MR IMAGING OF DIFFUSE LIVER DISEASE

Assessment of liver iron overload by 3 T MRI

PURPOSE

To evaluate the performance and limitations of the signal intensity ratio method for quantifying liver iron overload at 3 T.

METHODS

Institutional review board approval and written informed consent from all participants were obtained. One hundred and five patients were included prospectively. All patients underwent a liver biopsy with biochemical assessment of hepatic iron concentration and a 3 T MRI scan with 5 breath-hold single-echo gradient-echo sequences. Linear correlation between liver-to-muscle signal intensity ratio and liver iron concentration was calculated. The algorithm for calculating magnetic resonance hepatic iron concentration was adapted from the method described by Gandon et al. with echo times divided by 2. Sensitivity and specificity were calculated.

RESULTS

Five patients were excluded (coil selection failure or missing sequence) and 100 patients were analyzed, 64 men and 36 women, 52 ± 13.3 years old, with a biochemical hepatic iron concentration range of 0-630 µmol/g. Linear correlation between biochemical hepatic iron concentration and MR-hepatic iron concentration was excellent with a correlation coefficient = 0.96, p < 0.0001. Sensitivity and specificity were, respectively, 83% (0.70-0.92) and 96% (0.85-0.99), with a pathological threshold of 36 µmol/g.

CONCLUSION

Signal intensity ratio method for quantifying liver iron overload can be used at 3 T with echo times divided by 2.

Quantification of liver fat in mice: comparing dual-echo Dixon imaging, chemical shift imaging, and 1H-MR spectroscopy

We evaluated dual-echo Dixon in-phase and out-of-phase (IP-OP), chemical shift imaging (CSI), and (1)H MRS (hydrogen MR spectroscopy) in estimating fat content (FC) in phantoms and in livers of mice. Phantoms were made according to the volume percentage of fat ranging from 0% to 100%. The three MR methods were performed to measure FC in phantoms and in livers of obese leptin-deficient (ob/ob), human BSCL2/seipin gene knockout (SKO), and wild-type (WT) mice. The results were compared with known FC in phantoms and to a reference standard from mice by histological semiautomatic vacuole segmentation (HIS-S) procedure and liver lipid (LL) chemical analysis. In phantoms, CSI underestimated FC from 50% to 100%, to a lesser extent than IP-OP. In vivo, liver FC in ob/ob and SKO mice measured by the three MR methods were all significantly higher than that in WT mice. Liver FC measured by IP-OP were significantly lower than that measured by CSI and MRS, with no significant difference between CSI and MRS. CSI and MRS showed a linear correlation with LL analysis and with each other. IP-OP underestimated FC, whereas CSI and MRS were more accurate for quantifying FC in both phantoms and liver. CSI and MRS have the potential to replace HIS-S and LL analysis in longitudinal studies.

Methods for assessing intrahepatic fat content and steatosis

PURPOSE OF REVIEW

Intrahepatic fat content is increasingly being recognized as an integral part of metabolic dysfunction. This article reviews available methods for the assessment of hepatic steatosis.

RECENT FINDINGS

Apart from liver biopsy, there are several noninvasive radiologic modalities for evaluating nonalcoholic fatty liver disease. Ultrasonography, computed tomography, and traditional MRI remain largely qualitative methods for detecting mild to severe degrees of steatosis rather than quantitative methods for measuring liver fat content, even though novel attempts to collect objective quantitative information have recently been developed. Still, their sensitivity at mild degrees of steatosis is poor. Undoubtedly, most methodological advances have occurred in the field of MRI and magnetic resonance spectroscopy, which currently enable the accurate quantification of intrahepatic fat even at normal or near normal levels. Xenon computed tomography was also recently shown to offer another objective tool for the quantitative assessment of steatosis, although more validation studies are required.

SUMMARY

Several modalities can be used for measuring intrahepatic fat and assessing steatosis; the choice will ultimately depend on the intended use and available resources.

Nonalcoholic fatty liver disease: diagnostic and fat-grading accuracy of low-flip-angle multiecho gradient-recalled-echo MR imaging at 1.5 T

PURPOSE

To assess the accuracy of four fat quantification methods at low-flip-angle multiecho gradient-recalled-echo (GRE) magnetic resonance (MR) imaging in nonalcoholic fatty liver disease (NAFLD) by using MR spectroscopy as the reference standard.

