Object ID: 00000018WIA30982870GYZ
Topic ID: id_40023427 Version: 1.4
Date: Jul 11, 2022 8:01:13 PM

Tissue heating

Before the patient is scanned, the computer estimates the level of heating and compares it to the predetermined exposure limits. If the scan is expected to exceed these limits, the system then adjusts the scan parameters before starting the scan. The complete estimate is based in part on patient weight. Therefore, take care to enter the patient’s weight correctly to prevent excessive RF exposure or scan abortion.

When patient temperature is not changing, typical skin temperatures are about 33 °C while core temperatures are about 37 °C. Patients dissipate metabolic heat at the same rate it is generated so there are no skin or core temperature changes. Humans subjected to significant radio frequency power deposition (i.e., significant SAR) will normally attempt to dissipate the additional heat load through vasodilatation of skin blood vessels permitting skin to approach core temperature. This action typically causes the skin to flush (turn red) and enables the body to dissipate heat more rapidly. This skin flushing is a normal response to significant radio frequency power deposition. Skin reddening or to a lesser degree the report of a warming sensation without reddening regardless of the method it was created (SAR, Contact, Metal, etc) is not hazardous if it clears in a few hours.

Patient comfort module

The temperature inside the magnet room should be set at less than 69.8°F and the bore fan should be turned on at all times to keep air flowing inside the bore of the magnet.

Thermal hazards

The increase in tissue temperature caused by RF exposure depends on a variety of factors associated with the thermoregulatory system of the individual and the surrounding environment. Thermoregulation is the ability of the body to maintain regulated heat capacity levels. Observe the following warnings concerning tissue heating:

Warning

RF power deposition can heat the patient’s tissue if delivered faster than the patient’s tissues can dissipate the generated heat. The amount of tissue heating depends on the patient’s weight, type of pulse sequence, timing factors, number of slices, SAR, and the use of imaging options such as saturation. Power deposition will typically be lower when the NORMAL MODE is selected for SAR. FIRST MODE for SAR offers higher performance but also higher power deposition.

Warning

A rise in body temperature can be a hazard to a patient with reduced thermoregulatory capacity and increased sensitivity to raised body temperature. These can be caused by pre-existing conditions, such as cardiac impairment that has reduced circulatory function, hypertension, diabetes, old age, obesity, fever, pregnancy, or an impaired ability to perspire. A patient with these complications must be carefully monitored at all times. Consider scanning with NORMAL MODE for SAR for patients that may not tolerate the higher levels.

CAUTION

The MR worker who remains in the scan room during a study could be subject to tissue heating caused by RF energy exposure. Care should be taken to limit the time the MR worker remains in the scan room during a study and to keep a sufficient distance (the distance required to complete necessary tasks in the scan room) from the transmit RF coil.

Warning

RF can raise the magnet bore temperature and cause thermal stress; medical conditions can reduce a patient’s ability to cope with external temperature increases. If the temperature continues to rise, the scan stops until the temperature within the bore is lowered. When the sensor detects temperatures that may cause patient discomfort, the system posts messages. Read the messages on the screen or in the error log and respond accordingly.

When the temperature drops to a comfortable level, the message is cleared from the screen. If the temperature continues to rise, a second message appears on the screen that informs you the scan is stopped due to a patient comfort sensor trip. To facilitate a return to scanning, make sure the patient fan is ON, room temperature is normal, 21°C (70°F), and air flow through the bore is unobstructed.

When the magnet opening temperature decreases to a level that allows you to resume the scan, a new message is posted. Read the message and respond accordingly.

CAUTION

All patients should be monitored for increased temperature during the scan acquisition. If the patient reports discomfort due to warming, stop the scan. Patients should be provided with the hand-held Patient Alert bulb prior to scanning. The patient should be instructed to communicate any concerns through the intercom or by activating the Patient Alert bulb.

CAUTION

RF heating can be caused by:

Damp clothing.
Clothing containing metallic thread or components and all other metallic objects such as watches, coins, etc.
Contact of body or extremities against the RF transmit coil surface, contact with metal, tattoos or metallic eyeliner, contact with other body parts.
Scanning with an unconnected receive coil or other cables in the RF transmit coil during the examination.
Formation of loops with RF receive coil cables and ECG leads.
The use of MR Unsafe or MR Conditional (used outside of its conditions for use) ECG electrodes. Never use ECG electrodes past their expiration date.
The use of MR Unsafe or MR Conditional (used outside of its conditions for use) ECG leads. MR ECG leads have a very high impedance that limits current to below the level of concern.

CAUTION

IEC 60601-2-33: 201.7.9.2.101 (j)

Extra attention should be utilized when scanning patients who are unconscious, sedated, or may have loss of feeling in any body part (temporary or permanent paralysis). They may not be able to alert you to RF heating.

CAUTION

The coil selected should match the coil that is connected. When scanning with a transmit/receive only coil, DO NOT scan using the body coil (or use the Body coil configuration) at any time. Using the body coil can cause RF heating and could result in patient burns. In addition, scanning with the body coil can damage the transmit/receive only coil, making the coil unusable and requiring that the coil be returned to the factory for service.