Improving image quality by accounting for changes in water temperature during a photoacoustic tomography scan

Abstract

The emerging field of photoacoustic tomography is rapidly evolving with many new system designs and reconstruction algorithms being published. Many systems use water as a coupling medium between the scanned object and the ultrasound transducers. Prior to a scan, the water is heated to body temperature to enable small animal imaging. During the scan, the water heating system of some systems is switched off to minimize the risk of bubble formation, which leads to a gradual decrease in water temperature and hence the speed of sound. In this work, we use a commercially available scanner that follows this procedure, and show that a failure to model intra-scan temperature decreases as small as 1.5°C leads to image artifacts that may be difficult to distinguish from true structures, particularly in complex scenes. We then improve image quality by continuously monitoring the water temperature during the scan and applying variable speed of sound corrections in the image reconstruction algorithm. While upgrading to an air bubble-free heating pump and keeping it running during the scan could also solve the changing temperature problem, we show that a software correction for the temperature changes provides a cost-effective alternative to a hardware upgrade. The efficacy of the software corrections was shown to be consistent across objects of widely varying appearances, namely physical phantoms, ex vivo tissue, and in vivo mouse imaging. To the best of our knowledge, this is the first study to demonstrate the efficacy of modeling temporal variations in the speed of sound during photoacoustic scans, as opposed to spatial variations as focused on by previous studies. Since air bubbles pose a common problem in ultrasonic and photoacoustic imaging systems, our results will be useful to future small animal imaging studies that use scanners with similarly limited heating units.

Figure 1. Nexus 128 scanner.

(a–c) are reproduced with permission from http://www.endrainc.com. The green box delineates the spatial support of a representative reconstructed field of view. The dimensions of this reconstructed volume are typically set to 2×2×2 cm, and the isotropic reconstruction resolution typically to 0.2 mm.

Figure 2. Speed of sound as a function of water temperature.

Figure 3. Phantoms.

The vessel phantom in (b) is a print-out of a digital phantom provided in the k-Wave Toolbox .

Figure 4. Perfused and excised mouse brain.

(a) whole brain, (b–g) representative slices.

Figure 5. Dot and vessel phantom reconstructions from data acquired with the water heating system switched off.

All reconstructions in a single row are displayed on the same grayscale (arbitrary units). is the drop in water temperature during the scan. For reconstruction purposes, the water temperature was assumed to be the mean of the water temperature before and after the scan. The planes of the phantoms were not perfectly aligned with the horizontal planes of the reconstruction volumes. As a result, only parts of the phantoms are visible at a time. For example, only one column of dots is visible in the dot phantom, and only part of the vessel structures are visible in the vessel phantom. MIP images that show all structures are shown in Figs. 8 and 9. We show both slices and MIP images to illustrate the difference in appearance of the speed of sound artifacts in both types of images. The red arrow in (l) indicates artifact that is visually indistinguishable from true vessel structures.

Figure 6. Dot phantom and vessel phantom reconstructions from data acquired with the water heating system switched off.

All reconstructions in a single row are displayed on the same grayscale (arbitrary units). is the drop in water temperature during the scan. For reconstruction purposes, the water temperature was adjusted for each bowl position separately, based on the temperature record of the water heating system's thermometer.

Figure 7. Dot and vessel phantom reconstructions from data acquired with the water heating system switched on.

All reconstructions in a single row are displayed on the same grayscale (arbitrary units). During reconstruction, the water temperature was assumed to be the mean of the water temperature before and after the scan. Since the water pump was switched on, these temperatures were nearly identical and approximately equal to the target temperature of 38°C.

Figure 8. MIP projections of dot phantom reconstructions.

All reconstructions in a single row are displayed on the same grayscale: to (arbitrary units).

Figure 9. MIP projections of vessel phantom reconstructions.

All reconstructions in a single row are displayed on the same grayscale: 0 to 1 (arbitrary units).

Figure 10. MIP projections showing the effect of correcting for decreasing temperature and hence decreasing speed of sound.

By applying the speed of sound correction, the vessel bifurcation in the lower left quadrant becomes visible (red arrow). The vessel coming in from the top left also becomes clearer.

Figure 11. Slice through a reconstruction of a perfused and excised mouse brain.

The correction for the changing speed of sound (d) greatly focuses the anatomical structures present in the image, compared to when reconstructing with the initial (a), average (b) or final (c) water temperature. These results illustrate that the image quality cannot be maximized by simply optimizing a single speed of sound value. Also note the close resemblance of (d) to the physical slice shown in Fig. 4(d). All reconstructions are displayed on the same grayscale: to (arbitrary units).