The protons return to the equilibrium position parallel to the static main magnetic field. The protons then emit electromagnetic waves in the frequency range at which they were excited.
Receiver coils arranged around the patient serve as antenna and detect these weak RF signals. During excitation, and while the protons are returning to the steady state, specially varying magnetic fields, known as gradient fields, are switched on, so that the resonance frequency of the proton varies from one region of the patient to the next. This is how the special distribution of the protons in the patient is coded into MR signals. A computer program analyzes the MR signals and generates an image of the distribution of the hydrogen protons in the human body. The MR unit is housed in an RF enclosure.
This ensures that the very weak signals from the RF pulses are not contaminated with interference and cannot cause radio interference themselves outside the enclosure. Therefore, the patient is exposed to three different magnetic fields during the MR examination. The static main magnetic field that aligns the protons, the high-frequency alternating fields that disrupt the alignment, and the pulse gradient field that enables correlation of the MR. MR signals to the location of origin. All three fields affect the patient differently.
The static magnetic field used in MR does not cause any lasting side effects. The same is true for alternating fields when used properly. The rapidly switched magnetic field gradients in combination with the static main magnetic field are the cause of the considerable noise created during the operation.
MR examination. Since the coils generating the gradient field are located in the static magnetic field, a sudden force is exerted on them when the current is switched on or off. The noise increases with the strength of the static magnetic field and the gradient fields.
Canon's MR scanners are equipped with audio comfort features, which reduce the noise levels typical of other scanners. However, it's always good practice to wear hearing protection. In rare cases, quickly switching on and off the magnetic gradient fields can induce electrical voltages in the peripheral nerves.
These harmless voltages are expressed as a tingling sensation or mild muscular twitching. However, Siemens MR systems are equipped with safety devices for controlling the switching speed and the strength of the gradients to prevent nerve stimulation. The RF used in magnetic resonance imaging is absorbed by the body and can cause localized warmth. For safety reasons, control circuits in the MR imager prevent the energy absorbed by the by the body, SAR, from exceeding safe thresholds. Since the computation of SAR involves the patient's weight, it is important to enter this information correctly during registration.
The limit values are fixed by international safety standards. Now, let's take a look at the core of the MR system, the magnet. There are three types of magnets used in clinical imaging, permanent magnets, resistive magnets, and superconducting magnets. Permanent magnets always have a magnetic field. In case of an emergency, a permanent magnet cannot be switched off.
They do not require an external energy supply and can only reach limited field strengths. With resistive magnets, the field is generated by a constant external current supply. The current flows through the wire coils and generates a large amount of heat, in addition to the magnetic field. This heat is drawn off by a water cooling system.
resistive magnets only generate low field strengths since they are limited by the ability of the cooling water to dissipate heat. In case of an emergency, a resistive magnet can be switched off at any time. Superconducting magnets are the most common commonly used magnets in clinical imaging and are used to generate higher magnetic field strengths.
Electrical current flows through wire coils and are cooled by liquid helium to 4 Kelvin above absolute zero, which is minus 269 degrees Celsius. The current is provided by a power supply unit during installation, but once super conduction is established, it can flow without an external power supply for several decades. Once cooled, the current flows continuously through the circuit and generates the static magnetic field.
In case of an emergency, a superconductive electromagnet can be switched off at any time. For MR, the magnets are large and shielded, so that their fields only act inwardly on the patient and quickly fade in an outward direction. This differs for various systems and depends on the field strength and the design of the magnet. The use of magnetizable objects in the area of the fringe field, the stray magnetic field, is strictly prohibited since the MR system is not a magnetic field.
exerts an attractive force on them. The 0.5 mT fringe field line is extremely important. A control area for the field strength in question is established around the magnet.
The control area must be clearly marked and must adhere to the particular safety measures measured in the direction of the patient axis. The 0.5 mT line of this 0.2 Tesla system is 1.9 m from the magnet center. In the case of a 1.5T system, it is 4 m from the magnet center.