The liver appears as a wedge-shaped organ located in the right, upper section of the abdominal cavity underneath the diaphragm, and lying just above the stomach and intestines. The liver weighs approximately three pounds and cannot be felt through a physical examination because of protection by the rib cages. The liver is highly vascular, and this confers the characteristic dark, brown color. The liver has two distinct lobes, made up of many interconnected lobules.
At any given moment, the liver holds approximately 13 % of the body’s blood supply. The hepatic artery supplies the liver with oxygenated blood while the hepatic portal vein supplies the liver with nutrients-rich blood (both blood vessels enter the liver through the hilum). The main function of the liver is to filter the blood coming from the gastrointestinal tract (Mitra & Metcalf, 2009). The nutrient-rich blood supplied by the hepatic portal vein undergoes a processing procedure (in the liver) which breaks down nutrients and detoxifies chemicals in the food. Drugs within the food are also metabolized in the process. After the breakdown of harmful substances, byproducts of the liver get excreted in the form of bile. The lobules play a critical role in the removal of bile from the liver because they connect into small ducts, which eventually form the hepatic duct. The hepatic duct then discharges bile produced in the liver cells into the duodenum and the gallbladder (Mitra & Metcalf, 2009).
The liver is covered with a thin, double-layered membrane called visceral peritoneum; the peritoneum ligament is vital in reducing friction between the liver and other organs. Unlike other body organs which have several synonyms, the liver does not go by any other name and is universally referred to as the liver.
Diagnostic Methods
Computed Tomography
Since the liver does not have a definite outline shape, it is extremely difficult to know the outline measurements (Catalano et al, 2008). However, the surface area and volume of the liver can be determined. The linear hepatic measurements are expressed as means ± standard deviations. The surface area ranges from a minimum of 79.70 to 312.87 cm² (mean 183.13 cm² ± 47.07 cm²) and a maximum of 98.03 to 467.99 cm² (mean 265.58 ± 68.26 cm²) depending on the age and body size of the individual. The volume of liver ranges from 533 to 2417 cm3 (mean 1106 ± 392 cm3) and is also depended on the age and body size of the person. Volume of the liver is measured by use of a contrast enhanced CT axial images at 5-mm intervals (Catalano et al, 2008).
CT provides a reasonable spatial resolution and the capability to examine the whole liver in a single breath-hold. It serves as a perfect screening examination for the whole abdomen and pelvis. Iodinated contrast media, which are administered intravenously, are regularly used in the viewing of the liver. They advance the contrast-to-noise ratio which exists between focal liver lesions and normal liver and, therefore, aid in recognition of focal liver lesions.
Advances in CT technology like a multi-detector row helical CT and helical CT have improved CT scanners in terms of speed and resolution thus it is now possible to view the liver in various phases of contrast. It is also possible to view the liver in three-dimensions and view the liver vasculature (CT angiography). This allows mapping of the liver's anatomy as well as in defining the liver and the volume of any tumor in case it is present (Catalano et al, 2008).
Magnetic Resonance Imaging (MRI)
According to Cruite et al (2010), all MRI tests are performed under suspended respiration with a 1.5-T system and a phasedarray coil. The series involve two dimensional coronal and transverse single-shot fast spin-echo T2-weighted MR imaging (echo time, 180–200 msec), transverse fat-suppressed fast spin-echo T2-weighted MR imaging (repetition time msec/echo time msec, 2000–4000/70–90), and spoiled dual gradient-echo T1- weighted in- and out-of-phase MR imaging (120–200/2.3 and 4.6, 90° flip angle). The T1 images demonstrate higher signal intensity than the spleen and slightly lower than that of muscle. The liver has a lower T2 signal intensity and appears darker than the spleen.
Certainly, MRI is the best imaging test for liver studies and characterization. This is because it provides high lesion-to-liver distinction and it does not apply ionizing radiation. Improvement in MRI like rapid half-Fourier acquisition and breath-hold 3D imaging help in viewing the liver with an exceptionally strong spatial resolution. The chemical change imaging is remarkably effective in distinguishing pseudolesions like focal fatty sparings and focal fatty infiltrations, from pathologic liver lesions (Cruite et al, 2010).
