A liver protocol is used to evaluate most hepatic pathology, improving hepatic lesion identification and characterization [2, 3]. The protocol consists of non-contrast phase followed by multiphasic dynamic imaging with an intravenous iodinated contrast agent. A 16-detector scanner or greater needs to be used so that the entire liver can be covered during the late arterial phase. Multidetector scanners also allow the liver to be imaged at low energy or 80 kVp. An iodinated contrast material has a higher attenuation on 80 kVp images compared to high-energy or 140 kVp images, and low-kVp images can improve hepatic lesion detection [4]. Unfortunately in larger patients, a low kVp can produce images with too much noise to be diagnostic. We use automated exposure control (AEC) to adapt the tube current to the patient’s size to obtain quality images. AECs also lower radiation dose and have been shown to reduce the mean tube current by 20–68 % compared to exams that are obtained at constant tube current [5]. We also use reconstruction algorithms to enhance image quality of lower-dose scans.

At our institution, the images are obtained at 5 mm and reconstructed into 2.5 mm images.

Intravenous contrast should be administered using bolus tracking to ensure that accurate hepatic perfusional phases are obtained, regardless of a patient’s cardiac function, blood volume, or visceral perfusion. Bolus tracking monitors a target vessel or organ’s attenuation and begins the exam when the region of interest (ROI) reaches a designated threshold. Our institution places the ROI in the abdominal aorta near the celiac axis.

The late arterial phase is the first phase and is the most difficult phase to capture. Our liver protocol images the late arterial phase 17 s after the ROI reaches 100 HU, or approximately 30–35 s after the intravenous contrast was administered at a rate of 4–6 cc/s.

The late arterial phase is useful for identifying hypervascular lesions, including hepatocellular cancer (HCC), melanoma, and neuroendocrine metastases. The hepatic arterial anatomy can also be evaluated during this phase (Fig. 1.2).

The portal venous phase is the second phase and is acquired approximately 60 s after contrast administration. The portal venous phase is used to identify hypovascular metastases such as those from colon cancer (Fig. 1.3). This phase is also used to evaluate the portal venous system and to determine segmental anatomy.

The excretory phase is the final phase and is acquired 3–5 min after contrast administration. This phase allows for additional characterization of lesions identified in the previous two phases. For example, a hypervascular lesion in a cirrhotic liver that has contrast washout on this phase is suspicious for HCC, while a lesion with progressive, centripetal filling is consistent with a hemangioma.

Hepatic lesions that retain contrast, such as confluent hepatic fibrosis, cholangiocarcinomas, and giant hemangiomas, may need a 10 min delayed phase for additional evaluation [6, 7].

Magnetic resonance imaging (MRI) is increasingly utilized as the first-line modality for hepatic imaging. MRI’s prior limitations, which included long exam times and images with low temporal and spatial resolution and poor signal-to-noise ratios, have been improved with new advanced imaging techniques. The clear advantages MRI has over computed tomography (CT), which is currently the most common modality used for hepatic imaging, are MRI’s superb tissue contrast and lack of ionizing radiation. The recent introduction of hepatobiliary contrast agents has created new potential applications for MRI hepatic evaluation. The following sections will review the commonly used sequences for hepatic imaging. For these sequences, the normal liver signal relative to adjacent organs and the relative signal of the liver to common focal liver lesions and diffuse pathologies will also be presented.

Hepatic imaging is best performed at 1.5 or 3 T strength magnets using a multichannel phased-array coil. The basic MRI protocol for hepatic imaging has matured to include four sequences:

T1-weighted imaging (T1WI) evaluation of the liver uses spoiled gradient echo (SPGR) techniques (GE Medical Systems, Milwaukee, USA) GE nomenclature will be used throughout. Please refer to Table 1.1, Appendix A. SPGR sequences can be obtained during a single breath hold, which minimizes respiratory motion artifact. Parallel imaging permits the acquisition of both in-phase (IP) and opposed-phase (OP) images during a single breath hold, when using 5 mm slice thickness or 30 slices, eliminating registration errors [8]. Parallel imaging is able to speed up scan time because only a portion of the k-space data is sampled with an array of receiver coils. The undersampled data can then be reconstructed into images using various algorithms [9]. In a 1.5T scanner, OP images are usually acquired at a TE of 2.1 ms while IP images are acquired at a TE of 4.2 ms.

