Initial 8-channel experiment

8.2.1.1.1 Introduction

Prior to completion of the instrument, a preliminary test was performed on the basic cylindrical phantom described above. Since it resulted in the first published image acquired with the MONSTIR system, it is briefly described here for ‘historical’ reasons. A more detailed discussion of this experiment is provided by [Hebden 1999]. Section 8.2.1.2 below provides a much more in-depth discussion of the measurement data and images that were acquired using the same phantom, but with the completed and fully functional system.

At the time of this preliminary experiment only one MCP-PMT was installed, pro­ viding 8 physical detector channels. However, by swapping the source between two positions relative to the detector optodes, it was possible to ‘simulate’ a fan-beam geometry with 16 detectors located opposite to the source. Manual rotation of the fibre holder ring around the phantom to 16 different positions therefore allowed us to acquire data equivalent

photograph in Figure 8-6 shows a single source fibre that is opposed by the 8 detector fibre bundles.

Figure 8-6 Photograph of the basic cylindrical phantom (c.f. illustration in Figure 8-5). A source fibre can be seen on the left, and 8 detector fibre bundles on the right. No VOAs were employed in the experiment because they were not yet integrated into the system. This was acceptable, since the dynamic range of detected intensities in this fan- beam geometry was relatively low as compared to that for the full 32-channel system. A laser power of approximately 20 mW was applied for 60 sec at each source position.

8.2.1.1.2 Results

As described in section 8.1.3.1 above, data types are derived from the TPSFs. The mean time and variance around the mean were used here, and no pre-processing other than calibration of the temporal offset for variability in fibre and electronic cable lengths, was applied. The variance was derived from a diffusion model fit to the data in order to reduce the effect of the reflections in the polymer fibre. These data were then employed by the TOAST image reconstruction package to obtain two maps representing the distribution of scatter and absorption within the transverse plane.

(b) (c)

Figure 8-7 A cross-section of the phantom indicating the three regions of interest (a), and the absorption (b) and scattering (c) profiles corresponding to iteration 30 of the image reconstruction process.

The images shown in Figure 8-7 were produced using a 2D reconstruction starting with an initial guess obtained from a global fit of homogeneous scattering and absorption coeffi­ cients to all the data. The linearly scaled images clearly reveal the absorbing and scattering inhomogeneities at their known locations. The images are purely qualitative, and some cross talk between the absorbing and scattering objects, as well as other artefacts, are present.

8.2.1.2 32-channel experiment

8.2.1.2.1 Introduction

Figure 8-8 shows a photograph of the basic cylindrical phantom with all 32 source fibres and 32 detector fibre bundles attached around the circumference.

Figure 8-8 Photograph of the solid tissue-equivalent phantom with the fibre holder ring and all 2x32 fibres mounted. Note the small-diameter source fibres, which are interspersed between the large-diameter detector fibre bundles.

The laser power applied to the phantom surface in this imaging experiment was approxi­ mately 1 mW, producing count rates of up to around 30 kcps per channel. Data were recorded at a single wavelength X = 800 nm. The acquisition time per source position was arbitrarily chosen to be 30 sec, although this could have been significantly shorter as the recorded signal was later found to be more than adequate. VOAs were employed to equalise the detected intensities, and five detector channels on either side of the activated source are completely shut off in order to satisfy the finite dynamic range of the VOAs. However, data

from small source-detector separations are considered to contribute little to the reconstruc­ tion of deeper structure within the object. This acquisition geometry is equivalent to a fan beam configuration with each activated source opposed by 22 active detector channels, resulting in a total of 32x22=704 TPSFs.

The results presented here have been published in [Schmidt 2000a], and represent the first obtained following completion of the system. All 32 source and detector channels are utilised, the VOAs are integrated, and the data acquisition is performed fully automati­ cally without the need to manually rotate the fibre holder ring. Compared to the procedures outlined in the previous section describing the initial 8-channel experiment, these measures have resulted in considerable improvements in data collection efficiency, acquisition speed and ease of use of the instrument, all of which are important criteria for a clinical imaging instrument.

