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AJR 2000; 174:691-698
© American Roentgen Ray Society

Review

CT and MR Imaging of Hepatic Metastases

Gregory T. Sica1, Hoon Ji1 and Pablo R. Ros1,2

1 Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, 75 Francis St., Boston, MA 02115
2 Department of Radiologic Pathology, Armed Forces Institute of Pathology, 14th St. and Alaska Ave., N. W., Washington, DC 20306.

Received February 25, 1999; accepted after revision August 10, 1999.

 
Address correspondence to G. T. Sica.


Introduction
Top
 Introduction
CT
 CT During Arterial Portography
 MR Imaging
 Comparative Studies
 Practical Approach
 References
 
The liver is one of the most common organs to be involved with metastatic disease, which arises most frequently from primary sites in the colon, breast, lung, pancreas, and stomach [1].

The accurate detection of metastatic disease at the time of diagnosis or during the course of treatment remains crucial to patient management. Early identification provides the opportunity for resection, which, at least in cases of colorectal carcinoma, has been shown to prolong survival [2]. Imaging-guided interstitial therapies, including cryoablation, laser photo-coagulation, radiofrequency and microwave ablation, and percutaneous ethanol injection, are evolving and may play both a curative and a palliative role, but their success depends on accurate imaging of liver neoplasms.

Both CT and MR imaging have benefited from rapid technologic advances and MR imaging, in particular, from the advent of new contrast agents. Comparative studies must be carefully scrutinized. Studies may quickly become outdated as new methods are introduced. An ideal examination provides high sensitivity and specificity and is noninvasive, low in cost, and widely available. No single study currently fits that profile. Studies available include helical CT using uni-, bi-, and triphasic techniques, CT during arterial portography, and MR imaging either unenhanced or enhanced with extracellular or tissue-specific contrast agents.

Factors such as availability, technical and clinical expertise, cost, and patient tolerance affect the decision of study choice. Familiarity with the technique used is important because false-positive diagnoses can have a substantial effect on patient management. The choice of imaging study should be influenced by the clinical indications. Objectives of oncologic liver imaging include screening for the presence of disease, the characterization of liver lesions, anatomic localization, evaluation of interval change during a course of treatment, and assessment of vascular patency. When the study is tailored to the clinical objectives, imaging resources are used optimally. This review will discuss imaging options and their appropriate indications.


CT
Top
 Introduction
 CT
CT During Arterial Portography
 MR Imaging
 Comparative Studies
 Practical Approach
 References
 
CT, which is widely available and familiar, is the mainstay of hepatic imaging. When performed properly, CT will suffice for most clinical indications.

Extracellular Contrast Agents
Extracellular CT contrast agents consist of ionic and nonionic iodinated aqueous compounds. With bolus injection, three distinct phases of hepatic parenchymal enhancement are identified: arterial (bolus), portal (nonequilibrium), and equilibrium. During the arterial phase, aortic and hepatic arterial enhancement increases rapidly and quickly peaks after the end of the injection. The effects of arterial enhancement are seen approximately 20-30 sec after the initiation of contrast injection. Because most liver blood flow is from the portal system, hepatic parenchymal enhancement lags behind arterial enhancement. During the portal phase, beginning approximately 60 sec after injection initiation, arterial enhancement decreases, and parenchymal enhancement increases because of portal venous inflow. Both decreasing parenchymal and vascular enhancement, and minimal liver-lesion differential enhancement mark the equilibrium phase [3, 4].

Intrinsic and extrinsic factors affect hepatic enhancement. Intrinsic factors relate to physiologic and anatomic aspects of the patient and include weight, cardiac function, state of hydration, renal function, and state of the hepatic vasculature. Extrinsic factors include volume and concentration of the contrast agent, the rate of injection, and technical scan parameters [3].

