Magnetic Resonance Imaging of Liver Metastasis
Introduction
Liver metastasis, by definition, is a malignant lesion originating in an organ distant from the liver, which would secondarily disseminate and grow in the liver. Liver is a very common site of metastasis. Gastrointestinal tumors such as neuroendocrine tumors, colorectal cancers, esophageal and gastric tumors, and pancreatic cancers are among the most common sources for metastatic disease to the liver. Colorectal cancers (CRCs) are especially very common, with approximately 50% of the patients would have metastatic liver disease either at the time of diagnosis or in the follow-up period after surgical resection.1 Despite advances in surgery, targeted biologic therapies, and chemotherapy, the survival rates of patients with liver metastasis are still dismal, with a significant portion of these patients dying owing to their metastatic liver disease.
Disseminations through the portal venous and arterial systems are the most common routes for metastasis. Once the tumor cells detach from their original site, they travel into the vascular system and extravasate into the target organ parenchyma and proliferate. They are frequently found as multiple lesions of different sizes in both liver lobes, and several cancers often incite desmoplastic reaction when they metastasize, which gives them their hard consistency.2 Often times, internal necrotic changes give rise to their pseudocystic appearance. Central fibrous scar formation and capsular retraction (when they are located close to the liver capsule) may also be detected.3, 4
Section snippets
Clinical Presentation
Most liver metastases are clinically silent and detected with cross-sectional imaging. When they are symptomatic, the disease is most often at an advanced stage and the prognosis is poor. Metastasis from hormonally active tumors may present with symptoms related to the hormonally active metabolites secreted by the metastatic cells.
Role of Cross-Sectional Imaging in the Evaluation of Metastatic Liver Disease
Imaging plays a crucial and integral role in the diagnosis and monitoring of metastatic liver disease. Several imaging modalities including ultrasonography (US), computed tomography (CT), positron emission tomography (PET), and PET-CT scans can be used for this purpose. In this article, we are going to focus on the role of MRI, which is becoming the gold standard modality in the diagnosis of hepatic metastases.
MRI for Liver Metastases
MRI offers superior soft tissue resolution, providing several advantages over other cross-sectional imaging modalities for the detection and characterization of the focal liver lesions. This superiority becomes more prominent when it comes to detection of small-sized metastases. The reported sensitivity of contrast-enhanced MRI (CE-MRI) is 91%-97% compared with 71%-73.5% for CT.5, 6, 7 The sensitivity of MRI increases, especially, in the characterization of the lesions deemed to be ātoo small
Advanced MRI Techniques
Several new MRI techniques have entered clinical practice in the past few years, and they have now become essential components of standard liver MRI protocols.9 Diffusion-weighted imaging (DWI), hepatocyte-specific magnetic resonance (MR) contrast agents (HSMRCA), MR elastography, and MR perfusion may be counted among them. For the sake of brevity, we focus on DWI and HSMRCA. MR elastography and MR perfusion are less commonly used in clinical practice, and both are currently in the research
DWI of the Liver
DWI is a technique that can provide tissue contrast based on the measurement of diffusion properties of the water molecules within tissue.10 It is mainly based on the intravoxel incoherent motion and provides quantification of the water diffusion and microcapillary blood perfusion within the tissue in a noninvasive manner.11
DWI was mainly a tool for neuroimaging; however, it has become an indispensible part of the liver MRI studies. It is not only used in the detection of liver lesions but also
Hepatocyte-Specific Contrast Agents
Gadolinium chelates used as IV contrast media in MRI studies are crucial for increasing the sensitivity and specificity of the liver MRI examinations. With increasing use of MRI for the liver examinations, several HSMRCAs have been used in the past. Currently, there are 2 HSMRCAs available for clinical use, and both are gadolinium-based agents, which overcome the other shortcomings related to former HSMRCAs. These 2 agents currently used in clinical practice are gadobenate dimeglumine
Pharmacokinetics of the Gadoxetate Disodium
Owing to its unique properties, a brief discussion of the pharmacokinetics of gadoxetate disodium is warranted. It first distributes to the extracellular space after IV administration, and once in the extracellular space, it can either be excreted from the kidneys through glomerular filtration or taken up by the hepatocytes and excreted into the intrahepatic biliary canalicules.22 This dual-elimination pathway gives gadoxetate disodium its hepatocyte-specific aspect. In subjects with preserved
Use of Gadoxetate Disodium
Gadoxetate disodium was approved for clinical use in Europe in 2004, followed by United States in 2008 and Canada in 2010; there has been a massive flush of the literature regarding its clinical value and use. Several different specialties including radiology, hepatology, and surgery have contributed to the accumulation of this massive body of information.
