Acoustic Radiation Force Imaging (ARFI) Does Not Predict Abnormal Urodynamic Parameters in Children
Courtney L. Shepard, M.D., Ethan A. Smith, M.D., Jonathan R. Dillman, M.D., Kate H. Kraft, M.D..
University of Michigan, Ann Arbor, MI, USA.
Urodynamics is the gold standard for functional assessment of the bladder and can reliably identify patients at risk for upper tract deterioration due to poor bladder compliance. However, this invasive and uncomfortable test is particularly challenging in the pediatric population. Therefore, a non-invasive technique to evaluate bladder dynamics would be ideal. Attempts have been made to correlate ultrasound measurements with bladder compliance with mixed results. Acoustic radiation force imaging (“ARFI”) is a special type of ultrasonography that allows in-vivo measurement of tissue stiffness. ARFI has been proven to be able to detect fibrosis in the liver and other tissues. As a common cause of decreased bladder compliance is bladder fibrosis, this technology could be useful at predicting bladder compliance and perhaps avoiding the need for more invasive studies. We hypothesize that ARFI parameters significantly correlate with abnormal bladder compliance and capacity in patients at risk for neuropathic bladder.
Patients who presented to our pediatric urology clinic for routine outpatient urodynamic cystometry (CMG) to evaluate neuropathic bladder were offered prospective enrollment in this IRB-approved study. After informed consent was obtained, subjects underwent ARFI at the time of their cystometry. Imaging was performed by one of two pediatric radiologists. A total of six bladder wall shear wave speed (SWS) measurements were acquired using Virtual Touch Quantification (VTQ) and Virtual Touch IQ (VTIQ) modes (Siemens Accuson 3000, Erlangen, Germany). The mean for each mode was correlated to bladder compliance, which was calculated using the formula for standardized bladder compliance in children as described by Wahl et al in 2004 (100 NWahl-1= 100 (V/Vcap,NL)/(P/Pcap,NL). ARFI parameters were also correlated to bladder capacity in patients who had capacity less than expected bladder capacity (EBC) based on weight and age. Bivariate relationships were assessed using Spearman correlation for nonparametric data.
A total of 20 patients (7 girls, 13 boys) were enrolled from January 2013 to May 2016. Indications for CMG included myelomeningocele, lipomyelomeningocele, and posterior urethral valves. Mean age at time of enrollment was 4.96 4.13 years (range 0.42-15 years). There was no significant correlation between bladder compliance and VTQ shear wave speed (r = -0.28, p = 0.24 ) or VTIQ shear wave speed (r = -0.18, p = 0.45). A total of 14 patients had bladder capacity below EBC. There was no significant correlation between bladder capacity and VTQ shear wave speed (r = -0.04, p = 0.89) or VTIQ shear wave speed (r = -0.11, p = 0.70).
This is the first study that has evaluated the relationship between ARFI technology and urodynamic parameters. Prior studies have demonstrated a positive correlation between increased shear wave speed measurements using ARFI and tissue fibrosis in the liver. Our results failed to demonstrate a significant correlation between ARFI shear wave measurements and reduced bladder compliance or bladder capacity. Limitations include a small number of subjects, and further studies are warranted to determine whether ARFI may be utilized to predict abnormal urodynamic parameters in children.
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