Diagnosis of liver diseases in bird is challenging as other animals. Valuable diagnostic information can be gathered using US. In the present study the feasibility and capability of using 3D VOCAL US for etiologic diagnosis of liver disorders in common mynahs were evaluated. Eleven mynahs with clinical and radiographic signs of hepatomegaly were evaluated by US and their liver biopsy samples were histologically investigated. Different minor to major pathologic changes were diagnosed in these birds. MGR of two different quantitative measurements including MGL MGM was associated with moderate to severe hepatic lipidosis. On the other hand, quantitative 3D VOCAL US evaluation was not capable of diagnosis of iron storage, hepatitis, and fibroplasia of the liver.
The diagnosis of diffuse changes in hepatic parenchyma is conventionally based on the subjective assessment of liver echogenicity [5, 25]. Evaluation of the liver by US is highly operator dependent. Although quantitative methods for measuring tissue echogenicity have been reported in human, these methods are not yet widely applied in avian practice [26, 27]. Display and formation of images in US are related to the tissue characterization, thus objective evaluation is useful to assess any changes consists in differentiating subtle changes in ultrasound speckle patterns, which may remain perceptible to human visual inspection [28]. Overall, the quantification is an objective evaluation of these changes and pathological state of the tissue by medical imaging that is helpful in diagnosing the disease. Research has currently focused on suggesting non-invasive methods to diagnose fatty liver as an alternative to liver biopsy. Several methods such as CT, MRI and acoustic radiation force impulse (ARFI) imaging are presented as potential methods for objective evaluation of the liver based on the changes of the tissue microstructures [22, 29].
The echogenicity of some parenchymatous organs such as spleen or kidneys are usually used as reference for comparing them to the liver in mammalian species [30]. These reference organs are not easily accessible in avian species due to their special anatomic adaptation [5]. The heart or vessels were suggested in avian patient for comparison [5]. According to its good accessibility, the pectoral muscle mass was used in the present study for this purpose and the result was promising.
The liver biopsy, its histologic evaluation and also microbiological culture are often necessary for definitive diagnosis of hepatic disorders [4, 5, 25]. Taking liver biopsy is a painful technique and despite the local anesthesia, the pain was retained in human patient at least 24 hours post procedure [31]. It should be considered that avian patient with hepatic disorders have less tolerance facing with stress and many birds are commonly presented in a poor condition [32]. While no complications were observed in our cases, massive internal hemorrhage could be the other adverse consequence of core needle liver biopsy in birds considering the possible coagulopathy in hepatic diseases [33, 34]. The absence of commercial test for routine coagulation determination in avian species further limit the safety of biopsy taking in birds [1, 4]. In this situation any alternative non-invasive diagnostic technique could be helpful in severely sick and debilitated avian patients.
As our results showed there was no correlation between fibrosis and US characteristics of the liver. Interestingly it was shown that sound attenuation in human liver with connective tissue proliferation was also minimal to undetectable. In agreement with our result, more attenuation has been detected in human cases with domination of fat in the hepatic tissue [35]. In conclusion at least severe lipidosis can be presumptively diagnosed by US relying on different sound interaction in fatty liver [35]. Lu et al. (1997) showed that quantitative US is more sensitive than subjective evaluation in the detection of initial phase of diffuse liver pathologies including the accumulation of fat [36].
Water content of the liver in rats suffered from fatty liver and cirrhosis did not correlate with the sound speed. On the other hand, the nature of hepatic tissue including the fat content and cirrhosis were associated with the sound speed. Echogenicity alteration was obvious in fatty liver, but no significant differences were detectable among the different degrees and stages of the lipidosis in affected rats [37]. Furthermore, in another study, the quantitative US has been shown to be a feasible and efficient method for detection and follow up of the hepatic lipidosis and subjective US was a practical replacement for histopathological examination [38]. Therefore, in compliance with our study higher hepatic fat content made the liver brighter in US evaluation, while the normal appearance is the result of collagen and the water content [39].
In agreement with our result in the field of avian practice, quantitative US was also applied as a noninvasive technique for monitoring of hepatic lipidosis in dairy cattle [40, 41]. Digitalized analysis of the liver US images was shown to be a promising noninvasive screening technique for fatty liver detection in dairy herd [27].
Multiple factors including depth related attenuation of the measured area and concurrent fibrosis and fatty infiltration might influence quantitative US diagnosis [42].
However, increase in liver echogenicity is expected in several vacuolar diseases; Lu et al. showed that elevation of the attenuation and backscatter coefficients in fatty liver was greater than cirrhotic liver [43]. Regarding the relatively small liver size in passerines, the impact of the chosen ROI could be minimal or even negligible in these birds.
Matheson et al. (2007) presented no significant change in pixel intensity of the US images in pigeon’s hemosiderosis that is same as results of the present study [44]. Different signal intensity is reported in the different iron content of human liver as compared to the muscle and pigeon’s hepatic iron content using magnetic resonance imaging (MRI) [45, 46]. Thus, MRI seems to be prior to US for the detection of iron deposition in the liver.
Small sample size and lack of normal group in the present study was considered as limitations. These limitations are unavoidable in the field of exotic animal research. Thus, further validation and comparison with normal livers to suggest cut-off values are needed in patients and normal population.