No single MRI technique is optimal for imaging the cirrhotic liver, and individual preferences are usually based on the hardware and software available. We strongly encourage use of a body phased-array coil for the best signal-to-noise ratio. We prefer a 1 to 1.5-tesla magnet with a gradient rise time of at least 600 msec.
T2-weighted (or STIR, for short tau-inversion recovery) images are an essential component of the MR exam. While conventional spin-echo pulse sequences provide excellent image contrast, long acquisition times may result in substantial motion artifact. Many centers have abandoned this technique for faster echo-train fast spin-echo pulse sequences.4 These may be performed with respiratory gating or respiratory ordered phase-encoding, depending on the manufacturer.
By decreasing the repetition time (TR) and using a relatively long echo-train, T2-weighted images can be acquired in the time frame of a single breath-hold. Frequency-selective fat suppression may be used to augment image contrast, which is especially helpful if the liver is fatty. Alternatively, fat-suppressed images with additive T1 and T2 contrast can be performed with STIR methods when inversion times are selected to null fat. This technique is less dependent on a homogeneous magnetic field, and it results in fewer artifacts than seen with frequency-selective fat suppression techniques.
New developments that provide for long echo-train length, short echo spacing, and half-Fourier computations provide for single-shot imaging that can be performed in patients unable to suspend respiration. However, these methods may result in a decrease in T2 contrast for detecting solid lesions such as HCC.4,5
Breath-hold, T1-weighted images should be performed using a short TR/short TE gradient-echo pulse sequence with a large flip angle. The authors acquire these at two separate echo times for in-phase and out-of-phase imaging (4.4 msec in-phase and 2.2 msec out-of-phase at 1.5 tesla). This improves the detection of either diffuse or focal fatty infiltration and nodular lesions that contain fatty components.
Imaging at short echo times has both benefits and disadvantages. It minimizes the magnetic susceptibility artifacts from transjugular intrahepatic portosystemic shunts (TIPS) and certain embolization coils, which can severely degrade image quality. However, it also decreases the ability to detect susceptibility effects of iron within the liver, whether diffuse or within siderotic nodules. A third gradient-echo pulse sequence with increased echo time and decreased flip angle can be used to increase the magnetic susceptibility effects of iron. This sequence can also be used to determine direction of portal venous flow when saturation bands are placed above and then below the imaging volume.
For MRI of the cirrhotic liver, dynamic, breath-hold, gadolinium-enhanced imaging performed with either two- or three-dimensional gradient-echo pulse sequences is essential.6-8 Contrast-enhanced, frequency-selective fat-suppressed sequences can be used to improve conspicuity of liver lesions, particularly in the setting of fatty infiltration. Fat suppression is also helpful when evaluating the extrahepatic manifestations of cirrhosis, including varices and bowel edema. Other contrast agents are available, including agents that are specifically taken up by hepatocytes or by reticuloendothelial cells. A discussion of these agents is beyond the scope of this article, and the reader is referred to a recent review.9
Contrast-enhanced images should be acquired in at least three phases: the hepatic arterial, portal venous, and equilibrium phases. A successful arterial phase is critical for the detection of small HCC.6,7 Traditionally, hepatic arterial phase images have been obtained following a fixed delay after intravenous bolus of contrast. However, use of a fixed delay often results in a suboptimal hepatic arterial phase in the cirrhotic cohort because of the wide range in circulation times that result from hemodynamic alterations intrinsic to the disease. In these patients, recent developments suggest that using a test bolus to determine an individual's circulation time, or fluoroscopic "real-time" triggering, can help obtain arterial phase imaging more reliably. In addition, an MR-compatible power injector is helpful for injections of the test bolus and main bolus of contrast material, as it provides a precise infusion rate (typically, 2 mL/sec) and dose (0.1 mmol/kg) and allows a technician to perform the entire examination outside of the magnet.
When near-isotropic pixel size (such as lesser than or equal to 2 mm in all three dimensions) can be achieved, 3-D gradient-echo imaging has clear advantages over conventional 2-D. Image sets can be reformatted using multiplanar reconstructions without loss of in-plane resolution. Moreover, angiographic reconstructions such as maximum intensity projections can be obtained for each contrast-enhanced acquisition, resulting in a "free" angiogram and portogram.8
The equilibrium-phase, contrast-enhanced study can be helpful for distinguishing HCC from RN and DN, as some HCC will have a discernable capsule at this stage while RN and DN should not.
RN of cirrhosis have characteristic features on MR imaging that usually allow distinction from HCC, but not always from DN. They invariably have a portal venous blood supply with minimal or no contribution from the hepatic artery. RN are usually isointense with other background nodules on both T1-weighted and T2-weighted images. Less commonly, they may be hyperintense on T1-weighted images and hypointense on T2-weighted images. However, unlike some HCC, RN are almost never hyperintense on T2-weighted images, with the noted exception of those that occur in the setting of chronic Budd-Chiari syndrome10 or those that have undergone infarction.
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