Light shadowing effect of large breast lesions imaged by optical tomography in reflection geometry

Abstract

When a large, highly absorbing breast lesion is imaged by optical tomography in reflection geometry, most of the photons are absorbed by the top portion of the lesion. As a result, the lower portion of the lesion is not quantified correctly. This posterior light shadowing effect is similar to the sound shadowing effect frequently seen in pulse-echo ultrasound images. The presence of significant posterior shadowing of a lesion in ultrasound images suggests malignance. The light shadowing effect due to optical contrast is characterized using a simple measure and validated by the Monte Carlo photon-tracking method and phantom experiments. Clinical examples of large malignant and benign lesions are presented to demonstrate the shadowing effect and the utility of the measure. Understanding and quantifying the shadowing effect due to optical contrast is important for characterizing larger malignant cancers from benign lesions.

Figures

Figure 1

Illustration of the MC simulation when a target is embedded in the medium.

Figure 2

Illustration of different photon groups. (a) Illustration of photon groups when a single target is embedded in the medium. Target group (TG) consists of photons that enter the target; background group (BG) consists of photons that propagate in the background medium only. (b) Illustration of photon groups when the target is segmented into top and bottom layers. Upper target group (UTG) consists of photons that enter the upper half target only; upper and bottom target group (UBTG) consists of photons that enter both the upper and bottom parts of the target.

Figure 3

Simulated absorbed weight distribution of a single target inside the 3-D volume was summed in the y direction and projected to the x-z plane. (a) The background photon group (BG) consists of photons that do not enter the target. (b) The target group (TG) consists of photons that enter the target. (c) The upper target group (UTG) includes the photons entering the upper half target only, when the target is separated into the upper half and the bottom half. (d) The upper and bottom target group (UBTG) includes the photons that reach the bottom half target when the target is separated into two parts.

Figure 4

Probe geometry used for simulation and phantom experiments. The red stars are the locations of sources, and the blue circles are the locations of detectors. The middle slot is used for a commercial ultrasound transducer. (Color online only.)

Figure 5

An example of ratio Phigh∕Plow versus source–detector separation. The x axis is the source–detector distance, and the y axis is the Phigh∕Plow. In the simulation, a pair of high-contrast and low-contrast 2-cm targets was located at 2.0-cm depth.

Figure 6

Average ratio of Phigh∕Plow (intensity ratio) and Rhigh∕low (reconstruction ratio) versus target center location. The three groups of curves correspond to target diameter of 1 cm, 2 cm, and 3 cm. Both Phigh∕Plow and Rhigh∕low are independent of target location, as predicted by Eq. 8.

Figure 7

Experimental results of 3-cm-diam high-contrast target. The target center was located at (a) 2.0 cm, (b) 2.5 cm, and (c) 3.0 cm, respectively. Each slice is the tomographic display of reconstructed absorption distribution in 8 cm×8 cm spatial x-y dimensions. The depth of each slice is marked in the figure.

Figure 8

Experimental results of 3-cm-diam low-contrast target. The target center was located at (a) 2.0 cm, (b) 2.5 cm, and (c) 3.0 cm, respectively. Each slice is the spatial x-y image of 8 cm×8 cm, and the depth of each slice is marked in the figure.

Figure 9

Experimental results of 1-cm-diam high-contrast target. From (a) to (d), the target center was located at 1.5 cm, 2.0 cm, 2.5 cm, and 3.0 cm, respectively. Each slice is the spatial x-y image of 8 cm×8 cm, and the depth of each slice is marked in the figure.

Figure 10

Experimental results of 1-cm-diam low-contrast target. The target center was located at (a) 1.5 cm, (b) 2.0 cm, (c) 2.5 cm, and (d) 3.0 cm, respectively. The depth of each slice is marked in the figure.

Figure 11

The ratio Rhigh∕low versus target center location obtained from phantom experiments. The solid lines are the ratios computed using Rhigh and Rlow obtained from the first target layer over the second target layer.

Figure 12

An example of a 2.7-cm malignant lesion. (a) A co-registered ultrasound B-scan image of the cancer. (b). Reconstructed absorption map at 780 nm.

Figure 13

An example of a 3-cm benign lesion. (a) A co-registered ultrasound B-scan image of the lesion. (b). Reconstructed absorption maps at 780 nm.