<b>Comparison of MRI and CT scans for liver imaging</b>
from ONCOLOGY 14(Suppl 3): 21-28, 2000.

<b>CT and MRI scans are the most common scans for imaging the liver in cancer patients.  Both have undergone major improvements, and it is often not clear which technique is better suited to specific clinical situations.</b>

"The spatial resolution of CT scans remains superior, but 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."[1]

<b>CT is cheaper than MRI: the equipment and its upkeep cost less, and scan time is shorter, so there is not such a backlog of patients.  CT scans are widely available, and physicians feel comfortable with evaluating the images.  CT scans have the disadvantages of nephrotoxic contrast agents, and adverse reactions to the iodinated contrast agents, and exposure to ionizing radiation.  CT scans are less sensitive and specific than MRI scans in imaging hepatic foci of disease.

"Magnetic resonance imaging provides excellent contrast that can reveal subtle variations in tissues of differing histology."[1] There is no ionizing radiation or nephrotoxicity, and the usual contrast agent [gadolinium chelates] has a good safety profile.  The scan can be done in any direction, not just axially like CT scans.  MRI has better detection and descriptive powers than does CT.

MRI is usually a more expensive scan, is not so widely available, takes longer, and has more contraindications [e.g. implanted metal like: pacemakers, aneurysm clips, heart valves, vascular stents] than CT scans do.</b>

"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." [1]  Those lesions whose blood supply comes from the hepatic artery, like LMS, will show a different appearance during the arterial phase, than from the portal venous phase.  The lesions are detected as well as the source of their blood supply, which helps determine the nature of the lesion.  Dual-phase scans need higher injection rates of nonionic contrast.

For CT Angiography, a catheter is placed in an appropriate artery and a rapid injection of contrast results in a large contrast load reaching the liver.  It is more sensitive than noninvasive CT, but is invasive and not routinely used.

Fast MRI sequencing has reduced scanning time for the entire liver to 15 to 30 seconds-a single breath holding.   Specific lesions appear distinctively different, e.g. hemangiomas are distinctive.

<b>CT-MRI Comparison Studies
Many of the initial comparison studies between MRI and CT were done using early technology and found no significant difference between the two.  With further technological developments, this is no longer true.</b>

<b>MRI With RES-Specific Agents</b>
New contrast agents are being designed for liver imaging; they target the cells of the reticuloendothelial system [RES], so that the contrast accumulates and remains within the liver.  These agents allow longer imaging periods after administration, and increase the detection and characterization of focal liver lesions.
The RES agents are composed of iron oxide [ferumoxide] particles, which are not taken up by the metastases.  The difference in enhancement increases the sensitivity of the examination for lesions by making the focal liver lesions conspicuous.   Many well-controlled studies support the superiority of a ferumoxide-enhanced liver MRI over CT or unenhanced MRI, but not over intraoperative ultrasound or pathological examination.  Ferumoxide-enhanced MRI scans are statistically superior to other noninvasive liver scans.

"Economic factors also play an important role in determining which imaging modality will be used... In one study, in 95% of the patients imaged preoperatively with ferumoxides-enhanced MRI, there were no additional lesions detected using IOUS. Compared to contrast-enhanced CT, ferumoxides-enhanced MRI altered the clinical management in 67% of patients. These management changes corresponded to an overall net cost savings of $108,368 ($1,901 per patient). The authors concluded that ferumoxides-enhanced MRI is an economically feasible imaging method that will alter the clinical management in a substantial number of patients as compared with contrast-enhanced CT."[1]


<b>MRI vs CT Angiography</b>
CT Angiography had been considered the most sensitive exams for investigation of liver lesions.  Several studies have shown that noninvasive ferumoxides-enhanced MRI is at least as accurate as CT angiography, and could probably replace the invasive procedure.  
However, several authors have concluded that spiral CTAP is more sensitive than ferumoxides-enhanced MRI in patients who are to undergo liver resection, on a lesion-by-lesion analysis. 

<b>Conclusions</b>

<b>"Dual-phase spiral CT, CT angiography, ultrafast MRI, and MRI enhanced with liver-specific contrast agents have allowed for better detection and characterization of focal hepatic neoplasms."[1]

"Direct comparison studies have repeatedly confirmed that MRI has greater accuracy for detecting hepatic lesions, especially when the targeted contrast agents are employed. Angiographically assisted CT and MRI appear to have similar sensitivity for detecting hepatic neoplasms; however, MRI has greater specificity and the advantage of being noninvasive. Liver CT is most commonly employed as a routine screening tool because it is somewhat less costly and is more widely available"[1]

"State-of-the-art spiral CT and MRI are each highly accurate in detecting focal hepatic neoplasms. Magnetic resonance imaging has clear advantages when the highest possible accuracy is desired, such as when imaging patients prior to hepatic resection or intervention, or when the CT result is either inconclusive or conflicts with other clinical data."[1]</b>


1. ONCOLOGY 14(Suppl 3): 21-28, 2000
Comparison studies of CT and MRI in patients with hepatic metastases.
Earls JP.   Inova Fairfax Hospital Center, Falls Church, Virginia, USA.
The Johns Hopkins University School of Medicine, Baltimore, Maryland