Lesion conspicuity will depend on differential enhancement between lesions and the adjacent liver parenchyma. Vascular (hypervascular) metastases may show significant enhancement during the arterial phase. Most liver metastases are hypovascular and are best imaged during the portal venous phase. During the equilibrium phase, lesions may become less conspicuous or completely obscured. The time to reach the equilibrium phase is variable. Using standard parameters such as a 150-ml volume of contrast material and a 2 ml/sec injection rate, equilibrium phase effects can start as early as 100 sec after the initiation of contrast injection [5]. This places constraints on scanning parameters, particularly if dual-phase imaging is desired.

Tissue-Specific Contrast Agents
A variety of tissue-specific, hepatobiliary and reticuloendothelial system-directed CT contrast agents have been studied in preclinical and clinical trials, but none has been approved for routine clinical use. Tissue-specific contrast agents accumulate in normal tissue, thereby increasing the contrast of focal lesions. Improved sensitivity and specificity may result. Imaging results with two promising agents, perfluoroctyl bromide [6] and ethiodized oil emulsion (EOE-13) [7], were considered excellent but suffered from significant safety issues that percluded further clinical testing [8].

Technical Factors
With the advent and more common use of helical scanners, the liver can be imaged in less than 20 sec, and dynamic incremental scanning is considered suboptimal. A power-injected, single-phase bolus is accepted as the superior technique [9]. Helical scanning enables higher injection rates, which shorten the time to peak liver enhancement but have no effect on maximum liver enhancement [10]. Automated bolus detection methods are available that offer precise coordination of the scan initiation with the timing of the contrast bolus [11]. In 1998, Paulson et al. [12] reported that up to 35% of patients in a tertiary care hospital may not achieve a threshold of 50 H greater than the baseline by 60 sec after injection of contrast material and would require the use of a set time delay.

Much has been written concerning the indications for dual- and triple-phase scanning. The specific technique used will depend largely on the individual indication for the study. For example, in the follow-up of a patient with known hypovascular liver metastases, portal venous phase scanning is appropriate. In 1996, Kuszyk et al. [13] reported an overall sensitivity for portal venous phase scanning of 91% for detecting malignant tumors greater than 1 cm, but a sensitivity of 56% for lesions smaller than 1 cm. Several studies have reported improved detection and characterization using multiphase scanning. The incremental improvement is especially seen in detecting typical hypervascular lesions such as renal, thyroid, breast, carcinoid, islet cell, and melanoma metastases. In 1995, Bonaldi et al. [14] reported the detection of an additional 8% of lesions using dual-phase scanning. Also in 1995, Hollett et al. [15] showed an increase in detection and conspicuity of malignant lesions smaller than 1.5 cm using this technique. In 1997, Van Hoe et al. [16] reported an increased specificity overall (53% versus 70%) using a dual-phase technique, although the difference was not significant for the subgroup of hemangioma versus metastasis.

The added value of unenhanced images and triple-phase scanning was investigated in 1998 by Miller et al. [17], who found no lesions in patients with suspected or known metastases or hepatocellular carcinoma were visualized solely on the unenhanced images. In 1998, Paulson et al. [18] reported that six of 206 lesions were seen only on the unenhanced images in patients with metastatic carcinoid. An alternative form of triple-phase scanning was used in 1996 by Van Leeuwen et al. [19] to obtain a delayed (8-10 min) equilibrium phase enhanced scan. They reported that lesion characterization was aided with the equilibrium phase images but recognized the increase in examination time, cost, and radiation exposure to the patient.

The use of smaller interscan spacing (4 versus 8 mm) in the reconstruction of images from a helical CT data set increased confidence in and sensitivity of detection for focal primary and metastatic liver lesions, as shown in 1993 by Urban et al. [20].

