Computed tomography (CT) and magnetic resonance
imaging (MRI) are commonly used for hepatic imaging in oncology
patients. Both CT and MRI have advanced rapidly over the past 10
years as a result of improvements in computer hardware, software, and
other electronic components. These changes have increased the
accuracy of each technique in detecting and characterizing focal
hepatic neoplasms. However, the improvements in both modalities have
led to some confusion over their relative advantages.
Many studies have directly compared the diagnostic efficacy of MRI
and CT in patients with focal hepatic malignancies.[1-18] These
reports determined that, although the spatial resolution of CT
remains superior, MRI has better contrast resolution and has
repeatedly been proven to detect and characterize focal liver lesions
with greater accuracy than CT.
Despite this obvious advantage, CT remains the primary tool for
routine diagnostic evaluation of patients with known or suspected
liver malignancies in most centers. The relative underutilization of
MRI may be due, at least in part, to a lack of understanding of the
advantages and disadvantages of each technique.
This article will review the current literature on state-of-the-art
techniques for imaging the liver using both CT and MRI. Direct
comparison studies of CT and MRI will be reviewed, and the relative
advantages of each technique will be discussed. With this knowledge,
the reader will be better able to choose an imaging study for
evaluating oncologic patients with known or suspected liver disease.
Both MRI and CT of the liver have various relative advantages and
disadvantages (Table 1).
Careful consideration of these advantages and disadvantages is
warranted before selecting the study that is most appropriate for an
individual patient. In many cases, the choice may not be clear, and
either study would be considered the standard of care. However, a
closer examination will show that each has some fairly specific
benefits and drawbacks.
In general, CT is less costly than MRI because it involves lower
capital equipment costs and a shorter examination time. Computed
tomography is also widely available and often does not have the same
backlog of patients as does MRI. Most physicians are
relatively comfortable with evaluating CT images themselves, although
nearly all still rely on the interpretation of a diagnostic
radiologist for a final diagnosis.
The major limitation associated with the use of hepatic CT studies is
the need for iodinated contrast material. Both the conventional and
the newer nonionic contrast agents are nephrotoxic, and, as a result,
their use is restricted in patients with renal insufficiency.[19,20]
Iodinated contrast is also associated with a relatively high rate of
adverse reactions. In addition, CT uses ionizing radiation, the
potential harm of which is poorly understood. Most significantly,
studies have shown that CT is less sensitive and specific than MRI in
detecting and characterizing focal hepatic diseasea fact that
will be discussed in greater detail below.
Magnetic resonance imaging of the liver has several advantages over
CT of the liver. Magnetic resonance imaging provides excellent
contrast that can reveal subtle variations in tissues of differing
histology. No ionizing radiation or nephrotoxic contrast media is
used, and the most commonly administered contrast agents,
extracellular gadolinium chelates, have very favorable safety
profiles. Magnetic resonance images can be acquired in any
orientation and, unlike CT scans, are not limited to the axial plane.
Together, these advantages of MRI lead to better detection and
characterization capabilities than are available with CT.
Magnetic resonance imaging also has some important drawbacks,
however. In most regions, it is a more expensive examination than CT.
There are fewer MRI scanners than CT scanners, which can result in
scheduling difficulties. In general, the MRI examination takes longer
and has more contraindications than a CT scan. Finally, MRI has more
contraindications than does CT. These include, but are not limited
to, the presence of a pacemaker or implanted device (such as a
defibrillator or insulin pump), some aneurysm clips and heart valves,
and recently placed vascular stents.
Two types of CT technology are currently in clinical use: axial (or
conventional) and spiral (or helical) CT.
Axial vs Spiral CT
Axial CT has been available for more than 20 years, although it has
been improved continuously. In this technique, the patient is moved a
short distance incrementally and a single image is formed, followed
by another incremental move and another image acquisition, and so on.
Spiral CT uses a constant linear movement of the patient through the
scanner as the x-ray tube revolves continuously 360°, forming a
spiral around the patient. This allows for more rapid
scanning than can be achieved with axial CT, and in general,
overcomes many of the constraints of conventional CT of the
liver. Faster scanning is important because it makes possible the
acquisition of more images during a single period, reducing
respiratory and other motion artifacts. Multiple image sets can then
be obtained following infusion of a contrast agent.
Dual-Phase Contrast-Enhanced CT
Dual-phase contrast-enhanced CT is a recently developed improvement
in liver imaging. In this technique, a complete set of images is
obtained during the hepatic-arterial dominant phase of liver
perfusion, followed by a second set obtained during the portal-venous
phase. Some lesions, such as hepatocellular carcinoma (HCC) and
certain metastases, have blood supplied mostly from the hepatic
artery and will have higher attenuation values than the liver
parenchyma during the arterial phase (Figure
1). Most metastases have a portal-venous blood supply and are
hypoattenuating compared to the rest of the liver during the portal
phase of perfusion.
The dual-phase technique offers advantages for both lesion detection
and characterization over the single-phase studies, and many
dedicated CT liver studies are now performed using this
technique.[23-25] Dual-phase examinations require higher injection
rates of contrast agent than are routinely used; for this reason,
nonionic contrast is customarily administered to reduce patient
discomfort and the incidence of adverse reactions.
Angiographically assisted CT is an invasive technique used to
increase the sensitivity of CT.[15-18,26-30] In this study, a
catheter is placed either in the celiac or hepatic artery (CT hepatic
arteriography [CTHA]) or in the superior mesenteric or splenic artery
(CT during arterial portography [CTAP]). A rapid bolus of contrast is
delivered, which results in a large contrast load reaching the liver
either directly from the hepatic artery (CTHA) or indirectly via the
portal or splenic vein (CTAP). Although these angiographically
assisted techniques are more sensitive than noninvasive CT, they are
not routinely used because of their invasive nature.
Computed tomography during arterial portography has been recognized
as the most effective imaging technique for the preoperative
determination of hepatic tumor resectability, but this distinction
was challenged recently by MRI studies using liver-specific contrast
agents.[15-18] One significant drawback of CTAP, aside from its
invasive nature, is its lowered specificity due to the false-positive
perfusion defects seen in the liver. The perfusion
defects are caused by altered hepatic vasculature and can appear
identical to focal hepatic metastases. Delayed images can help reduce
the number of false-positive focal lesions, but CT angiography
remains a sensitive, but less specific, technique.
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