The gallbladder is an area where ultrasound imaging shows excellent spatial resolution. Unusual presentations of disease can mimic other conditions. For example, acute hepatitis is known to cause marked gallbladder wall edema. This can mimic tumor (Teefey et al. 1991) (Fig. 15.11a). The use of higher-frequency transducers improves axial resolution and allows demonstration of the striated thickened wall (Fig. 15.11b), a feature that is more commonly seen in inflammation (Joo et al. 2013). Certain pathologies are well demonstrated with ultrasound imaging. The typical features of adenomyomatosis can be seen with ultrasound imaging (Fig. 15.12a). The lower resolution of CT means that the dilated Rokitansky-Aschoff sinuses are not clearly seen (Fig. 15.12b). MR images show greater detail due to the ability to use different pulse sequences that are sensitive to different types of tissue (Fig. 15.12c). The use of the twinkling artifact (Fig. 15.12d) is a unique technique that helps to confirm the presence of small stones in the dilated Rokitansky-Aschoff sinuses (Yu et al. 2012).

Due to the superior contrast resolution of ultrasound imaging, small gallbladder masses can be better depicted. A small fundal mass that may be difficult to appreciate on CT (Fig. 15.13a) is obvious on ultrasound imaging (Fig. 15.13b). Appropriate weight should be placed on the most worrying imaging feature seen in clinical decision making. Attention to scanning parameters is important. The presence of bile provides excellent contrast between the lumen and the wall. The excessive use of overall gain to increase signal can worsen the signal-to-noise ratio and obscure pathology. Even gallstones, which have inherently high signal-to-noise ratio, can be overlooked if the scanning parameters are poor (Fig. 15.14). The use of harmonic imaging can restore the signal-to-noise ratio by reducing side lobe artifacts (Oktar et al. 2003).

The pancreas is usually partly obscured in transabdominal ultrasound scans. This is due to the stomach and bowel loops that often move in and out of the field of view as they undergo peristalsis. Despite this, the head and body can be visualized in most patients. A failure to remember a fixed mass should be evaluated further can lead to a failure to appreciate a large pancreatic head mass (Bondestam 1983) (Fig. 15.15a). It is worthwhile to spend time examining the pancreas in detail as it is often a location of symptomatic pancreatic and biliary disease (Fig. 15.15b).

The kidney is relatively easier to evaluate with ultrasound imaging compared to the pancreas and the liver. Usually, the entire kidney can be seen, as the acoustic window available for imaging does not get obscured by the ribs or bowels. Many of the same pitfalls that occur in the liver can also occur in the kidney (Kang and Chandarana 2012). A renal tumor may be difficult to detect when it merges with the rest of the renal tissue (Fig. 15.16). Optimization of scanning parameters and a high index of suspicion are required to avoid such errors. Although there was a pertinent history of gross hematuria, the initial interpretation of the kidney was normal as it was erroneously believed that a renal tumor would be hypoechoic and mass-like.

The renal pelvis is often evaluated with ultrasound imaging for the diagnosis of hydronephrosis (Jeffrey and Federle 1983). Errors in diagnosis are often due to an overcall, i.e., making a diagnosis when the disease is not present. In patients with acute pyelonephritis, mild separation of the renal pelvis and mucosal edema should not be mistaken for true obstruction (Fig. 15.17). Hydronephrosis should be evaluated before and after the patient has emptied the urinary bladder. Otherwise, reversible pelvicalyceal distension will be mistaken for hydronephrosis (Fig. 15.18).

Lesion detection on CT relies on a differential attenuation between the lesion and the surrounding visceral organ parenchyma or intra-abdominal fat. Intravenous (IV) administration of an iodinated contrast agent is indicated in most instances, with the exception of CT KUB where the primary aim is to look for calculi within the urinary systems. In multiphasic CT studies of the solid visceral organs, specifically of the liver, kidneys, and pancreas, a non-enhanced CT is mandated. This serves as a baseline for evaluation of the presence of enhancement within a lesion (Fig. 15.19). The need for oral contrast administration is controversial; high accuracy rates of diagnosis have been shown in patients with acute appendicitis and blunt abdominal trauma without using oral contrast in CT, as well as in oncology patients undergoing imaging follow-up (Lee et al. 2006, 2013; Anderson et al. 2009; Harieaswar et al. 2009) (Fig. 15.20). Modern-day CT scanners all provide high spatial resolution, thus allowing for multiplanar reformatted images to enable better appreciation of the anatomic relations, particularly for large masses and for lesions that lie in close proximity to vascular structures (Fig. 15.21). CT is the imaging modality of choice for most acute conditions because of its high temporal resolution.

MRI, by virtue of its superior soft tissue contrast, is able to depict soft tissue lesions, often without the use of gadolinium contrast agent. Nevertheless, routine IV administration of gadolinium chelate contrast agents for imaging of the abdomen should be performed, unless the patient has renal impairment at risk for nephrogenic systemic fibrosis (Willicombe and Cunningham 2008). For rectal and prostate imaging, IV contrast administration is not necessary, unless it is for perfusion imaging. MRI is usually better able to depict subtle enhancement; this is by virtue of its better soft tissue contrast. Like on CT, a pre-contrast T1-weighted image should also be performed, since proteinaceous or hemorrhaging contents within lesions result in inherently high relaxivities and T1 signal intensity (Fig. 15.22). MRI may also allow for assessment of diffusivity of water molecules (diffusion-weighted imaging, DWI) that is usually, but not always, reduced in malignant lesions that tend to be more cellular in nature (Padhani et al. 2009) (Figs. 15.23 and 15.24). Spatial resolution on MRI can be comparable to CT, although through plane resolutions are typically poorer, due to limitations of scan time. This is countered by the fact that MRI allows for multiplanar acquisition, thus allowing the in-plane resolution to be good in all orthogonal planes. MRI scans take significantly longer than CT. MRI is considered to be the “problem-solving” modality for characterization of lesions found on ultrasound imaging or CT.