Imaging Appearance
Most metastases are revealed as low- or isoattenuating masses on CT. Depending on lesion size, the margins tend to be irregular, and necrosis may be present, but margins can be sharp and well defined. Central low attenuation may be the result of necrosis or cystic change. Calcification may be present with metastases from mucinous gastrointestinal tract tumors and from primary ovarian, breast, lung, renal, and thyroid cancer (Fig. 1). Most metastases are hypovascular and during the arterial phase show a complete ring of enhancement. Hypervascular metastases have diffuse enhancement. During the portal venous phase of imaging, a thickened rind enhances progressively but to a lesser extent than liver (Fig. 2). Both hemangiomas and metastases may fill in (centripetally) with contrast material over time, although complete filling in is more characteristic of hemangiomas. Hemangiomas typically present with a broken globular or nodular ring of enhancement. Globular enhancement has a sensitivity of 62-88% and a specificity of 84-100% (100% when enhancement is isoattenuating to the aorta) for the diagnosis of hemangioma [21, 22]. Small, homogeneously enhancing hemangiomas may be difficult to differentiate from hypervascular metastases on arterial phase imaging [23, 24]. Hemangiomas, though, tend to remain enhanced during the portal venous phase, whereas hypervascular metastases tend to wash out [21, 23]. In the equilibrium phase, metastases are often isoattenuating but on a 4- to 6-hr delayed phase contrast-enhanced CT scan may again appear to be of low attenuation. Some metastases will show peripheral areas of low attenuation surrounding an enhanced center on delayed images [25]. This appearance is thought to represent contrast material washing out of the viable tumor periphery while remaining in the extracellular space of the center.



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  		Fig. 1. —56-year-old man with metastatic colonic carcinoma. Unenhanced CT scan shows multiple liver lesions with amorphous calcifications.

 


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  		Fig. 2. —51-year-old man with metastatic colonic carcinoma. Contrast-enhanced CT scan during portal phase shows multiple hypovascular liver lesions, largest of which has central necrosis. Note enhancement (less than that of adjacent liver) of complete, thickened rind of tissue.

 

An uncommon secondary finding, capsular retraction adjacent to a mass, has been reported to be highly specific for malignancy [26]. In 1994, Apicella et al. [27] reported five cases in which hepatic vessels coursed undisturbed through low-attenuation areas proven to be metastases. Previously this sign was thought to indicate areas of steatosis.


CT During Arterial Portography
Top
 Introduction
 CT
 CT During Arterial Portography
MR Imaging
 Comparative Studies
 Practical Approach
 References
 
The specific technical parameters for CT during arterial portography are beyond the scope of this review and will not be discussed. CT during arterial portography has been the most sensitive nonsurgical imaging technique for the detection of hepatic metastases, with an overall detection rate of 81-94% [28,29,30]. This sensitivity may be diminished in patients with portal hypertension and cirrhosis [18]. CT during arterial portography appears to be particularly strong in detecting greater numbers of smaller lesions. Disadvantages of this technique are low specificity with variable ability to differentiate benign from malignant lesions, the presence of scan-specific artifacts (e.g., perfusion anomalies or abnormalities) (Fig. 3A,3B), the need for technical expertise in performing and interpreting studies, cost, and the relative level of invasiveness compared with conventional CT [31, 32]. One study reported a false-positive rate of 15%, which the authors believed was significant because resection could be denied in patients who are otherwise surgical candidates [33]. The addition of CT during hepatic arteriography to CT during arterial portography has been reported to aid in characterization [34]. Performing 4- to 6-hr delayed CT may also be useful for differentiating tumors from perfusion defects [35]. The use of CT during arterial portography has been largely replaced by multiphase helical scanning and tissue-specific enhanced MR imaging.



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  		Fig. 3A. —58-year-old man with colonic carcinoma and liver lesion suggestive of metastasis. Preoperative CT during arterial portography (CTAP) image shows focal low-attenuation lesions in medial segment of left lobe (arrowhead) and adjacent to falciform ligament (arrow). Their nonround shape and typical location suggest perfusion abnormalities.

 


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  		Fig. 3B. —58-year-old man with colonic carcinoma and liver lesion suggestive of metastasis. Contrast-enhanced CT scan shows no lesions in those locations, consistent with perfusion defects on CTAP.

