Liver imaging can be performed with sonography, computed tomography (CT) and
magnetic resonance imaging (
MRI). Ultrasound is, caused by the easy access, still the first-line imaging method of choice; CT and
MRI are applied whenever ultrasound imaging yields vague results. Indications are the characterization of metastases and primary liver tumors e.g., benign lesions such as focal nodular hyperplasia (FNH), adenoma, hemangioma and malignant lesions (cancer) such as hepatocellular carcinomas (HCC).
The decision, which
medical imaging modality is more suitable,
MRI or CT, is dependent on the different factors. CT is less costly and more widely available; modern multislice scanners provide high
spatial resolution and short scan times but has the disadvantage of radiation exposure.
With the introduction of high performance
MR systems and advanced
sequences the
image quality of
MRI for the liver has gained substantially.
Fast spin echo or single
shot techniques, often combined with
fat suppression, are the most common
T2 weighted sequences used in liver
MRI procedures.
Spoiled
gradient echo sequences are used as ideal
T1 weighted sequences for evaluating of the liver. The
repetition time (TR) can be sufficiently long to acquire enough sections covering the entire liver in one pass, and to provide good signal to
noise. The TE should be the shortest
in phase echo time (TE), which provides strong
T1 weighting, minimizes
magnetic susceptibility effects, and permits acquisition within one breath hold to cover the whole liver. A
flip angle of 80° provides good
T1 weighting and less of power deposition and tissue
saturation than a larger
flip angle that would provide comparable
T1 weighting.
Liver
MRI is very dependent on the administration of
contrast agents, especially when detection and characterization of focal lesions are the issues. Liver
MRI combined with
MRCP is useful to evaluate patients with hepatic and biliary disease.
Gadolinium
chelates are typical non-specific extracellular agents diffusing rapidly to the extravascular space of tissues being cleared by glomerular filtration at the kidney. These characteristics are somewhat problematic when a large organ with a huge interstitial space like the liver is imaged. These agents provide a small temporal imaging window (seconds), after which they begin to diffuse to the interstitial space not only of healthy liver cells but also of lesions, reducing the
contrast gradient necessary for easy lesion detection. Dynamic
MRI with multiple phases after i.v. contrast media (Gd chelates), with arterial, portal and late
phase images (similar to CT) provides additional information.
An additional advantage of
MRI is the availability of liver-specific
contrast agents (see also
Hepatobiliary Contrast Agents). Gd-EOB-DTPA (gadoxetate disodium,
Gadolinium ethoxybenzyl dimeglumine, EOVIST Injection, brand name in other countries is
Primovist) is a gadolinium-based
MRI contrast agent approved by the FDA for the detection and characterization of known or suspected focal liver lesions.
Gd-EOB-DTPA provides dynamic phases after intravenous injection, similarly to non-specific
gadolinium chelates, and distributes into the hepatocytes and bile ducts during the hepatobiliary
phase. It has up to 50% hepatobiliary excretion in the normal liver.
Since ferumoxides are not eliminated by the kidney, they possess long plasmatic half-lives, allowing circulation for several minutes in the vascular space. The uptake process is dependent on the total size of the particle being quicker for larger particles with a size of the range of 150 nm (called
superparamagnetic iron oxide). The smaller ones, possessing a total particle size in the order of 30 nm, are called
ultrasmall superparamagnetic iron oxide particles and they suffer a slower uptake by RES cells.
Intracellular contrast agents used in liver
MRI are primarily targeted to the normal liver parenchyma and not to pathological cells. Currently, iron oxide based
MRI contrast agents are not marketed.
Beyond
contrast enhanced MRI, the detection of fatty liver disease and iron overload has clinical significance due to the potential for evolution into cirrhosis and hepatocellular carcinoma. Imaging-based liver fat quantification (see also
Dixon) provides noninvasively information about fat metabolism;
chemical shift imaging or T2*-weighted imaging allow the quantification of hepatic iron concentration.
See also
Abdominal Imaging,
Primovistâ„¢,
Liver Acquisition with Volume Acquisition (LAVA),
T1W High Resolution Isotropic Volume Examination (THRIVE) and
Bolus Injection.
For Ultrasound Imaging (USI) see
Liver Sonography at
Medical-Ultrasound-Imaging.com.
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