Review of imaging test phantoms

Abstract

Significance: Photoacoustic tomography has emerged as a prominent medical imaging technique that leverages its hybrid nature to provide deep penetration, high resolution, and exceptional optical contrast with notable applications in early cancer detection, functional brain imaging, drug delivery monitoring, and guiding interventional procedures. Test phantoms are pivotal in accelerating technology development and commercialization, specifically in photoacoustic (PA) imaging, and can be optimized to achieve significant advancements in PA imaging capabilities.

Aim: The analysis of material properties, structural characteristics, and manufacturing methodologies of test phantoms from existing imaging technologies provides valuable insights into their applicability to PA imaging. This investigation enables a deeper understanding of how phantoms can be effectively employed in the context of PA imaging.

Approach: Three primary categories of test phantoms (simple, intermediate, and advanced) have been developed to differentiate complexity and manufacturing requirements. In addition, four sub-categories (tube/channel, block, test target, and naturally occurring phantoms) have been identified to encompass the structural variations within these categories, resulting in a comprehensive classification system for test phantoms.

Results: Based on a thorough examination of literature and studies on phantoms in various imaging modalities, proposals have been put forth for the development of multiple PA-capable phantoms, encompassing considerations related to the material composition, structural design, and specific applications within each sub-category.

Conclusions: The advancement of novel and sophisticated test phantoms within each sub-category is poised to foster substantial progress in both the commercialization and development of PA imaging. Moreover, the continued refinement of test phantoms will enable the exploration of new applications and use cases for PA imaging.

Keywords: photoacoustic; test phantom.

Fig. 1

A diagram depicting the basics of PA. A light source shines a focused and pulsing laser into the body. The area of interest goes through rapid thermal expansion creating an acoustic signal. The acoustic signal can then be received by an acoustic receiver.

Fig. 2

An image containing a simple tube phantom. A tube full of fluid was pulsed with a syringe pump and collected in a container. The flow of the fluid through the tube could be measured using PA.

Fig. 3

An example of a simple naturally occurring phantom. Francis et al. (Ref. 36) image the leaf phantom with three different methods: (a) optical photograph, (b) PA, and (c) ultrasound. (d)–(h) The resolution of PA was compared to the photograph when the number of angles was changed.

Fig. 4

A simple block phantom is depicted. PVA was used as a base material. (a) The clear PVA with dark PVA gel spheres representing breast cancer. (b) The final block phantom after going through four freezing and thawing cycles.

Fig. 5

An example of an embedded tube/channel phantom. PVC pipe was used to create four channels within a ballistic gel. The phantom was then sealed and used for ultrasonic measurement. (a) A top view of the phantom and (b) the channels from the side.

Fig. 6

An example of an intermediate naturally occurring test phantom. A model tissue was used for a reference and is labeled “tissue in vivo.” A manufacturing process is then described by first creating a base of collagen. Fibroblasts were then added followed by blood cells. Finally, a layer of epithelial cells was dispersed on top to create the simulant tissue.

Fig. 7

An example of two ML block phantoms. (a) A human breast phantom. The shape of the human breast was created through a flexible latex shell. The center of the breast was filled with a PVA liquid. (b) A human baby head phantom. This head was created with a latex shell and PVA interior. Scattering material could be added to the inside PVA material to represent cancer sites.

Fig. 8

An example of an ATC phantom. This phantom mimics the human pulmonary artery. This phantom was 3D printed using a PolyJet^© printer. The bottom left hand of the image shows an example of a catheter being placed into the phantom.

Fig. 9

An example of an advanced layered phantom. Three different phantoms are depicted. On the far left, a 3D-printed phantom face is created from a 2D image reconstruction. Both phantoms in the middle are created using EcoFlex 0030 and Dragon skin rubber. They have layers for different features of the face. This phantom was then used to spoof the iPhone Face ID technology.

Fig. 10

Two phantom cartoons that can be implemented for PA imaging. (a) The cross section of a phantom containing a series of wires embedded in a surrounding tissue. The wires are a set diameter and allow for a quantification of resolution as a function of depth. (b) A lp/mm phantom embedded in a surrounding tissue. This phantom allows for the quantification of elevation and lateral resolutions.