The terms in-phase and out-of-phase refer to the relative phases of the signal of water and fat protons. In the out-of-phase series, the fat signal will cancel with corresponding water signal. In contrast, the in-phase series will result in the addition of both proton signals.

The normal liver will have the same signal intensity on IP T1WI and OP T1WI. The liver is slightly hyperintense to the spleen, muscle, and kidneys. The signal is dependent on the liver’s fat and iron content. In the setting of fatty infiltration, the liver signal increases on IP T1WI and decreases on OP T1WI (Fig. 1.4). This is also useful to detect microscopic fat within lesions. In the setting of iron overload in the liver, the liver is brighter on the OP relative to the IP images. This is due to the T2*-related signal loss on the longer echo time [10]. Similar susceptibility effects from iron and other metals can be identified with IP and OP imaging. Susceptibility artifacts occur at interfaces of substances with different magnetic susceptibilities. Magnetic susceptibility artifacts are also commonly seen at interfaces between air and tissue, such as in the bowel. These artifacts are more pronounced on IP images secondary to the longer echo time [11]. Spin echo (SE)-based techniques can be used to decrease susceptibility artifacts, but these SE series are very long.

In addition to fat, protein and blood are bright on IP T1WI, and IP T1WI may be useful in identifying posttraumatic hematomas and hemorrhagic malignancies [6]. Melanin, copper, and glycogen are also bright on IP T1WI. Lesions that are hypointense to the normal liver on IP T1WI include metastases, cysts, hemangiomas, and areas of fibrosis. Finally, IP T1WI is not fat saturated and provides excellent definition of the various fat planes in the evaluation of hepatic anatomy.

There are a variety of techniques that can be used to obtain T2-weighted imaging (T2WI) of the liver. The most commonly used is fast spin-echo (FSE) technique. A variant of FSE are FRFSE and SSFSE (GEMS) sequences. Another T2WI imaging technique is diffusion-weighted imaging. In this section we review the imaging parameters for these various series.

Diffusion-weighted imaging (DWI) is also a variant of T2WI. The images are obtained with echo planar imaging (EPI), using breath-hold, free-breathing, or respiratory-triggered techniques. Respiratory-triggered EPI is recommended over breath-hold EPI because its multiple acquisition technique produces images with a better SNR and allows for the use of more b values [18]. It is best to obtain diffusion gradients in three orthogonal planes to produce a stronger gradient with shorter TEs and less susceptibility, but adequate images can be obtained with only one axis (z-axis) gradient [19].

DWI uses two or more identical diffusion-sensitizing gradient pulses separated by a 180° refocusing pulse to change the phase of normal, unrestricted water protons so that they lose signal while abnormal, restricted water protons retain their phase and signal. The amount of signal loss is inversely related to the degree of water proton motion or apparent diffusion coefficient (ADC) and the b value. The b value is the sum of the diffusion-weighted gradient amplitude, applied gradient duration, and the time interval between the applied gradients or gradient separation.

Changing the b value can alter diffusion sensitivity and typical b value range from 400 to 800 s/mm². The larger the b value, the more the image signal is based on diffusion properties, while smaller b values will have more signal contribution from T2 effects. The b values used for hepatic lesion detection should be less than 100 s/mm². This low b value creates images with high tissue contrast that aides in lesion detection. Higher b values, 500 s/mm² or greater, provide more information about a lesion’s diffusion properties and are used for lesion characterization.

DWI technique has been reported to be used to: