GenIII MRI Platform Design and Implementation

The MR components of the GenIII Optical-MR Imaging platform were developed by Dr. Thomas J. Con- nick, building off his earlier work on the MR platform in the GenII system238_{(Section4.2.2), while adapting}

the design for 3T.

This platform contains an 8-coil SENSE (sensitivity encoding) surface coil array (2 lateral and 2 medial coils on each breast). The medial coils have local preamplifiers; the lateral coils currently use an external preamplifier box. Preamplifiers will be added to the lateral coils in the near future; we prioritized using our limited available supply of these specialized and long lead time components on the medial coils as the

Figure 6.1: Photos of GenIII Optical-MR Imaging platform. A detector fiber module is mounted on the left breast lateral compression plate; a source fiber module is mounted on the medial plate (source fiber tips are visible as white dots on the medial plate in the left image).

Figure 6.2: Several views of the GenIII Optical-MR Imaging platform. (right) Platform without padding or linens. (left) Platform with minimal padding and linens in place; with the linens in place the weight bearing elements of the platform are colored white to minimize the possibility of damage to the platform through excessive load on the mobile compression plates.

Figure 6.3: Images of the lateral compression plates in the GenIII system. (left) Left lateral plate with a fiber optic module blank (i.e. before machining for fiber optic mounting) in the center position. (right) Right lateral plate with a biopsy plate mounted. The cable visible in the bottom of the images connects the right lateral posterior coil to the external preamplifier box out of the frame to the right. The copper lateral coils are visible through the clear Plexiglas cover surrounding the biopsy plate.

coupling between these coils is problematic.

Portions of the RF electronics are visible through the clear Plexiglas covers of the lateral plates in Figure6.3. A schematic of the space allotted to electronic and optical components is shown in Figure6.4

and the entire platform in the MRI suite in Figure6.5. The limited space for electronics created a number of practical difficulties and considerable effort was expended to maintain the large area of access for the optical modules.

MR phantom imaging (Figure6.6) demonstrated that the system has sufficient resolution for clinical research use by our collaborators Drs. Angela DiMichele and Mark A. Rosen in the ISPY-2 triali_.

As mentioned above, the GenIII platform utilizes fiber optic modules, permitting fast and simple reposi- tioning of fibers. These modules also greatly reduce system set-up time in the clinic, a critical concern with tightly scheduled clinical instruments; designes are shown in Figure6.7and Figure6.8. We have currently constructed one pair of source and detector modules to fully validate the design; we will construct the others after we have recruited several human subjects for simultaneous DOT and MRI. These initial modules (pho- tos shown in Figure6.9and Figure6.10) contain 5 TD and 54 CW detector fiber positions and 32 source

170

8

8

120

25

67.5

67.5

Figure 6.4: A schematic of the compression plates for the GenIII system. Blue areas are available for the RF electronics; red areas are cutouts for the placement of optical fiber modules; white areas were reserved for mechanical support and mounting. Note the upper optical access region, which permits optodes to be placed a few mm from the chest wall. Dimensions are in mm.

Figure 6.5: Photos of the GenIII Opt/MR platform during a phantom imaging study. The fiber optic modules were not mounted when these photos were taken.

Figure 6.6: Example MR Imaging with GenIII platform. (left) Images of copper sulfate phantom and human forearms, showing thefiducialmarkers used for image co-registration. (center) A phantom composed of ground turkey, vegetable shortening (Crisco), and saline (phantom suggested by Dr. Sarah Englander, based on earlier work on breast tissue quantification243) atop a phantom of cooking oil and saline. (right) Joint Optical/MR phantoms composed of gelatin, Liposyn, and ink, with inclusions containing Gd chelate (MultiHance) and ICG.

Figure 6.7: Example detector fiber module for the GenIII system. (left, top) Schematic of side view, showing (blue) optical fiber, mounting plate, and protective cover; bottom view, showing placement of 1 mm (CW) and 3 mm (TD) fiber optics. (left, bottom) Top view of detector module, showing the layout of fiber optics the larger, 3 mm, holes are for the TD detectors. The walls of the protective cover (magenta) are made of 1/16thinch PVC; 1/4thinch PVC is used for the end pieces. (right) Photos of fiber optic modules without protective back cover. The tissue face of this module is shown in Figure6.9. Dimensions are in mm.

positions, a significant improvement over other reported systems (Table3.2). Note, the current source mod- ule has additional fibers pre-positioned to be utilized for additional CW/TD sources (12) and single mode fibers for use as DCS detector fibers (12).

In addition to the changes in the fiber optical mounts, we attempted to utilize the information gathered over the course of our study to improve subject comfort during the exam: not only would this reduce the strain on our subjects, but comfortable subjects are less likely to move during the exam, improving our data quality. The most common complaint from our subjects was neck pain due to laying on the right or left side

Figure 6.8: Example of GenIII source fiber module. Source fibers were inserted at 20 or 30 degree angles to reduce the bending radius of the fibers. The walls of the protective cover (blue) are made of 1/16thinch PVC; 1/4thinch PVC is used for the end pieces. The tissue face of this module is shown in Figure6.9. Dimensions are in mm.

Figure 6.9: GenIII source and detector fiber modules, both 3.5x18 cm. The source plate includes thirty two CW/TD multi-mode source fibers and ten single mode DCS detector fibers; the detector plate holds fifty four 1 mm CW detector, ten 1 mm DCS source, and five 3 mm TD detector fiber bundles. Schematics of the detector and source modules are shown in Figure6.7and Figure6.8.

of the head. We addressed this by installing a ‘face-pillow’ from a massage table in the GenIII platform. The second most common complaint focused on the portion of the subject’s weight which rested on her sternum. We addressed this by supporting both the shoulders and the abdomen in the updated platform (see Figure6.2and Figure6.5) and by widening the sternum support from 5 to 5.5 cm.

The GenII platform was constructed of white PVC plastic which permitted considerable light reflec- tion/scattering. Unfortunately, most black plastics utilize carbon black as a pigment; when tested, they all proved to cause artifacts in MR images. Note, this effect is related to the quantity of plastic used and the few grams of black Delrin and PVC in our fiber coverings have little effect on the MR signal. We therefore chose to construct the GenIII system out of gray PVC as a compromise between MR compatibility and stray light reduction.

Figure 6.10: GenIII detector modules and compression plates. (left, top) a schematic of 5 detector modules (A, B, C, D, and a fixed module near the chest wall) mounted in the lateral compression plate. (right, top; bottom) Photos of a detector module (A in the left, top schematic) mounted in the imaging platform. This module is also shown in Figure6.7and6.9. A source module is visible in the lower left and upper right images.