Medical applications of low speed and resolution electrical impedance tomography

Abstract

Urinary incontinence (UI), detrusor overactivity and prostatic hyperplasia are pathologies that cause urinary tract infections due to loss of sphincter control. UI is a common condition in elderly people, and between 15% to 35% of all adults suffer from severe UI. Additionally, people suffering from spinal cord injuries are highly prone to urinary tract infection due to the loss of micturition sensation. According to the World Health Organization, the prevalence of urinary tract diseases ranges from 40 to 80 people per million. Hence, UI is a public health problem. UI affects self-esteem and quality of life, and it is associated with higher rates of depression. UI also affects daily activities such as work, travel, social interaction, physical activity, sexual function, and sleep. Currently, catheterization is the most commonly used method for evacuating the bladder for people with UI, but the periodicity of the associated emptying procedures depends on an empirical estimate of the time it takes to fill the patient's bladder. Underestimating this period unnecessarily increases the number of catheterization procedures, thus increasing the risk of infections, cancers, and kidney stones. Conversely, overestimation of the catheterization period increases the risk of complications such as over-distension of the bladder wall, hydronephrosis, and autonomic dysreflexia. Ultrasound-based estimates are the gold standard to estimate bladder volume, with advantages of high portability and accuracy. However, ultrasound measurements must be performed by professionals, precluding continuous monitoring and limiting application to bladder volume monitoring. Due to the electrical conductivity of the urine, several bioimpedance techniques have been considered for bladder volume monitoring. Among these, Global Impedance (GI), which presents a high accuracy in volume estimation for the following configurations: (1) simple ring electrode arrangement with adjacent patterns for injection and measurement, (2) two vertical lines V2L electrodes arrangement with opposite patterns for injection and measurement. Other proposed approaches for bladder monitoring are: Voltage Change Ratios (VCR), Impedance Ratio Method (IRM) and Focused Impedance Method (FIM). Which present high sensitive to changes in the conductivity, just like GI. Therefore, these approaches are not suitable for long-term monitoring of the bladder, because the conductivity of urine varies with health status and diet. The robustness of a new proposal based on FIM was evaluated in a bladder phantom, presenting a low sensibility to the conductivity uncertainty of the phantom; being a promising technique for long-term monitoring of the bladder and would support the assisted bladder emptying process.