The Effects of Fatty Desposits on the Accuracy of the Fibroscan Liver Transient Elastography Ultrasound System

A new generation of ultrasound transient elastography (TE) systems have emerged which exploit the well-known correlation between the liver’s pathological and mechanical properties through measurements of the Young’s elastic modulus; however, little work has been carried out to examine the effect that fatty deposits may have on the TE measurement accuracy. An investigation was carried out on the effects on the measurement accuracy of a transient elastography ultrasound system, the Fibroscan®, caused by overlaying fat layers of varying thickness on healthy liver-mimicking phantoms, simulating in vivo conditions for obese patients. Furthermore, a steatosis effect similar to that in non-alcoholic fatty liver disease (NAFLD) and alcoholic liver disease (ALD) was simulated to investigate its effect on the TE system. A range of novel elastography fat-mimicking materials were developed using 6-10wt% poly(vinyl alcohol) cryogel capable of achieving a range of acoustic velocities (1482-1530m/s) and attenuation coefficients (0.4-1dB/MHz/cm) for simulating different liver states. Laboratory-based acoustic velocities and attenuation coefficients were measured while the Young’s modulus was established through a gold standard compression testing method. A significant variation of the Young’s elastic modulus was measured in healthy phantoms with overlaying fat layers of thicknesses exceeding 45mm, impinging on the scanners region of interest, overestimating the compression tested values by up to 11kPa in some cases. Furthermore, Fibroscan® measurements of the steatosis phantoms showed a consistent overestimation (~54%), which strongly suggests that the speed of sound mismatch between that of liver tissue and that assumed by the scanner is responsible for the high clinical cut-offs established in the case of ALD and NAFLD

Comparative Review of Imaging Methods Used for Diagnosing Renal Artery Stenosis (RAS)

This comparative review examines the efficacy of different imaging methods to detect and quantify renal artery stenosis (RAS). Detection of RAS is important because it can lead to renovascular hypertension which is the most common form of secondary hypertension. Furthermore, it is important that a RAS is detected as early as possible as it is a potentially correctable cause of hypertension.1 If detected at an early stage RAS can potentially be treated using a minimally invasive drug treatment regimen rather than the more invasive percutaneous transluminal renal angioplasty without or with stent placement or surgery. Currently there are a number of different types of modalities used to image the renal artery and determine whether a stenosis is present. Each of these imaging modalities has its own positive and negative aspects, which will be discussed in turn

Evaluation of CIRS String Doppler Phantom as a Test Tool for Use in a Doppler Ultrasound Quality Assurance Program

Ultrasound Doppler systems are routinely used to perform blood flow velocity measurements which assist in the clinical assessment and diagnosis of vascular. Doppler measurements of peak velocities for vascular applications provide an indication of the degree of the stenosis which will ultimately assist in deciding how a patient is managed. It is imperative that Doppler systems are capable of accurately measuring blood flow velocities to ensure correct diagnosis and appropriate patient treatment; therefore such systems should be evaluated regularly as part of a Quality Assurance program. Although a range of Doppler test phantoms have been developed for quality control (QC) purposes to establish the measurement accuracy and stability of Doppler systems only a limited number of such test phantoms are commercially available, the easiest of these devices to operate is the String Phantom. Currently, only one string Doppler phantom is commercially available, namely the CIRS Model 043. In this study an evaluation of the performance of this test device was carried out as a number of problems currently exist with it such as the filament type, the fact that the filament passes through a water–air interface and vibrations from the motor. This study has established that the braided-silk filament, provided with the phantom, should not be used as it introduces errors of as much as 24% for the mean velocity accuracy and 20% for the intrinsic spectral broadening (ISB) depending on the soak time of the filament. Rather, to avoid such errors it is advised that the phantom be retrofitted with a filament made from an O-ring rubber. While this eliminates the temporal changes in backscatter seen with the braided-silk filament, further discrepancies were observed, even with an O-ring filament, when the filament velocity was set in the range 26–44 cm/s, where a resonance effect significantly increased the variability of the maximum velocity accuracy and ISB measurements. This was most likely as a result of the imposed vibrations from the motor, which is mounted directly onto the tank wall; hence, it would prove practical to avoid taking measurements in this velocity range where resonance effects are observed

