To assess the most accurate method and frequency probe required to assess bladder wall thickness (BWT) accurately.

We harvested eighteen bladder dome specimens from cadavers after ethical approval. The bladder dome tissues were scanned with 3 ultrasound probes [AB27D (TA 2D, 7MHz), RAB25D (TA 2/3D, 5MHz), RIC59D (TVS 2/3D, 9MHz)], using GE Voluson E8 scanner. All the specimens were measured for the minimum and maximum width with a micrometer calliper. Eighteen bladder specimens were scanned on 2 different occasions by a specialist and a second operator. Both were trained in measuring the BWT according to the protocol.

BWT varied due to the presence of trabeculations. The bladder tissue was soft and compressible. Putting pressure on the tissue caused deformation and variation in readings. The peritoneal surface caused a bright echo with the TA probes (curved array), but less so with the TVS probe (endo array). Spearman’s rho correlation coefficient was used to assess the agreement on the readings between the different probes (See Table 1).
The calliper measurements varied widely. The calliper thinnest measurements ranged from 1mm to 10mm (mean 4.9mm). The thickest calliper measurements ranged from 3mm to 13mm (mean 6.02mm).

The 7MHz curved array probe gave reliable measurements when measuring both the Full thickness image as well as the Detrusor only image. The 7MHz curved array measurements of the Detrusor muscle only, gave the most highly correlated measurements compared to the calliper measurements (=0.864, P<0.00001). The 5MHz curved array probe gave reliable measurements only when measuring the Detrusor muscle only (=0.719, P=0.001). The 5MHz TA probe caused the thickest acoustic shadows on the peritoneal surface.  The 9MHz endo array probe gave reliable results when measuring the Detrusor muscle only (=0.641, P=0.004), despite the fact that visually it cast the least acoustic shadows. There was good inter-observer variability (Cohen’s Kappa 0.89).

We assessed 3 probes with different frequencies (5MHz, 7MHz, and 9MHz). The 9MHz TVS probe cast the least acoustic shadows. Higher frequency probes delineate more tissue detail giving a sharper image for measurements to be taken. The 5MHz and 7MHz probes were curved arrays. The 9MHz probe was an endo array probe. The inherent technological differences in transmitting the ultrasound waves may have also influenced the measurements. 
To our knowledge, this is the first study assessing ultrasound accuracy for BWT using different ultrasound probes and different measurement methods. Ultrasound measurement of the BWT is a non-invasive method of predicting detrusor over-activity. It has been suggested that BWT assessment may reduce the need for invasive urodynamic investigations, that are costly and invasive. In men, BWT is measured abdominally, where peritoneal acoustic shadows can be very prominent. In clinical practice, the thickness of the Detrusor muscle only is measured most commonly, without the acoustic shadows of the peritoneal and epithelial surfaces that appear on either side of the detrusor. When scanning patients, the acoustic shadows are not always seen, depending on which probe is used and on the method of imaging employed. In research studies and clinical practice, BWT has been measured with a variety of probes, methods and scanning equipment. Possible artefacts, such as the peritoneal surface of the bladder casting an acoustic shadow leading over-estimation of BWT were investigated.

The data suggests that detrusor muscle thickness is more reliably measured and using a higher frequency of probe is more reliable. Also using a 5MHz probe is not reliable.