 


MR Imaging
Top
 Introduction
 CT
 CT During Arterial Portography
 MR Imaging
Comparative Studies
 Practical Approach
 References
 
MR imaging has become increasingly more attractive as a primary imaging technique rather than being used solely for problem solving. It offers high intrinsic soft-tissue contrast, technical versatility for pulse sequence selection, biochemical and anatomic information, sensitivity to blood flow and contrast enhancement, and multiplanar capability. Several factors account for its increasing use including more wide-spread availability and technical familiarity, lower cost, ability to perform entire examinations in a reasonable time (<30 min for hepatic studies), and—using the most current software—the ability to perform entire breath-hold examinations, thus limiting motion artifacts. Currently, though, its use lags far behind that of CT as a technique for general liver imaging.

Extracellular Contrast Agents
Extracellular agents for MR imaging include a variety of paramagnetic chelates of gadolinium. They exert their predominant effect on T1 relaxation, resulting in increased signal intensity in the affected tissues. These contrast agents are administered by bolus injection and show biodistribution and pharmacokinetics similar to those of the iodinated CT extracellular agents. One difference, though, is the lack of substantial hepatocyte uptake with the MR agents [36]. Maximum liver-to-tumor contrast is seen within the first 2 min after administration for most lesions. Rapid imaging is thus required to maximize enhancement between the liver and focal lesions.

Tissue-Specific Contrast Agents
Two tissue-specific MR liver contrast agents are approved for clinical use: mangafodipir (Mn-DPDP) (Teslascan; Nycomed-Amersham, Princeton, NJ), a hepatobiliary-positive agent; and ferumoxides (Feridex; Berlex Laboratories, Wayne, NJ), a reticuloendothelial system-negative agent.

Positive contrast agents show their enhancement effect on T1-weighted sequences. Teslascan is composed of manganese bound to a pyridoxyl phosphate ligand. Although designed to have specific hepatocyte uptake, the compound dissociates in the blood and liver [37, 38]. Manganese becomes bound to macromolecules (explaining its T1 shortening effect), is taken up by hepatocytes, and is mainly excreted in the bile [38]. After injection, an enhancement plateau in liver signal intensity is reached within 10 min and persists for several hours [39]. In animal studies, the elimination of manganese was impaired when biliary obstruction was present [40, 41]. In patients with cirrhosis, enhancement may be diminished and heterogeneous [42]. Other hepatobiliary agents being investigated include small molecular weight lipophilic compounds bound to gadolinium—gadoxetate disodium (Gd-EOB) (Eovist; Schering, Berlin, Germany) and gadobenate dimeglumine (Gd-BOPTA) (MultiHance; Bracco Diagnostics, Princeton, NJ)—that have partial selective hepatocyte uptake and a biphasic enhancement profile, and hepatocyte receptor-targeted agents that consist of polysaccharide- or glycoprotein-coated iron oxide particles [43].

Particulate agents with sizes ranging from 30 to 5000 nm are cleared mainly by the reticuloendothelial system [43]. Iron oxide particles, or ferumoxides, have been coated with various materials, including dextran and starch. When these agents are placed in a magnetic field, they create strong local field inhomogeneities with shortening of both T2 and T2* relaxation. Larger particle agents (<50 nm), such as Feridex, are cleared predominantly by the liver (Kupffer's cells) and spleen (macrophages). The intravascular half-life of Feridex is less than 10 min, and clearance from the liver and spleen has a half-life of 3-4 days [44]. A plateau of decreased signal intensity can be observed from 30 min to 6 hr [45]. Ultrasmall particles (<50 nm; ultrasmall superparamagnetic iron oxide) have a prolonged intravascular half-life of 200 min and are distributed more widely within the reticuloendothelial system (liver, spleen, bone marrow, and lymph nodes). In addition to the T2 effects, they exert intravascular T1 shortening and act as blood pool agents. Studies have shown that in the presence of underlying diffuse liver disease, hepatic enhancement and the tumor-to-liver contrast-to-noise ratio can be impaired [46].