Characterisation of the Colour Doppler Twinkle Artefact

This investigation involved the development of a range of Colour Doppler Twinkle Artefact phantoms to characterise and quantify the “Twinkle” artefact which is often present when an irregular structure is encountered in the imaged field of view. The artefact occurs in both colour and power Doppler ultrasound imaging and manifests as a false depiction of colour velocity information in stationary soft tissue and therefore can cause significant misdiagnosis of areas of flow within the patient. It has been hypothesised that it is generated due to a strongly reflecting medium composed of individual reflectors and therefore becomes a clinical concern when parenchymal calcifications are encountered. (Tsao et al., 2006). The aim of this study was to investigate the occurrence and magnitude of this artefact across a range of ultrasound scanners and to monitor the effects on the artefact of varying image acquisition parameters. A range of phantoms were produced that could reproducibly recreate the Twinkle artefact, the presence of which was quantified in a range of scanners (Zonare, Siemens Antares, Philips HDI and IU22). These phantoms included both fine and coarse structures as well as a flow channel in one of the phantoms, through which blood mimicking fluid was pumped. A semi-quantitative grading system was implemented and instrument controls such as pulse repetition frequency (PRF), colour write priority, greyscale gain and depth of focal zone were varied in order to determine their impact on the Twinkle artefact. Instrument control settings were found to significantly affect the intensity of this artefact, predominantly PRF showed a significant increase in the presence of this artefact. Furthermore, the extent of the artefact varied greatly across the range of scanners with Siemens Antaries and Zonare being most sensitive to the artefact. The implication of this study has shown the Twinkle artefact to be dependent on scanner specifications and instrumental parameters. With careful image optimisation, a reduction or elimination of the artefact can be achieved

Comparative Imaging Study in Ultrasound, MRI, CT and DSA Using a Multi-Modality Renal Artery Phantom

A range of anatomically realistic multimodality renal artery phantoms consisting of vessels with varying degrees of stenosis was developed and evaluated using four imaging techniques currently used to detect renal artery stenosis (RAS). The spatial resolution required to visualize vascular geometry and the velocity detection performance required to adequately characterize blood flow in patients suffering from RAS are currently ill-defined, with the result that no one imaging modality has emerged as a gold standard technique for screening for this disease. Methods: The phantoms, which contained a range of stenosis values (0%, 30%, 50%, 70%, and 85%), were designed for use with ultrasound,magnetic resonance imaging,x-raycomputed tomography, and x-raydigital subtraction angiography. The construction materials used were optimized with respect to their ultrasonic speed of sound and attenuation coefficient, MR relaxometry(T1,T2)properties, and Hounsfield number/x-ray attenuation coefficient, with a design capable of tolerating high-pressure pulsatile flow. Fiducial targets, incorporated into the phantoms to allow for registration of images among modalities, were chosen to minimize geometric distortions. Results: High quality distortion-free images of the phantoms with good contrast between vessel lumen, fiducial markers, and background tissue to visualize all stenoses were obtained with each modality. Quantitative assessments of the grade of stenosis revealed significant discrepancies between modalities, with each underestimating the stenosis severity for the higher-stenosed phantoms (70% and 85%) by up to 14%, with the greatest discrepancy attributable to DSA. Conclusions: The design and construction of a range of anatomically realistic renal artery phantoms containing varying degrees of stenosis is described. Images obtained using the main four diagnostic techniques used to detect RAS were free from artifacts and exhibited adequate contrast to allow for quantitative measurements of the degree of stenosis in each phantom. Such multimodality phantoms may prove useful in evaluating current and emerging US,MRI,CT, and DSA technology

Tissue Mimicking Materials for Multi-Modality Breast Phantoms

The paper proposes two different Tissue Mimicking Material (TMM) techniques for the development of breast phantoms which are suitable for multi-modality imaging. In particular, the focus is on the behavior of dielectric and acoustic properties when fat, sodium chloride and sugar are added to the mixtures