Parameter Space Mapping for Blood Oxygenation Measurement with Low Field NMR

Abstract

Blood oxygenation is a critical physiological parameter for patient health. The clinical importance of this parameter means that measurement of blood oxygenation is a routine part of care. Magnetic resonance provides a way to measure blood oxygenation through the paramagnetic effect of deoxy-haemoglobin, which decreases the T2 relaxation time of blood. This effect has been well characterised at high fields (>1:5 T) for use in Magnetic Resonance Imaging, and it is a contributing factor to the Blood Oxygenation Level Dependent contrast used in functional MRI. However there are relatively few studies of this effect at low magnetic fields, and these have only looked at extreme levels of oxygenation/deoxygenation. To study this effect for potential application in a low-field device, we measured this effect to determine how factors such as oxygenation, field strength and CPMG echo time affect the T2 of blood.

A continuous flow circuit, similar to a cardiopulmonary bypass circuit, was used to control parameters such as oxygen saturation and temperature, before the blood sample flowed into a variable field magnet (set at fields between 5-40 MHz), where a series of CPMG experiments with echo times ranging from 1 ms to 20 ms were performed to measure the T2. Additionally, the oxygen saturation was continually monitored by an optical sensor, for comparison with the T2 changes. This allowed us to test the sensitivity of this effect at low fields.

These results show that at low fields, the T2 relaxation change still follows the trends shown in the literature, with a dependence on B0 squared, and on the fraction of deoxyhaemoglobin squared. Additionally, these results were also compared with two theoretical models for the dependence on echo time, which have previously been tested at high fields: the Luz-Meiboom equation, and the Jensen and Chandra model. Both models gave good agreement with the data measured at low fields. These experiments show that the T2 changes in blood due to oxygenation are still visible at low field, and that this technique should be feasible in a low field device.