An interesting approach is the combination of an iron particle-based tissue-specific agent with a gadolinium-based extracellular agent. This combination offers both the high sensitivity for lesion detection of the tissue-specific agents and the familiar dynamic enhancement patterns used for characterization of the extracellular agents. The cost of such a study, though, may limit its appeal.

Technical Factors
The body of literature discussing parameter optimization for individual pulse sequences and the appropriate combination of pulse sequences for a liver protocol is enormous. Some lesions will not be detected or will be more conspicuous on T2-weighted sequences than on T1-weighted sequences and vice versa [47,48,49]. Contrast-enhanced sequences can aid in both detection and characterization of lesions [50, 51]. Some studies, though, report similar sensitivities for detecting liver metastases on both the unenhanced and contrast-enhanced sequences [52, 53]. As a general principle, a complete liver examination should include sequences providing T1, T2, and dynamic contrast enhancement information. The dynamic contrast-enhanced sequence is performed as a multiphase acquisition. In 1990, Steinberg et al. [54], using a combination of T1- and T2-weighted sequences, showed overall sensitivity for lesion detection will be similar at both.5 T and 1.5 T. Specific pulse sequences used will depend on the vendor of the scanner, the need for resolution, time constraints, and the clinical question being addressed. The application of fat suppression with T2-weighted imaging has been reported to increase lesion conspicuity, yet it may diminish overall image quality [50, 55]. There also appears to be more widespread use of breath-hold T2-weighted sequences, including breath-hold fast spin-echo, single-shot fast spin-echo, and half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequences [56, 57]. Although the T2 lesion characterization of these breath-hold sequences has not been proven, instances occur in which their use would suffice. For example, in the follow-up of a patient with known metastases that previously were well seen on T2-weighted images, a contrast-enhanced sequence may be unnecessary. In that instance, all that may be needed is a simple 30- to 40-sec breath-hold T2-weighted sequence.

Tissue-specific agents can improve detection and characterization. Mn-DPDP can be taken up by focal nodular hyperplasia, regenerating nodules, and, to a variable degree, hepatocellular carcinoma. Metastases lack the ability to take up Mn-DPDP and will appear hypointense compared with enhancing liver parenchyma on T1-weighted sequences. The variable uptake in malignant and benign hepatocellular tumors may complicate image interpretation [58]. Gadolinium-based hepatobiliary agents, Gd-EOB and Gd-BOPTA, show enhancement on both the early perfusion phase images and delayed hepatobiliary phase images. Those agents can provide information on both characterization, from the early enhancement, and lesion detection, on the delayed images [59].

Because metastases do not contain Kupffer's cells or a capacity to phagocytose iron-oxide particles, their signal intensity remains unchanged, whereas a marked reduction occurs in signal in normal liver, resulting in a significant increase in liver-to-lesion contrast [60] and in improved detection of lesions. Focal nodular hyperplasia, some adenomas, and well-differentiated hepatocellular carcinoma show variable ability to phagocytose the iron-oxide particles, whereas hemangiomas may show diminished signal intensity and contrast agent pools within their vascular lakes [60,61,62].

Imaging Appearance
The T1 and T2 signal intensities of metastases are variable but are usually prolonged, resulting in hypo- to isointensity on T1-weighted sequences and iso- to hyperintensity on T2-weighted sequences. This feature is exploited in multiecho T2-weighted imaging by comparing moderately T2-weighted (TE<120) and heavily T2-weighted (TE > 160) sequences. Metastases become progressively less intense (Fig. 4A,4B) and may be differentiated from nonsolid benign lesions such as cysts and hemangiomas. Metastatic tumors with liquefactive necrosis, cystic neoplasms, and hyperplastic neoplasms, though, may show higher signal intensity with long TEs. On T2-weighted images, 25% of metastases, and in particular those from colorectal carcinoma, may show a hyperintense rim or halo (viable tumor) surrounding central hypointensity (coagulative necrosis, fibrin, and mucin) [63, 64]. The doughnut sign, seen on T1-weighted images, and the corresponding target sign seen on T2-weighted images are most commonly seen with metastases [63]. The doughnut sign shows a low-signal-intensity rim surrounding an irregular or ovoid center of even lower signal intensity. The target sign consists of a hyperintense center (liquefactive necrosis) surrounded by a less intense rim of viable tumor (Fig. 5).