Ultrasound
A curved array of 2 to 6 MHz is best suited for sonographic examination of the liver depending on the size of the person being examined. In case of nodularity, in the border of the liver, a linear array with a 7 to 12 MHz frequency is preferred for better results. There should be change of focal frequency and zone in order to assess both deep and superficial structures adequately (Piyasena & Allison, 2008).
Ultrasound is cheap but commonly available method of examination. It is a superb test to examine for biliary obstruction and diseases of the gall-bladder. It is, however, not highly sensitive like the CT scan and the MRI. Ultrasound, however, has some limitations like high operator dependency and it is also unable to detect small lesions especially those less than one centimeter or those with low specificity (Piyasena & Allison, 2008).
Appropriateness of imaging
The American College of Radiology (ACR) has come up with appropriate criteria to which other healthcare workers can find out the most diagnostic imaging examination for the clinical condition of the patient. The ACR determines a panel to evaluate the appropriateness of imaging for each clinical condition based off of evidence in a literature search of peer reviewed articles. The “appropriateness criteria provide guidance to supplement the clinician’s judgment as to whether a patient is a reasonable candidate for the given treatment, test, or procedure”. The panel classifies the appropriateness of a diagnostic examination or treatment on a scale of between 1 and 9 where 1 is the least appropriate while 9 is the most appropriate.
Since the liver is located in the abdominal cavity which is highly vascularized, the Gastrointestinal Imaging Criteria is examined, and Blunt Abdominal Trauma is mostly the one chosen. According to the ACR (2011), for the stable patient, CT abdomen and pelvis with contrast has appropriateness rating of 9. For an unstable patient, a chest x-ray, ultrasound chest and pelvis (FAST scan) and x-ray abdomen and pelvis (KUB) had an appropriateness rating of 8. It is commonly agreed with the ACR criteria for blunt abdominal trauma since the patient is unstable the fastest, most diagnostic tests should be ordered to examine the extent of damage of the trauma. The use of the FAST scan is extremely user dependent and should be carried out by an experienced expert to prevent missed diagnosis or anatomy. If the patient is stable, a more thorough examination should be performed to detect the extent of injury and structures involved (American College of Radiology, 2011).
Appendix
Normal Anatomy seen in the Transverse View of the Left Lobe on Ultrasound
Retrieved from http://www.ultrasoundpaedia.com/normal-liver2/
Top view of the liver by MRI Retrieved from http://www.medscape.org/viewarticle/488088_5
References
American College of Radiology. (2011, March 01). Liver imaging reporting and data system . Retrieved October 15, 2012, from American College of Radiology: http://www.acr.org/Quality-Safety/Resources/LIRADS
Catalano, O. A., Singh, A. H., Uppot, R. N., Hahn, P. F., Ferrone, C. R., & Sahani, D. V. (2008). Vascular and biliary variants in the liver: Implications for liver surgery. Radiographics, 28, 359-378 doi: 10.1148/rg.282075099.
Cruite, I., Schroeder, M., Merkle, E. M., & Sirlin, C. B. (2010). Gadoxetate disodium–enhanced MRI of the liver: Part 2, protocol optimization and lesion appearance in the cirrhotic liver. American journal of roentgenology, 195, 25-41 doi: 10.2214/AJR.10.4538.
Mitra, V., & Metcalf, J. (2009). Functional anatomy and blood supply of the liver. Anaesthesia and intensive care medicine, 10 (7), 332-333 doi: 10.1016/j.mpaic.2009.03.012.
Piyasena, R. V., & Allison, S. J. (2008). Ultrasound imaging of liver and renal transplantation. Applied Radiology, 37 (3), 10-12.
Umphrey, H. R., Lockhart, M. E., & Robbin, M. L. (2008). Transplant ultrasound of the kidney, liver, and pancreas. Ultrasound Clinics , 3 (1), 49-65 doi: 10.1016/j.cult.2007.12.012.
