MR image brightness very depends on relaxation time value. Relaxation time hugely depends on temperature. So the way to improve the contrast is to heat up or down a human tissue investigated in MR scanner. Are there any works on it for clinical MRI?
Heating is used all the time in small-animal MRI to avoid hypothermia, i.e. cooling of the (anesthetized) animal below its normal body temperature. (Obviously, constant body temperatures are also extremely important for reproducible measurements of MR parameters such as ADC, T1, etc.) Thus, several "MR compatible" heating devices are available for body temperature control (e.g. based on heated air or liquids, or based on lamps).
Heating of tissue is also performed in the context of induced hyperthermia (applied e.g. in the treatment of cancer) - this can be done also in MR systems equipped with (additional) radio-frequency (RF) applicators; an alternative approach is tissue heating by focused ultrasound.
Heating of tissue can actually be monitored based on temperature-dependent MR parameters (relaxation times, proton resonance frequency, ADC, ...). However, I'm not aware of any applications of these devices to modify/improve the intrinsic tissue contrast in vivo: since one has to stay close to physiological temperatures, there wouldn't be any substantial changes of relaxation times.
So if I heat up my hand from 36,6 to 45 0C then water relaxation time (consider a water in your kitchen) increases in 20 %. Say, from 2 s to 2,5 s. Then let us look at T2 weghted image in MR scanner. The brightness of pixel of this MR image is proportional to exp(T2/techo). Say techo=0.05 s. Then digital value of brightness after heating changes in exp((T2after-T2before)/0.05) =exp(10) times. It is just my calculations :). Am I right? What is your opinion?
If we consider more really the blood. So T2before=0.2 s, T2after=0.24 s, techo=0.01 s then the expected change in the brightness digital value of the pixel is exp(4). Again very big. Where am I wrong? Or right?
First, it might not be very enjoyable to heat up your hand to 45°C. Blood perfusion is constantly cooling your tissue (back to 37°C), so you'll need even higher temperatures at the skin to obtain 45°C in the center of your limb ... I wouldn't want to try.
Then: are we talking about T1 or T2? There are quite a lot of publications about the temperature dependence of T1 of tissue (order of magnitude: 1 to 2 %/K in the relevant temperature range), and also some reports about T2 (e.g., typical T2 changes in fat of 5 to 7 ms/K). But let's (rather optimistically) assume an effective change of 20%.
In your (pure) water example, you then assume a T2 increase from 2.0 to 2.4 s. This doesn't really change the signal at a TE of 50 ms, which is proportional to exp(-TE /T2): The signal increases by a factor of about 0.005 (it's exp(-TE * (1/T2a - 1/T2b))).
In your blood example, it is a factor of 0.008. The effect becomes somewhat larger at longer echo times. But before "optimizing" the acquisition, you should find out about actual changes of T2 (for your tissues of interest). And then I'm still not quite sure if you are trying to improve the signal (SNR) or the contrast (CNR) between different tissues.
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Thaink you very much for very helpful articles and corrections. I confused the formula :)))) Yes the contrast is of few percents.
What do you think? May this contrast be used in small vessels imaging (angiography), peripherical vessel of legs, arms, coronary artery imaging or the like? Maybe blood velocity in small vessel network is slow and I could heat up the blood? E.g, in a heel, calf or elsewhere (I am not a physician)..
Good luck to you!
I'm skeptical ... but do the maths: first clearly define the CNR (or SNR) you are interested in, then find out about the T2 temperature dependence of blood (and the surrounding tissues), use realistic TEs (for T2w MRI), discuss potentially disadvantageous effects such as the decrease in physical density and the decrease in magnetization due to the Boltzmann factor exp(-µB/kT) at increasing temperatures as well as potential SNR losses due to the influence of the heater hardware (which might require less optimal receive coil setups) ...
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