Object ID: 00000018WIA30A92870GYZ
Topic ID: id_40021321 Version: 1.13
Date: Jul 26, 2022 12:09:42 PM

Specific Absorption Rate (SAR) limits

It is necessary to measure the RF absorption in tissues since RF exposure cannot be measured by the system. Energy dissipation through absorption by the body can be described in terms of Specific Absorption Rate (SAR). SAR is a rate, meaning it is a measure of the amount of RF power absorbed per unit of mass of an object in watts per kilogram (W/kg). There are several types of SAR, listed in the table below, that must be understood.

Table 1. SAR definitions
SAR type	Definition
Whole body	SAR averaged over total patient body mass over any six-minute period for Body and Receive Only surface coils, which use body transmit.
Partial body	SAR averaged over the exposed mass in the coil averaged over any six-minute period.
Head	SAR averaged over the mass of the patient’s head over any six-minute period.
Extremity	SAR averaged over the estimated mass of the patient’s extremity over any six-minute period for small volume and Transmit/Receive coils.
Short term	The SAR averaged over any 10-second period.

System SAR operating modes:

Normal Operating Mode: mode of operation of the MR equipment in which none of the outputs have a value that can cause physiological stress to patients.
First Level Controlled Operating Mode: mode of operation of the MR equipment in which one or more outputs reach a value that can cause physiological stress to patients which needs to be controlled by medical supervision.
Second Level Controlled Operating Mode: mode of operation of the MR equipment in which one or more outputs reach a value that can produce significant risk for patients, for which explicit ethical approval is required (i.e. a human studies protocol approved to local requirements).

CAUTION

Continuous patient observation and contact is required in all modes of operation. Medical Supervision is required in the First or Second Level controlled operating modes.

CAUTION

SAR may be controlled by Local Approval.

CAUTION

For sites which need to comply with IEC60601 2nd or local safety standard YY 0319-2008, this MR equipment shall not be used in the normal operating mode when the scan room temperature is greater than 24 °C or relative humidity about 60%.

Warning

The magnet room temperature shall not be more than 21°C per the manufacturer’s requirements and the relative humidity shall not be more than 60%. Temperatures above 21°C and humidity above 60% could result in lowering the system SAR limit. The derating temperature is 25°C for relative humidity less than 60%. For each 10% increase of the relative humidity in excess of 60%, the temperature is reduced by 0.25°C, e.g., 24°C at 100% relative humidity. For each degree of environmental temperature that exceeds the SAR-derating temperature, the whole-body SAR limit is reduced by 0.25 W/kg until the SAR is 2 W/kg or 0 W/kg for the First Level controlled operating mode or for the Normal mode, respectively.

Warning

IEC 60601-2-33: 201.7.9.2.101

Errors in patient weight entry may result in the following adverse effects: negatively impacted parameters, which potentially impact image quality, or a false-positive UPM trip. Regardless of the error, the SAR controls regulatory limits remain within the IEC 60601-2-33 specification.

Warning

IEC 60601-2-33: 201.7.9.2.101

The SAR algorithms for the MR systems calculate SAR values and set a limit on the number of slices/echoes per second in order to limit RF power deposition. The power monitor and SAR algorithm limit SAR, regardless of the patient weight or pulse sequence used. SAR limits are conservatively estimated from worse-case patient positioning as a function of weight. The legacy power monitor module limits the RF amplifier output power thus limiting the patient SAR in case of a catastrophic failure. This module monitors peak power based on the patient’s weight, duty cycle, and pulse sequence parameters. The peak power limits prevent you from using incorrect patient weights.

Warning

IEC 60601-2-33: 201.7.9.2.101

The average power monitor and SAR algorithm limit SAR based on patient weight and RF transmit coil used. SAR limits are conservatively estimated from worse case patient positioning as a function of weight. The power monitor limits the RF power, which in turn limits the patient SAR to within controlled limits over time.

Pulse sequence SAR predictions (estimated SAR) are based on patient weight at the worst-case landmark. To minimize nuisance power monitor trips caused by patient-to-patient variability, pulse sequence predicted SAR is the mean plus 1.96 standard deviations (typically the normalized standard deviation is about 18% at the worst-case landmark). The expected worst-case nuisance trip rate is approximately 2.5%. If you experience a significant number of power trips above the 2.5% frequency, please consult your local field service representative. The power monitor measures actual power and limits SAR appropriately. The power measuring accuracy of the power monitor is about +/-12%.

To help avoid injury, enter the patient’s correct weight, set the appropriate operating mode, and prevent excessive RF exposure.

SAR limits

The MR system’s RF power monitor helps prevent excessive RF exposure due to equipment failure. Since the monitor protects the patient, it must be operational at all times, even when a scan is not in progress. If it detects an equipment failure, it immediately disables the RF system. This system must be repaired or adjusted by qualified service personnel.

Table 2. SAR operating limits
System	Normal mode (W/kg)	First level (W/kg)	Second level (W/kg)
1.5T	Head = 3.2 Body = 2.0	Head = 3.2 Body = 4.0	Head > 3.2 Body > 4.0

The IEC also allows Short Term limits that allow a higher Short Term SAR for 10 second periods of time. These limits are 2 times higher than the 6 minute limitations per IEC guidelines. This allows short term bursts of RF energy for very short scans.

Note: Patient acceptance of High SAR scanning can be increased by giving the patient breaks to cool down, providing light clothing, and limiting room temperature to 18 ± 3 °C, and by maximizing air flow.

Maximum B₁+ RMS for Transmit Coils

The following table provides a bound for maximum B1rms (in micro-tesla) for body transmit coils and head transmit coils at 1.5 T. Values are shown for the limits at 1.5 T for the head transmit coil and for the body transmit coil with the patient’s umbilicus at isocenter.

IEC 60601-2-33: 201.7.9.2.101 (b)

Table 3. B₁+ RMS limit (µT)
Coil	1.5T
Body Coil Umbilicus Landmark	3.6
Body Coil Chin Landmark	N/A
Head Coil	7.2

SAR limits

Maximum B1+ RMS for Transmit Coils

Maximum B₁+ RMS for Transmit Coils