MATERIALS AND METHODS

In this institutional review board-approved, HIPAA-compliant prospective study, 110 subjects (29 with biopsy-confirmed NAFLD, 50 overweight and at risk for NAFLD, and 31 healthy volunteers) (mean age, 32.6 years +/- 15.6 [standard deviation]; range, 8-66 years) gave informed consent and underwent MR spectroscopy and GRE MR imaging of the liver. Spectroscopy involved a long repetition time (to suppress T1 effects) and multiple echo times (to estimate T2 effects); the reference fat fraction (FF) was calculated from T2-corrected fat and water spectral peak areas. Imaging involved a low flip angle (to suppress T1 effects) and multiple echo times (to estimate T2* effects); imaging FF was calculated by using four analysis methods of progressive complexity: dual echo, triple echo, multiecho, and multiinterference. All methods except dual echo corrected for T2* effects. The multiinterference method corrected for multiple spectral interference effects of fat. For each method, the accuracy for diagnosis of fatty liver, as defined with a spectroscopic threshold, was assessed by estimating sensitivity and specificity; fat-grading accuracy was assessed by comparing imaging and spectroscopic FF values by using linear regression.

RESULTS

Dual-echo, triple-echo, multiecho, and multiinterference methods had a sensitivity of 0.817, 0.967, 0.950, and 0.983 and a specificity of 1.000, 0.880, 1.000, and 0.880, respectively. On the basis of regression slope and intercept, the multiinterference (slope, 0.98; intercept, 0.91%) method had high fat-grading accuracy without statistically significant error (P > .05). Dual-echo (slope, 0.98; intercept, -2.90%), triple-echo (slope, 0.94; intercept, 1.42%), and multiecho (slope, 0.85; intercept, -0.15%) methods had statistically significant error (P < .05).

CONCLUSION

Relaxation- and interference-corrected fat quantification at low-flip-angle multiecho GRE MR imaging provides high diagnostic and fat-grading accuracy in NAFLD.

Biliary complications after living donor adult liver transplantation

The highest rate of complications characterizing the adult living donor liver transplantation (ALDLT) are due to biliary problems with a reported negative incidence of 22-64%. We performed 23 ALDLT grafting segments V-VIII without the middle hepatic vein from March 2001 to September 2005. Biliary anatomy was investigated using intraoperative cholangiography alone in the first five cases and magnetic resonance cholangiography in the remaining 18 cases. In 13 cases we found a single right biliary duct (56.5%) and in 10 we found multiple biliary ducts (43.7%). We performed single biliary anastomosis in 17 cases (73.91%) and double anastomosis in the remaining six (26%) cases. With a mean follow up of 644 days (8-1598 days), patient and graft survivals are 86.95% and 78.26%, respectively. The following biliary complications were observed: biliary leak from the cutting surface: three, anastomotic leak: two, late anastomotic strictures: five, early kinking of the choledochus: one. These 11 biliary complications (47.82%) occurred in eight patients (34.78%). Three of these patients developed two consecutive and different biliary complications. Biliary complications affected our series of ALDLT with a high percentage, but none of the grafts transplanted was lost because of biliary problems. Multiple biliary reconstructions are strongly related with a high risk of complication.

MR imaging of the liver

MR imaging is establishing a role as a primary diagnostic technique, with increasing evidence showing MR imaging to have advantages over CT regarding diagnostic sensitivity and specificity for many pathologies of solid organs, bile and pancreatic ducts, bowel, peritoneum, and retroperitoneum. In addition, there are increasing concerns regarding the risks of radiation and iodinated contrast associated with CT imaging of the abdomen. The incidence of contrast-induced nephropathy associated with iodinated contrast used for CT scanning is difficult to ascertain because reporting is spurious and variable in interpretation.

Magnetic Resonance Imaging of the Liver: Review of Techniques and Approach to Common Diseases

MR imaging examination of the liver should use a combination of single-shot T2W and breath-hold T1W images, and include gadolinium enhancement with acquisition of multiple phases. MR provides superior characterization of liver masses than CT, and multi-phase gadolinium enhancement including a properly timed arterial phase is critical. The T1 weighted pre-contrast images must include in-phase/out-of-phase acquisitions, to assess hepatic lipid and or iron content, and dynamically enhanced post-gadolinium images. Timing of the arterial phase images is also critical for demonstration of acute hepatitis. The timing of the venous and equilibrium phase images are less critical, and are important for grading more severe acute hepatitis, demonstration of fibrosis, and for delineating vascular abnormalities. In cirrhosis, dynamic post-gadolinium images are critical for detection and characterization of regenerative or dysplastic nodules, and HCC. The same sequences useful for liver evaluation provide a comprehensive evaluation of all the soft tissues of the abdomen, and allow depiction of most of the important diseases, thus facilitating use of a universal protocol for abdominal imaging.