Different studies have demonstrated the superiority of MRI with hepatocyte-specific contrast agents, in particular gadoxetic acid, relative
Colorectal Cancer Metastases
CRC is a very common cause of morbidity and mortality, and liver is a very common site for tumor metastasis. Approximately 30% of the cases have metastatic liver disease at initial presentation, with almost 14.5% of the cases showing development of liver involvement during chemotherapy.46, 47 Early detection and characterization of liver metastases are of crucial importance for optimal triage of patients who may benefit from hepatic resection from those who need chemotherapy, to improve 5-year
Imaging After Locoregional Therapy for Liver Metastases
Locoregional treatments with ablative technologies and intraarterial chemoembolization or radioembolization are commonly used methods in the treatment of liver metastases. Cross-sectional imaging is the main tool for the evaluation of early-stage assessment of procedural success and follow up. US, CT, PET-CT, and MRI may all be used for this purpose. As other modalities except for MRI is out of the scope of this article, we focus on the role of MRI. Subtraction images may be helpful, especially
Imaging Findings After Ablative Locoregional Liver Therapy
The imaging specialist should be familiar with the postprocedural imaging findings to prevent the potential diagnostic confusion. Radiofrequency ablation and microwave ablation are the 2 most commonly used local ablative technologies.
The term āablation zoneā refers to the area undergoing coagulative necrosis after ablative therapy. This encompassess the actual tumor zone as well as the 5-10Ā mm around the tumor, which is akin to disease-free resection margin aimed after conventional surgical
Imaging Findings After Transarterial Chemotherapy or Radiotherapy
Intraarterial treatments take advantage of the dual vascular supply of the liver. As the metastatic foci in the liver mostly derive their blood supply from hepatic artery and the rest of the liver from the portal vein, theoretically, by using the transarterial way, the metastases may be selectively treated by sparing the liver parenchyma. Metastatic liver disease from several different malignant neoplasms may be treated by transarterial approaches, but most of the information accumulated is on
Atypical Presentations of Liver Metastases and Challenging Findings
Typical imaging findings in metastatic liver disease may not be challenging to the imagers, and diagnosis is relatively straightforward. However, it is well known that liver metastases may mimick other benign and malignant clinical conditions, and awareness to these findings is crucial for the right diagnosis.
Diffuse infiltrative metastatic disease may be highly challenging to recognize in some patients. This pattern is less common, and hematologic malignancies are known to metastasize in this
Role of MRI in Prognosis and Predicting Response of Liver Metastases to Chemotherapy
In addition to its high sensitivity in diagnosing liver metastases, MRI may also have a role, by using DWI and other sequences, in predicting prognosis and tumor response to chemotherapy. Koh et al107 have demonstrated in their study that the detection of high ADC values in pretreatment metastatic liver foci secondary to CRCs may be an indicator of poor prognosis. The authors postulated that the presence of necrosis and loss of cell membrane integrity, causing high ADC values, in the
Future Directions
Several new state-of-art developments in MRI have a potential of opening new horizons for imaging specialists including more quantitative approaches in MRI (MR fingerprinting), the combination of MRI with PET (PET-MRI), and metabolic approaches such as glucose chemical exchange saturation transfer (glucoCEST) assessing the glucose metabolism with MRI.
In MR fingerprinting, the quantitative assessment of the tissue T1 and T2 relaxivity has a great potential, in addition to conventional
Conclusion
Metastatic liver disease is very common in the course of cancer. The presence of hepatic metastatic disease is commonly a sign of advanced stage malignancy, and early detection and proper follow up is mandatory for optimal management. Although all cross-sectional imaging modalities including US, CT, and PET-CT may be used, MRI appears to be most successful modality with its superior soft tissue resolution. This advantage of MRI became even more pronounced with the advances in
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