Muscle Imaging

The first imaging experiments on human subjects using MONSTIR were performed in order to study the haemodynamics of arm muscle in response to exercise [1]. A ring of sources and detectors were placed around the arms of volunteers as shown in figure 1 below. TPSFs were recorded for 15 seconds at each source position, using a Ti:S laser tuned at wavelengths of 780 nm and 820 nm. Data were acquired while the arm was at rest, and then again during a simple finger flexion exercise. The latter involved the volunteer gripping a tension meter between the palm and the index and middle fingers, as shown.

Figure 1: The 32 sources and detectors arranged around the forearm of a volunteer.

Cross-sectional images of the arm representing the internal absorbing and scattering properties were generated from both relative and absolute values of mean flight time. Images acquired for two volunteers at a wavelength of 780 nm are shown in figure 2 below.

Subject A
Subject B
	MRI image	Scatter image	Absorption image
Figure 2: MRI and optical images of two volunteer forearms.

The optical images above are compared to images of the same forearms obtained later using magnetic resonance imaging (MRI). Reconstruction using the TOAST algorithm employed a 2D FEM mesh, and the data were corrected to account for the fact that the image reconstruction is based on a 2D forward model [2]. The bones are clearly visible as localised regions of increased scatter. Note that subject B had a smaller arm than subject A, and therefore the ring was positioned further up the arm. The absolute values of the transport scatter coefficient are between 0.6 mm^-1 and 2.0 mm^-1. The absorption images do not reveal much high resolution detail, with highest absorption observed near the locations of arteries. The absorption coefficient in both cases varies within the range 0.025 mm^-1 to 0.054 mm^-1. TOAST was also employed to generate images which exhibit the differences in absorption coefficient produced by the finger flexion exercise. The scatter is assumed to remain unchanged. The differences in mean time measured for subject B were used to produce the images shown in figure 3 below.

780 nm
820 nm			Anatomy of the forearm. Muscles activated by finger flexion exercise are highlighted in red.
	Difference image	Difference image + MRI
Figure 3: Images revealing the differences in absorption produced by the finger flexion exercise at two wavelengths (left). Also shown are the results superimposed (in red) on the MRI images (centre), and the anatomy of the forearm (right).