Patients are required to fast for more than 8 h or at least 4 h before the scan and take 1,000–1,500 ml of water orally half an hour before the scan to fill the gastrointestinal tract, and this can overcome air artifact, so that the gut lumen and wall can be displayed clearly and the relationship between the intestinal tract and its adjacent structures can be known. Normally liver scan includes plain and enhanced scan. Enhanced scan is very important and almost indispensable. Enhanced scanning is performed after the injection of nonionic iodine contrast agent from peripheral vein, which then is mainly distributed in the artery, portal vessels (including vein), and organ parenchyma. Accordingly, the enhanced scan is called the scan of arterial phase, portal venous phase (venous phase), and the substantial balance of the entire liver (layer thickness ≤5 mm, pitch ≤1.5). The following must be emphasized:(1) the total amount of nonionic iodine contrast agent must be calculated at 1.5–2 ml/kg of total body weight, and injection rate should be 3 ml/s; (2) the scan delay time of each phase (calculated when the injection of nonionic iodine contrast agent begins) (arterial phase, 30 ± 5 s; portal venous phase,80 ± 5 s; substantial balance phase, 180 s); (3) iodine contrast agents should be used with caution to patients who have history of drug allergy or severe renal insufficiency; (4) to display the hepatic artery and portal vein and hepatic vein-based CT angiography, the appropriate layer thickness should be ≤1.5 mm; and (5) during the scan, the patient should hold the breath in order to avoid scan plane beating and missing lesions; therefore, the patient’s breathing should be trained before the examination. They are required to hold the breath in a calm state in order to maintain the consistency of each breath. This point is very important.

Patients are only required to fast more than 4 h before the scan. There is no need to make special preparation under normal circumstances, and can also oral negative MRI contrast agent (such as dilute barium, etc.). Spin echo or fast spin echo (SE/FSE) and gradient echo (GRE) techniques are commonly used in liver check with MRI. The following can be used if necessary:

SE T1-weighted (T1W) and FSE T2-weighted fat suppression (T2W + FS) axial images (layer thickness ≤5 mm, interval ≤3 mm) are normally used in liver scan. If jointed with real-time navigator and trigger technology, a better image can be collected. Liver examination with MRI focuses more on plain + enhanced scanning with GRE T1-weighted fat suppression sequences. After the injection of magnetic resonance contrast agent (usually gadolinium–diethylenetriamine pentaacetic acid, Gd-DTPA) from peripheral vein, enhanced 2D or 3D fast axial scanning is performed through breath-hold GRE T1-weighted fat suppression (2D layer thickness ≤7 mm, interval of ≤3 mm; 3D layer thickness ≤5 mm), and breath-hold requirements are the same with CT. GRE T1-weighted fat suppression sequence scanning is fast, especially the latest high-strength-field, high-gradient, high-switching-rate, and multi-channel sampling of the aircraft, as with the spiral CT, after injection of Gd-DTPA, and can finish the whole liver scan in artery, portal vein, and substantial balance phase of the liver. Generally, Gd-DTPA total injection is 0.4–0.5 mmol/kg of body weight (in total about 30 ml), 2–3 ml/s of injection rate. At the same time, according to the clinical condition of patients, the coronal enhanced scanning which mainly shows hepatic vessel can be used (using the 3D scan of GRE T1-weighted fat suppression, layer thickness ≤1.5 mm).

In addition to commonly used extracellular Gd-DTPA, the liver-specific contrast agents currently used in clinical area are mainly three types: (1) Mn-DPDP, which is absorbed by the hepatic cells and excreted by biliary, is mainly used to shorten the T1 relaxation; (2) SPIO and USPIO, absorbed by mesh endothelial cells (including macrophages and Kupffer cells), mainly shorten the T2 and T2 * relaxation; and (3) contrast agents of dual functions, absorbed by extra cells and hepatic cells and excreted by biliary, such as Gd-BOPTA (MultiHanceR), Gd-EOB-DTPA (PrimovistR), etc., are used to shorten the T1 relaxation. (1) and (2) liver-specific contrast agents are used mainly as the supplementary to contrast agents in extracellular Gd-DTPA examination, while the contrast agents of dual functions are expected to replace Gd-DTPA and become the common or preferred contrast agents in MR examination.

The liver with CT and MRI examination may help clinicians accurately understand:

Hepatic metastasis: Hepatic metastasis can be single or multiple. The most common metastasis is gastrointestinal tumor metastasis, followed by lung cancer, breast cancer, pancreas cancer, gallbladder cancer, ovarian cancer, renal cancer, thyroid cancer, and nasopharyngeal cancer. The features of colorectal liver metastasis are similar to that of other liver metastasis. They display low-density, occasionally slightly calcified, or mucus-like image in the center of lesions through the plain CT scan, which are of great help to the diagnosis of hepatic metastasis. Most hepatic metastasis are lack of blood supply, while a small number of lesions are rich in blood supply. So enhanced scans in arterial phase generally enhance the unobvious or only enhance the edge by rim enhancement, while the typical ring enhancement of the edge of lesions is often visible by the enhanced scans in portal venous phase, in particular, the concentric “bull’s-eye sign” or “target sign” is typical in diagnosing hepatic metastasis (see case 1). With MRI, SE sequence, the lesion often shows low signals on T1W image and slightly higher signals onT2W image. If there is clear necrosis or cystic change in tumors, it can show high signals on T2W image clearly. With enhanced scans, it can be presented with edge-enhanced “bull’s-eye sign” or “target sign” and sometimes similar to the “petallike” structure (see cases 2 and 3). MRI, in particular DWI, T2W image, and enhanced image, can detect lesions with high sensitivity. Besides, hepatic metastasis is less likely to cause liver subcapsular rupture and portal vein tumor thrombus. In the mean time, the hilar, abdominal, and retroperitoneal enlarged lymph nodes are also found to show ring enhancement, which is helpful for diagnosis.