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  		Fig. 4A. —48-year-old woman with metastatic uterine leiomyosarcoma. Dual-echo fast spin-echo MR sequence (TE, 102 msec) shows mildly hyperintense focal lesion in right lobe of liver.

 


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  		Fig. 4B. —48-year-old woman with metastatic uterine leiomyosarcoma. Dual-echo fast spin-echo MR sequence (160 msec) shows liver lesion again but also shows relative loss in lesion signal intensity with this long TE sequence, consistent with metastasis.

 


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  		Fig. 5. —66-year-old woman with metastatic colonic carcinoma. T2-weighted fast spin-echo MR sequence shows lesions with target sign (arrow). Periphery of lesions (viable tumor) is relatively hypointense compared with center (liquefactive necrosis).

 

High signal intensity on T1-weighted sequences has also been described for various metastatic lesions and is presumably related to the internal content of a paramagnetic substance. High T1 signal intensity is not specific for malignancy. This appearance can be seen with metastases from melanoma (melanin, extracellular methemoglobin), colonic adenocarcinoma (hemorrhage or coagulative necrosis), ovarian adenocarcinoma (protein), multiple myeloma (protein), and pancreatic mucinous cystic tumor [65].

After the administration of IV contrast material with dynamic imaging, enhancement findings are similar to those on CT. Peripheral globular enhancement with centripetal filling is seen with hemangiomas, although as discussed earlier with CT, small hemangiomas may show immediate uniform enhancement. Large hemangiomas may have central areas of hemorrhage or fibrosis that do not fill in with contrast material. Hypervascular metastases show marked early enhancement as a continuous ring that on later images fills in centrally, or they may show early uniform enhancement. During the portal venous phase, hypervascular metastases may become iso- or hypointense. Hypovascular metastases are seen as hypointense masses that may have an enhancing peripheral rim best visualized during the arterial phase. Progressive centripetal fill-in may occur on delayed phases (Fig. 6A,6B). The peripheral washout sign, in which the peripheral rim is hypointense to the center of the lesion, can also be seen on delayed enhanced images [66]. That study reported a sensitivity of 25% and a specificity of 100% for the diagnosis of malignancy using this sign.



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  		Fig. 6A. —48-year-old woman with metastatic uterine leiomyosarcoma Contrast-enhanced fast spoiled gradient-echo (FSPGR) MR image during arterial phase shows thin enhancing rim.

 


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  		Fig. 6B. —48-year-old woman with metastatic uterine leiomyosarcoma Contrast-enhanced FSPGR MR image during portal phase shows peripheral rim less conspicuously, and central portion of lesion has become progressively more enhanced than in A.

 

Using tissue-specific agents, metastatic lesions should appear unenhanced against a background of either positively (Teslascan) or negatively (Feridex) enhanced liver.


Comparative Studies
Top
 Introduction
 CT
 CT During Arterial Portography
 MR Imaging
 Comparative Studies
Practical Approach
 References
 
Before the introduction of ferumoxides-enhanced MR imaging, CT during arterial portography was considered the most sensitive nonsurgical imaging technique for the detection of liver metastases, particularly those smaller than 2.0 cm. A recent study showed the accuracy of helical CT to approach that of CT during arterial portography [67]. In 1996, Semelka et al. [68] compared helical CT during arterial portography with MR imaging in patients being considered for hepatic resection and concluded that MR imaging has higher diagnostic accuracy, has greater effect on patient management, and is 64% less expensive than CT during arterial portography. Several studies have reported MR imaging to be more sensitive and more specific than dynamic incremental CT [69, 70] and helical CT [71].

