**7. Noninvasive evaluation of liver steatosis using Controlled Attenuation Parameter (CAP)**

Even though liver stiffness provides an alternative to liver biopsy for fibrosis staging, it can identify very important histologic features such as macrovesicular steatosis, ballooned hepa‐ tocytes, inflammation, etc [68].

**Figure 4.** Boxplots representing mean SSM values according to the esophageal varices grade using the original (A) or the modified (B) calculation algorithm.

Knowing that fat interferes with ultrasound propagation, a novel attenuation parameter has been developed to detect and quantify liver steatosis. This parameter is based on the ultra‐ sonic properties of the radio-frequency back propagated signals acquired by the Fibroscan [92]. It is called controlled attenuation parameter (CAP). This ultrasonic attenuation coeffi‐ cient is an estimate of the total ultrasonic attenuation (go-and-return path) at the central fre‐ quency of the regular or M Fibroscan® probe, i.e. at 3.5 MHz, and is expressed in dB.m−1. CAP is evaluated using the same radio-frequency data and the same region of interest, as the region used to assess the LSM. CAP is only appraised if the acquisition is ''valid''. There‐ fore, CAP is guided by vibration-controlled transient elastography (VCTE), which ensures that the operator automatically obtains an ultrasonic attenuation value of the liver [92, 93]. The device is used to assess, at the same time, LS (which is related to liver fibrosis) and CAP (which is related to liver steatosis).

**•** The liver volume used to evaluate fibrosis is 150-400 times higher than the volume ob‐

Non-Invasive Evaluation of Liver Steatosis, Fibrosis and Cirrhosis in Hepatitis C Virus Infected Patients Using

Unidimensional Transient Elastography (Fibroscan®)

http://dx.doi.org/10.5772/52621

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As far as the evaluation of liver steatosis is concerned, in comparison to other modalities, CAP is non-invasive, quantitative, non-ionizing, and inexpensive. Furthermore, the proce‐ dure is easy to perform, even by an operator who does not have any radiological skills and provides immediate results. The procedure is also machine-independent and does not re‐ quire corrections to be made for gain, frequency, focusing or beam diffraction, and is also not subject to operator interpretation. In addition, CAP has been shown to efficiently detect steatosis at a level of ≥ 10%, which is more sensitive than other imaging modalities. Com‐ pared to a liver biopsy, CAP is less prone to sampling error as it explores a liver volume

Liver fibrosis can not be evaluated by TE in 5-8 % of the cases. Some of the possible causes

**•** obesity (an ultrasound machine may be used in order to find the best window and thus

**•** large vascular structure present in the acquisition window (may lead to false results).

The failure of TE varies according to different authors from 2.4% to 9.4% [16, 21, 24, 36, 74, 64, 100]. In a study performed on 2114 patients [101], liver stiffness could not be determined in 4.5% of the cases and multivariate analysis showed that the only element associated with measurement failure is a body mass index over 28. Yet, with more experience, one may real‐ ize that a thick thoracic wall is more likely to be a limiting factor for a failed measurement

Technical solutions regarding the design of the probe were investigated lately, in order to overcome these limitations. Recently a new probe became available, that was specially de‐ signed for obese patients, with a central frequency of 2.5% MHz (compared with the 5MHz probe that is usually used), and that is able to determine liver stiffness on a distance of 35-75 mm from the skin (while the normal probe is able to do that on distance of 25 to 45 cm). With the help of this new transducer, it was possible to obtain valid measurements in 49% of the patients with a BMI ≥30 kg/m2, in which the usual probe failed to determine the LS

As far as predicting steatosis in HCV patients is concerned, CAP has further validation in larger populations and by independent teams, since there are rather few studies published

increase the ability to measure liver stiffness in overweight patients);

**•** ascites (vibrations are not transmitted through fluid);

than the growth of the body mass index in itself [102].

**•** the quality of the liver parenchyma and other liver structures;

tained through liver biopsy.

∼100 times larger [92, 93].

**9. Limitations of TE**

for this are listed below [16]:

**•** a narrow intercostal space;

[103].

Even though relatively few studies have been published on this topic [92, 93, 94,95] the pre‐ liminary results showed that CAP is a promising non-invasive tool to detect steatosis in CHC patients.

In the study conducted by Sasso et al, the CAP performance was appraised on 115 patients, taking the histological grade of steatosis as reference. CAP was significantly correlated to steatosis with an AUROC equal to 0.91 and 0.95 for the detection of more than 10% and 33% of steatosis, respectively.

A study performed recently on 615 HCV patients, who underwent both Fibroscan (®) and liver biopsy showed in multivariate analysis, that CAP was related to steatosis, independ‐ ently of fibrosis stage (which was related to LS. The AUROCs of the were 0.80, 0.86 and 0.88 respectively, for predicting a fatty overload of more than 11%, 33%, and 66%, respec‐ tively. CAP also exhibited a good ability to differentiate steatosis grades (Obuchowski measure = 0.92) [96].

CAP is evaluated using the same radio-frequency data and the same region of interest, as the region used to assess the liver stiffness for fibrosis quantification. Preliminary studies performed in our department have found significantly different CAP values for different steatosis grades and AUROCs of 0.830 and 0.85 respectively, for the prediction of a hepatic fat content over 33% and 66%, respectively [97].