8.2.1.2.2 Results

The 22 TPSFs recorded for the first of 32 sources are shown in Figure 8-9. As expected, the mean time and width of the TPSFs increase with optode separation.

7 8 9

S ource

1000 2000 3000 4000 5000 6000

(a) (b) Time [ps]

Figure 8-9 TPSFs (b) recorded by the 22 active detectors opposite source no 1 (a). Each TPSF consists of up to -10^ photons, and the curves are normalised to their re­ spective maximum values.

Three data types, mean time, variance and the normalised Laplace transform (with a coefficient of 0.005 p s ' \ were derived from the full set of 22x32 TPSFs. The calibrated and corrected data type values corresponding to the TPSF raw data of Figure 8-9 (h) are shown in Figure 8-10. It is evident that the mean time increases with optode separation; hence detectors furthest from the source have the largest values (c.f. also with Figure 7-16 which shows the mean times for a homogenous phantom). The variance depends on the width of the curve, and therefore also increases with optode separation. The normalised Laplace transform is sensitive to early light, and increases as the number of shorter flight­ time photons increases. It is therefore largest for detectors that are adjacent to the source. The ‘fingerprint’ of the three embedded cylinders is manifested by the asymmetry and irregularity of these data type curves, although some of that may be artefactual due to instrumental and/or calibration errors.

(a) 3,000 2,500 « ’ 2,000 a I . . . . P 1,500

I

1,000 500 0 5 10 15 20 25 30 Detector C hannel 600,000 500,000 a 400,000 « 300,000 200,000 100,000 (b) 0 5 10 15 20 25 30

D ete ctor C hannel

0.025

I

I

1 0.020 0.015 ■o 0.010 . S 2

I

_{o 0.005} 0.000 (C) _{0 5 10 15 20 25 30} D e te ctor C hannel

Figure 8-10 The three commonly used data types, mean time (a), variance (b) and Laplace transform (c) extracted from the TPSF curves presented in Figure 8-9 (h).

The 2D/3D correction of the measurement data, combined with the fact that the optodes were arranged in a single plane, enabled TOAST to employ the computationally faster 2D reconstruction. Using the three types of measurement data (mean time, variance and Laplace transform) described above, and starting with homogeneous background values of |Lta = 0.01 and tis' = 1 .0 (the nominal optical properties of the phantom), the absorption and scattering maps corresponding to the phantom’s cross section were simultaneously reconstructed. The circular FEM mesh used in the forward solver consisted of 7392 triangular elements (c.f. Figure C-1 in the TOAST appendix), and the image was recon­ structed into a 16x16 pixel basis^*. The Robin boundary condition was used with the non­ linear conjugate gradient method, and median filtering after each update. No régularisation schemes were employed. Figure 8-11 shows the first 35 iterations of the TOAST image reconstruction process. There is little improvement after about 10 iterations. However the scattering feature on the right-hand side of the images appears to become artefactually strong at higher iterations.

The absorption and scattering images from iteration 17 are shown in Figure 8-12, and clearly reveal the 3 embedded perturbers at their known locations. The absorption image is dominated by region B, while region A is clearly revealed in the scattering map. The weaker region C can be identified in both images. Although the separation of the absorption and scattering profiles is good, there is some identifiable cross talk, noticeably in the scattering image. This cross talk and the inherently lower resolution of the absorption image are, at least partly, due to the choice of data types. There are also some artefacts, such as the presence of a ring, in the absorption map. This residual ring artefact is largely due to 2D/3D errors that are not fully eliminated by the Green’s function based correction method described in section 8.1.3.2 above (see also [Ftilhnan 2000] for a discussion of this type of artefact). These images are purely qualitative, but it should be stressed that they are absolute images which were obtained without any reference measurements (i.e. without a measurement of the object minus the perturbers) from purely time-resolved data. The optical properties for this iteration range from = 0.62 to 1.36 mm \ and Pa = 0.007 to 0.017 m m '\ The lack of quantitation and relatively low contrast can be attributed to several factors which are examined in the discussion in chapter 9.

Hence the noticeable pixelation as compared to other images in this chapter that were reconstructed into an element basis.

ABSORPTION

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