Ferumoxides-enhanced MR imaging has been shown to detect more metastatic lesions than dynamic contrast-enhanced MR imaging [61] and to have a detection rate similar to that of CT during arterial portography [72] (Fig. 7A,7B). The increased detection is most prominent for small lesions. Vogl et al. [61] also reported higher specificity (93% versus 82%) with superparamagnetic iron oxide-enhanced MR imaging than with gadolinium-enhanced MR imaging, although their study included patients with benign and malignant primary tumors (but no hemangiomas or cysts) and metastases. Characterization criteria for common benign lesions, such as hemangiomas and cysts, have not been well defined, and the false-positive rate is unknown. Small lesions, which are detected at a greater frequency with this technique, likely will be particularly difficult to accurately characterize. Gadolinium-enhanced MR imaging may be helpful in the characterization of some of those lesions.



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  		Fig. 7A. —63-year-old woman with metastatic colonic carcinoma. CT during arterial portography image shows three lesions.

 


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  		Fig. 7B. —63-year-old woman with metastatic colonic carcinoma. T2-weighted fast spin-echo MR image enhanced with Feridex (Berlex Laboratories, Wayne, NJ) shows same three lesions and no additional lesions.

 

Using Mn-DPDP, Hamm et al. [73] reported increased detection of focal lesions compared with unenhanced MR imaging. Clinical trials have shown that Mn-DPDP-enhanced MR imaging was superior to unenhanced MR imaging and contrast-enhanced CT for the detection of focal lesions and differentiation of metastases from hepatocellular carcinoma [74]. Vogl et al. [75] compared MR imaging with Gd-DTPA and Gd-EOB and found improved lesion detection and comparable characterization with the latter agent.


Practical Approach
Top
 Introduction
 CT
 CT During Arterial Portography
 MR Imaging
 Comparative Studies
 Practical Approach
References
 
General screening for liver disease is well performed with helical CT during the portal venous phase using a bolus injection. In patients with a clinical suspicion of hypervascular lesions, the addition of unenhanced and, particularly, arterial phase images is beneficial. Although many lesions can be characterized with a uniphasic approach, multiphase scanning may also aid in lesion characterization. Small lesions can be problematic because they may have atypical appearances and may require follow-up or biopsy for more accurate characterization. Unenhanced CT has a limited role in evaluating metastases. Although its use has been reported in evaluating lesion interval change, margins may be too obscure for that assessment.

The cost of MR imaging has decreased and its use is increasing as a primary imaging technique. MR imaging has some advantages compared with CT in supplying T1 and T2 signal information for detection and characterization, in addition to multiphase enhancement. This additional information may improve confidence in detection and characterization of focal lesions. In our experience, MR imaging has also been helpful with equivocal CT studies. The options for MR imaging include unenhanced, gadolinium-enhanced, and tissue-specific enhanced studies. Unenhanced MR imaging may be used in follow-up studies in which change in lesion size, for instance, is being evaluated. For most general liver imaging, including the characterization of lesions such as metastases and hemangiomas, a dynamic contrast-enhanced sequence using a nonspecific gadolinium-based extracellular agent should be included along with T1- and T2-weighted sequences.

We reserve the use of tissue-specific MR contrast agents for when the goal is to achieve the highest detection rate of focal lesions, as with a patient being evaluated for curative hepatic resection. To date, there is more clinical experience with the iron-based reticuloendothelial system agents. CT during arterial portography and ferumoxides-enhanced MR imaging appear to be of equivalent value (Fig. 7A,7B). Both of these examinations have limited specificity, particularly for small lesions, and intraoperative sonography may be required to confirm a malignant diagnosis.

The reticuloendothelial system agents also have a role in differentiating focal nodular hyperplasia, which can show negative enhancement from both metastases and primary malignant tumors in which uptake of the agent is not expected. However, if the clinical question is the differentiation of a primary malignant tumor from a metastasis, we recommend MR imaging using a nonspecific extracellular agent.


References
Top
 Introduction
 CT
 CT During Arterial Portography
 MR Imaging
 Comparative Studies
 Practical Approach
 References
 

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