Contrast-enhanced magnetic resonance body imaging 5

Many new techniques and applications in magnetic resonance imaging of the body have been introduced in the last decade and, at the same time, a wide variety of contrast media have become available for different imaging strategies. The aim of this article is to review the current use of contrast agents in body MRI. Extracellular and hepatobiliary gadolinium chelates, as well as iron oxide-based contrast media, are discussed and their use in different areas of the body highlighted. Topics to be covered include breast imaging, imaging of the thorax and the mediastinum, and imaging of the upper abdomen, kidneys, and pelvis. Established applications as well as new emerging indications are discussed, and the impact on improved detection and characterization of pathologies is demonstrated.

Non-invasive assessment of hepatic iron stores by MRI

BACKGROUND

MRI has been proposed for non-invasive detection and quantification of liver iron content, but has not been validated as a reproducible and sensitive method, especially in patients with mild iron overload. We aimed to assess the accuracy of a simple, rapid, and easy to implement MRI procedure to detect and quantify hepatic iron stores.

METHODS

Of 191 patients recruited, 17 were excluded and 174 studied, 139 in a study group and 35 in a validation group. All patients underwent both percutaneous liver biopsy with biochemical assessment of hepatic iron concentration (B-HIC) and MRI of the liver with various gradient-recalled-echo (GRE) sequences obtained with a 1.5 T magnet. Correlation between liver to muscle (L/M) signal intensity ratio and liver iron concentration was calculated. An algorithm to calculate magnetic resonance hepatic iron concentration (MR-HIC) was developed with data from the study group and then applied to the validation group.

FINDINGS

A highly T2-weighted GRE sequence was most sensitive, with 89% sensitivity and 80% specificity in the validation group, with an L/M ratio below 0.88. This threshold allowed us to detect all clinically relevant liver iron overload greater than 60 micromol/g (normal value <36 micromol/g). With other sequences, an L/M ratio less than 1 was highly specific (>87%) for raised hepatic iron concentration. With respect to B-HIC range analysed (3-375 micromol/g), mean difference and 95% CI between B-HIC and MR-HIC were quite similar for study and validation groups (0.8 micromol/g [-6.3 to 7.9] and -2.1 micromol/g [-12.9 to 8.9], respectively).

INTERPRETATION

MRI is a rapid, non-invasive, and cost effective technique that could limit use of liver biopsy to assess liver iron content. Our MR-HIC algorithm is designed to be used on various magnetic resonance machines.

MR imaging of diffuse liver disease

MR imaging is able to demonstrate and distinguish the full variety of benign and malignant diffuse liver diseases.

Magnetic resonance imaging of diffuse liver diseases

The intrinsic soft tissue contrast and exquisite sensitivity to contrast agents are unique attributes of magnetic resonance imaging that are beneficial when evaluating diffuse liver disease. Much like a pathologist uses different tissue or cell marker stains, the magnetic resonance imager can use a variety of imaging strategies to elucidate pathologic liver processes in vivo, including processes leading to abnormal lipid metabolization, iron deposition, perfusion abnormalities related to inflammation, fibrosis, vascular occlusion, or infarction and hemorrhage. This article reviews the most important diffuse liver diseases and the corresponding magnetic resonance imaging features.

Diffuse liver disease

During the last decade, the role of the radiologist in evaluating patients with diffuse liver disease has increasingly expanded. In many cases, the management choices for the hepatologist in the imaging work-up of a patient with suspicion of a diffuse liver disease have significantly widened. In some instances, imaging may point directly to the diagnosis; in many instances, imaging helps narrow the differential diagnosis or is crucial in the follow-up of patients. Although some rare entities still have nonspecific radiologic features, the imaging pattern, in combination with appropriate clinical information, may provide the most likely diagnosis.

