Temperature Calculations

655

Table 4

Maximum SAR in Any 10 cm3 Region for Head Average SAR of 3.2 W/kg for Comparison to IEC Limits

Maximum local SAR in head

Maximum local SAR in brain only

RF coil

Value

Value

Location

Location

(W/kg)

(W/kg)

Birdcage coil at 64 MHz

Muscle tissue in masticator

10.7

Superior cerebrum, partially

8.92

space

including SSS

TEM coil at 300 MHz

Superior cerebrum, partially

8.73

Superior cerebrum, partially

8.73

including SSS

including SSS

Surface coil at 300 MHz

Mid-posterior cerebrum, partially

28.7

Mid-posterior cerebrum, partially

28.7

including SSS

including SSS

superior sagittal sinus.

SSS

sue, whether at 64 or at 300 MHz. Our calculations

tory limits on head-average SAR, it may be reasonable

indicate that T may increase by more than 1°C within

to neglect the physiological response to temperature, as

the head but outside of the brain. Because local heating

was done in this study. For other cases, including the

in some regions of the brain (especially in the hypothal-

case of the quadrature surface coil driven to induce a

amus) may result in confusion of the whole-body ther-

3.0 or 3.2 W/kg SAR averaged over the head, results of

moregulatory system (23), it is important to ensure that

the methods used here would indicate a temperature

no significant temperature increase occurs in the brain.

change larger than expected in the presence of a func-

But, just as tissues in the extremities are expected to

tioning thermoregulatory system.

withstand local T of up to 3°C (1), tissues in the head,

In all calculations considering the head, we assumed

other than the brain (e.g., skin, muscle, fat, bone, and

perfusion rates comparable to those in the healthy hu-

tissues in the eye) (6,24), should be able to withstand

man. It is important to note that temperature increases

T of up to a few °C with no irreversible effects. For this

might be higher in patients with impaired cerebral per-

reason, we present both maximum  T in the entire

fusion, such as those with a stroke, tumor, or arterio-

head and in the brain only, though the IEC guidelines

venous malformations. The present head model was

place the limit for T at 1°C anywhere in the head.

based on an anatomy typical for relatively young adults

When the quadrature surface coil is driven to induce

and for tissue properties from the literature. The results

3.2 W/kg SAR averaged over the entire the head, the

may vary with age as the geometry, composition, and

maximum  T within the brain tissue is 1.85°C, well

material properties of human tissues change (22), and

above the 1°C limit. In agreement with the IEC guide-

will also vary between individuals. The flow of CSF was

lines (1), this indicates that when using transmit sur-

not taken into account in the present simulation.

face coils, whole-head SAR limits are not a good guide

In agreement with previous publications (3,4), the

and extra caution should be exercised. Because they

results in Table 2 indicate that the nonsignificant risk

are exciting much smaller volumes, however, surface

FDA limit on SAR in any 1 g region in the head (8 W/kg)

coils generally require much less power than do volume

will be exceeded before the nonsignificant risk FDA

coils. For example, proton decoupling can be accom-

limit on head-average SAR (3 W/kg) is exceeded for both

plished with the quadrature surface coil, modeled here,

volume and surface coils intended for head imaging.

with only 4 W of power and a 2.4% duty cycle (unpub-

Similarly, the results in Table 4 indicate that the nor-

lished experimental results). But even with 4 W of

mal mode IEC limit on SAR in any 10 g region in the

power and a 100% duty cycle (about 0.55 W/kg aver-

head (10 W/kg) will be exceeded before the normal

aged over the head), the maximum T within the brain

mode IEC limit on head-average SAR (3.2 W/kg) is ex-

is only 0.32°C, the maximum T in the head is 0.785°C,

ceeded, except in the TEM coil at 300 MHz.

and the maximum SAR in any 1 g of tissue in the head

The results in Tables 3 and 5 indicate that when a

is 9.90 W/kg, as was calculated using the numerical

volume coil is driven to produce 3.2 W/kg average SAR

methods presented here.

in the head, it is unlikely that any increase in T much

The highest local SAR in the brain is consistently

above 1°C (IEC normal mode limit for increase in tem-

found adjacent the superior sagittal sinus. This is be-

perature anywhere in the head) will occur in brain tis-

Table 5

Maximum Temperature Increase ( T) for Head Average SAR of 3.2 W/kg for Comparison to IEC Limits

Maximum T in whole head

Maximum T in brain only

RF coil

Value

Value

Location

Location

(°C)

(°C)

Birdcage coil at 64 MHz

Muscle tissue in

2.2

Frontal lobe at anterior

0.9

masticator space

and inferior surface

TEM coil at 300 MHz

Muscle tissue in

2.6

Gray matter nearest

0.7

masticator space

masticator space

Surface coil at 300 MHz

Scalp behind cerebellum

4.5

Posterior cerebellum

1.8