Sample heating

Experiments in§2.4.8showed howT2 for water varied with temperature. A concern

with using small samples was the possibility of the sample being heated by the B1

coil leading to inaccurateT2 measurements. This was especially of concern with

water-based samples, since the required number of RF refocussing pulses is often significant due to long T2 times and the requirement for short pulse spacing to

minimize diffusive effects. A water sample at 25◦C on the Mole probe requires about 10.5s for theT2 signal to decay away to noise levels. For 500µs pulse spacing,

about 21,000 (π) refocussing pulses are required in order to sample the full decay. The power delivered to theB1 coil can be calculated if a good impedance match

between the HPA andB1 circuit is assumed. In this case all incident power from the

HPA is dissipated as heat in the B1 coil resistance. For the following experimental

parameters: 500µs echo time, 26µs RF pulse duration, −10dB (π) refocussing pulse amplitude (equating to 51.6VRMS into 50Ω), 32,000 echoes, and 30s repetition time,

2.4.10. Sample heating 57

the average RF power dissipated in the B1 coil is:

(51.6V)2

50Ω ×

32,000×26µs

30s ≈1.5W. (2.4) The number of echoes in this calculation is around 50% above that required for measuring water at 25◦C, thus it provides a worst case scenario. If all of this power was delivered into 10mL of water with heat capacity 4.186 J/(g·K) the number of seconds required to raise the sample temperature by 1◦C would be:

4.186 J/(g·K)×10g×1◦C

1.5W ≈30s. (2.5)

An experiment was run over several hours using the parameters described above to test the effects of long RF pulse sequences upon a 10mL water sample. A Pt100 sensor was located just below the surface of the water and attached to a digital multimeter. The Mole environmental chamber lid was sealed and the Mole and ambient temperatures were recorded. The results are shown in Fig. 2.15.

The temperature of the sample increased immediately after RF power was applied (around 25 minutes in Fig. 2.15) at an initial rate of about 0.13◦C/minute. At this rate, after 8 minutes the temperature would have increased by approximately 1◦C and the water T2 would have increased by around 32ms. Evidently about

one-sixteenth of the RF power delivered to the probe ended up as heat in the water sample. The environmental chamber heater elements stayed off during the sustained RF heating period as more heat was being added to the inside of the environmental chamber than was being lost to the outside, while the ambient temperature hovered around 20◦C.

Initially it was suspected that sample heating was due to the water absorbing RF energy from the oscillating fields. Some mobile phones and MRI systems transfer radio frequency energy from their antennas to the human body causing localized heating. The quantity of power absorbed is measured in terms of the specific absorption rate (Jin (1998) [88]):

SAR (Watts/kg) = total RF energy dissipated in the sample (Joules)

exposure time (s)·Sample weight (kg) . (2.6) At the low operating frequency of the Mole probe, it was unlikely that RF absorption was the heating mechanism. Jin argues that for a B0 below 0.5T, the

interaction between the B1 field and the human body can be neglected. It seemed

more likely therefore that sample heating was via conduction from the B1 circuit

board copper traces through the Perspex vial holder and glass vial to the sample. This could be tested by removing the conductive heat pathway between the RF coil and sample, and repeating the experiment.

58 Chapter 2. Experimental apparatus 0 2 5 5 0 7 5 1 0 0 1 2 5 1 5 0 1 7 5 2 0 0 2 2 5 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1 N O T E : E v e r y e v e n n u m b e r e d d a t a p o i n t r e m o v e d f o r c l a r i t y . S a m p l e : 1 0 m L M i l l i - Q w a t e r E x p t : C P M G R o o m t e m p : ~ 2 1°C M o l e t e m p s e t : 2 4°C n r S c a n s : 4 n r E c h o e s : 3 2 , 0 0 0 e c h o T i m e : 5 0 0 µs r e p T i m e : 2 7 . 5 s I r r a d i a t i o n t i m e / s c a n : 1 6 s S a m p l e t e m p e r a t u r e ( P t 1 0 0 a n d F l u k e 8 0 5 0 A d i g i t a l m u l t i m e t e r ) C o o l i n g p e r i o d T em p er at u re ( ° C ) T i m e ( m i n u t e s ) A m b i e n t T e m p e r a t u r e ( a l c o h o l t h e r m o m e t e r ) M O L E T e m p e r a t u r e ( P t 1 0 0 ) H e a t i n g p e r i o d S a m p l e h e a t i n g d u e t o B ₁ c o i l

Figure 2.15: Temperature versus time for 10mL water sample. The first 25 minutes

show the water temperature approaching the environmental chamber temperature. Upon application of RF pulses the water and Mole temperatures increase before falling again at the cessation of the RF pulses. The repTime parameter was set to 27.5s in the backdoor macro to provide scan beginning every 30s on average while also providing time for

recording the three temperatures; (4×27.5s) + 10s = 120s, thus an average of 30s per

scan. The estimated uncertainty in sample and Mole temperature measurements was