MR imaging in chronic hepatitis and cirrhosis

The role of magnetic resonance imaging (MRI) in the evaluation of diffuse parenchymal abnormalities of the liver has been expanded by recent technical advances of MR systems as well as the evolution of intravenous contrast media. Currently, MR is undoubtedly the most useful imaging modality for detecting the presence of chronic liver disease. Tailored sequences allow acurate depiction of specific disorders, including steatohepatitis and iron-overload states. Morphologic changes and signal intensity effects not only facilitate the diagnosis of chronic liver disease with MRI but they also help to distinguish between different etiologies, and they assist in staging the histologic severity of certain chronic conditions. Moreover, the faster MRI scanning techniques presently available permit the dynamic assessment of contrast enhancement, which permits improved characterization of focal hepatic lesions, including regenerative nodules, dysplastic nodules, and hepatocellular carcinoma (HCC). Although overlap in MRI findings still may exist among different types of chronic liver disease and among focal liver lesions, familiarity with certain specific imaging features may be diagnostic in the proper clinical setting. Finally, comprehensive MRI examination, including MR angiography and MR cholangiography, is the most sensitive and cost-effective technique for detecting extrahepatic disease, diagnosing vascular disorders, and evaluating the patient before or after liver transplantation. This article focuses on the current role of MR imaging in patients with chronic liver disease. The subjects covered include the detection and characterization of chronic hepatitis and cirrhosis, specific findings seen in steatohepatitis and certain metabolic diseases, the evaluation of extrahepatic vascular complications of cirrhosis, and patient assessment before and after liver transplantation. The characterization of hepatic masses is also included briefly. This subject is covered in greater depth elsewhere in this issue.

T1 relaxation times for viability evaluation of the engrafted and the native liver in a rat model of heterotopic auxiliary liver transplantation: a pilot study

Following a heterotopic auxiliary liver transplantation, commonly used measurements are either invasive or non-indicative of individual viability of the coexisting engrafted and native livers. Magnetic resonance imaging (MRI) was therefore tested for its potential to monitor the post-transplant hepatic viability in a rat model. Thirteen Wistar rats were systematically evaluated with MRI and serum biochemical liver parameters. Post-transplant complications and the causes of animal death were identified by autopsy and histo-pathological examinations. The data of the healthy survivors were compared with those of the rats that developed complications. On MRI, the hepatic complications could be depicted in the individual livers. A specific pattern of signal evolution was found in the livers of the healthy survivors: the mean T1 relaxation times of the engrafted livers increased immediately after transplantation (476 +/- 64 ms, mean +/- standard deviation, pre-operative; 730 +/- 48 ms, week 1) and then declined steadily to a 3 month value of 489 +/- 246 ms, while, following a transient first rise (476 +/- 64 ms, pre-operative; 589 +/- 28 ms, week 1), the mean T1 value of the native livers increased again 4 weeks after surgery and reached a 3 month value of 859 +/- 43 ms. However, in the rats with various complications, the mean T1 relaxation times of the engrafted livers continued to increase throughout the first post-operative month (760 +/- 48 ms, week 1; 922 +/- 76 ms, week 4), while that of the native liver only varied mildly (546 +/- 25 ms, week 1; 473 +/- 25 ms, week 4). After the first post-transplant week, the healthy engrafted livers could already be distinguished from those with complications by a significant decrease in T1 relaxation times. These data suggest that, besides demonstrating major complications, MRI may allow one to monitor the viability of each liver by analysing the relative signal intensity and T1 relaxation times after a heterotopic auxiliary liver transplantation.

Use of magnetic resonance contrast media in body imaging

A large range of new techniques and applications in magnetic resonance imaging has been introduced in the last decade. Correspondingly, a wide variety of contrast media has become available for different imaging strategies. This article reviews the main indications and techniques in magnetic resonance imaging and magnetic resonance angiography of the body with special regard to contrast media applications and its impact on improved detection and characterization of pathologies in thoracic and abdominal MRI.

MR imaging techniques of the liver

This article reviews the currently available MR imaging techniques that are useful for the detection and characterization of focal and diffuse liver pathology. The implementation and clinical utility of various T1-weighted, T2-weighted, T2*-weighted, and MR angiographic sequences are described.

Magnetic resonance imaging of focal and diffuse hepatic disease

MRI is a powerful tool in the detection and characterization of both focal and diffuse liver pathology. Because of superior soft tissue characterization, direct multi-planar capabilities and lack of ionizing radiation, current state of the art MRI is useful when contrast CT is relatively contraindicated or not definitive. This article reviews the MRI findings of the most common focal and diffuse liver diseases encountered in clinical practice. Reviews of current MR techniques and MR contrast agents used in liver imaging have been recently published. For this article, discussion of specific techniques and use of contrast is addressed for each pathological entity discussed.