Oral Presentation Australian & New Zealand Society of Magnetic Resonance Conference 2017

Measurement of transverse relaxation properties of blood at low magnetic fields (#75)

Dion G Thomas 1 , Petrik Galvosas 1 , Yu-Chieh Tzeng 2 , Paul D Teal 3 , Sergei Obruchkov 1 4
  1. School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand
  2. Wellington Medical Technology Group, Centre for Translational Physiology, University of Otago, Wellington, New Zealand
  3. School of Engineering and Computer Science, Victoria University of Wellington, Wellington, New Zealand
  4. Robinson Research Institute, Victoria University of Wellington, Wellington, New Zealand

The relationship between the oxygenation of blood, and the transverse relaxation rate of water protons has been previously investigated by many researchers e.g. Thulborn and Ogawa [1,2]. This blood oxygen level dependent (BOLD) effect is caused by the susceptibility change of oxy/deoxy-hemoglobin, creating magnetic field inhomogeneities, which affect the relaxation of water protons diffusing through those regions. Because the oxy/deoxy-hemoglobin fraction is linked to the partial pressure of oxygen (pO2) in blood, the relaxation rate of blood, which increases quadratically with the deoxyhemoglobin fraction [1,3], can be used as indirect proxy of blood and therefore tissue oxygenation.

This relaxation effect has been well characterised at fields greater than 1.5 T, mainly due to its importance in clinical studies (e.g. fMRI). It is known that the size of the relaxation change is proportional to B02, and that the observed relaxation rate follows a similar dependency on CPMG echo time to chemical exchange type systems (Luz-Meiboom equation)[1,4-7]. However, at lower fields there have only been a handful of experimental studies of this effect, and they have been limited to measurements at extreme levels of oxygenation/ deoxygenation [6,7].

In this study, we investigated this oxygenation dependent relaxation of blood at fields ranging from 0.1 to 1.5 T using a variable field magnet system. Whole human blood was continuously pumped through a circuit containing an oxygenation membrane, which provided dynamic control of the oxygenation and other physiologically important parameters before it travelled into the magnet. This allowed us to measure small variations in the blood oxygenation, rather than the two point studies which have been done previously. Additionally, we characterised the effect of different CPMG echo times on the observed relaxation rate, and have compared the results to models which have been developed and tested at higher magnetic fields.

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