Like all devices, ultrasound probes, even those that are new and unused are subject to varying imaging quality. This results from minor variations in their manufacturing such as the imprecise nature of gluing the acoustic lens to the probe. Ageing and constant use introduce further variations which affect imaging quality and therefore diagnostic accuracy. The degradation of image quality is initially small, however progressively increases over time. These changes are barely perceptible to the naked eye compounding the problem.

Hypothesis 1: Initial probe imaging quality and the rate at which it degrades is not the same for all probe models from all manufacturers. There are clinically significant differences between different models ^{1, 2, 3, 4}.

Hypothesis 2: The loss of quality is pronounced in the side coils suggesting that imaging quality loss may be detected and quantified by observing the side coils ^{1, 5}.

Hypothesis 3: The novel Random Void Phantom (RVP) can be used to asses probe imaging quality in an objective test system. Reductions in grayscale resolution and maximum acoustic contrast of an ultrasound probe can be quantified. Criteria based on these measured parameters can be used to determine the suitability of a probe for clinical use. Probes deemed to be of inadequate imaging quality can be removed from further use in order to preserve diagnostic accuracy ^{6, 7}.

Measurement of 26 different transducer models from five manufacturers was performed using the RVP. The RVP for grayscale resolution consists of an artificial water filled foam sponge submersed in a water filled tank, Figure 1.

Figure 1. Random Void Phantom: Water-filled artificial foam immersed in a water tank. The 3D image shows the ultrasound image of the sponge acquired with a matrix probe.

Synthetic foam has an acoustic absorption similar to echo free cysts (voids) in living tissue. The voids are filled with pure water being free of reflective contaminants. The remaining background noise in the voids is caused only by the side and grating lobes. The tolerances of the foam inhomogeneities for maximum acoustic contrast are in the range +/- 1.5 dB ^{1, 6}.

The transducers to be measured were motor-driven, or passed over the RVP manually. The main and side lobes were recorded. The ultrasound image sequences were documented in 3-sec loops as AVI files and subsequently processed as 3D-data blocks.

Maximum acoustic contrast and grayscale resolution of the various ultrasound probes was evaluated with software from Tissue Characterization Consulting (TCC), Figure 2. The higher and narrower the amplitude, the sharper the acoustic contrast ^{6, 8, 9}.

Probe measurement was performed at four sites; Trnava, Bratislava, Ludwigsburg and Tübingen. Data analysis was performed by the author J.S. using the TCC software developed by J.S. (Medical Technology Lange, Rostock & SonoGalerie, 18211 Bargeshagen, Schliemannstraße 6).

Acoustic contrast, using the RVP, was documented with linear arrays and matrix transducers. Low intensity contrast differences are indistinguishable and appear nearly black to the naked eye because the human eye can only discern 16 gray levels. These small differences in the low intensity range can however be visualized and evaluated when projected onto the 8-bit grayscale (256 shades of gray) of modern equipment. This is possible when the dynamic range (DR) is properly set and results in a smooth contrast graph, Figure 2.

Figure 2. measurement side lobes in different depths. A high and narrow amplitude indicates a sharp acoustic contrast. (azimuthal = lateral; elevational = perpendicular to lateral = transversal)

The maximum acoustic contrast and dynamic range of 26 transducers were measured. A surprisingly broad range of acoustic contrast values from 14.6 dB to 36.6 dB was observed, Table 1.

Probes with high acoustic contrast (32.63 dB to 36.66 dB) generated high quality images, Figure 3. Those with acoustic contrast values from 20 dB to 25 dB produced images of lower quality, Figure 4. A typical image produced by a damaged transducer measured as having an acoustic contrast of only 14.68 dB is shown below, Figure 5.

Figure 3. Random Void Phantom - transducer with high contrast resolution (36.66 dB, Philipps IU22.L12.5 MHz). Original 3D rendered image (left), with increased image contrast (right). Even voids within the RVP with weak contrast are discernible.

Figure 4. Random Void Phantom - medium contrast resolution (21.61 dB, Alloca SSD 5412). Original 3D rendered image (left), with increased image contrast (right).

Figure 5. Random Void Phantom) - damaged transducer with inadequate contrast resolution (14.68 dB, Alloca SSD P52105). Original 3D rendered image (left), with increased image contrast (right). Small voids within the RVP with weak contrast cannot be visualised.

This work highlights the importance of transducer maximum acoustic contrast for the imaging of non-homogeneous tissues. The best grayscale differentiation and highest acoustic contrast were achieved by matrix probes.

Based on our observations, applications demanding high grayscale differentiation as abdominal and breast examinations should only be performed with transducers measured as having a maximum acoustic contrast greater than 25 dB in order to optimally detect different tissue types. Ultrasound probes with an acoustic contrast between 15 dB and 25 dB are only appropriate for examinations where grayscale differentiation is not critical, such as is the case for orthopaedic examinations. Probes with a measured acoustic contrast of less than 15 dB should be retired to preserve diagnostic accuracy.

With the simple free hand scan transducer testing on RVP with 3D-data acquisition, quantitative determination of probe contrast quality and detection of small sound probe errors are possible.