Optical instrumentation for respiration measurement in magnetic resonance scanners

Magnetic resonance imaging (MRI) is a non-invasive technique for visualising the internal structure of the body. The method involves placing the subject in a strong static magnetic field (c. 1.5 T) and using various combinations of radiofrequency energy and pulsed magnetic field gradients, known as pulse sequences, to produce images. Most MRI pulse sequences acquire data over an extended time period in order to produce an image. Any physiological motion, e.g. movement of the heart, respiration etc., during this time period will cause artefacts and blurring. Various approaches have been employed to try and reduce this sensitivity to motion, including the synchronisation of data collection with the subjects electrocardiogram (EGG) to minimise cardiac motion and the development of fast pulse sequences to acquire the data within multiple breath-hold periods, between 10 and 20s. We have been employing both techniques together to acquire dynamic images of cardiac function. The requirement for data collection over multiple breath-holds however, is that each breath-hold must be reproducible, so that the heart is in the same position within the thorax each time. Even with highly motivated subjects it is difficult for them to breath-hold in a consistent manner. This paper outlines the adaptation and development of the Fibre Optic Respiratory Plethysmograph (FORP) to produce a MRI gating system (FORP-GS). This consists of a PC based, non-invasive respiratory sensor and a joint optical feedback system to the subject, allowing accurate monitoring of the thorax position in real time without any electro-magnetic interference