MR imaging of the liver: breath-hold T1-weighted MP-GRE compared with conventional T2-weighted SE imaging--lesion detection, localization, and characterization

Diffusion-weighted MRI and liver metastases

Liver metastases are the most frequently encountered malignant liver lesions in the Western countries. Accurate diagnosis of liver metastases is essential for appropriate management of these patients. Multiple imaging modalities, including ultrasound, CT, positron emission tomography, and MRI, are available for the evaluation of patients with suspected or known liver metastases. Contrast-enhanced MRI has a high accuracy for detection and characterization of liver lesions. Additionally, diffusion-weighted MRI (DWI) has been gaining increasing attention. It is a noncontrast technique that is easy to perform, could be incorporated in routine clinical protocols, and has the potential to provide tissue characterization. This article discusses the basic principles of DWI and discusses its emerging role in the detection of liver metastases in patients with extrahepatic malignancies.

Water excitation MPRAGE: an alternative sequence for postcontrast imaging of the abdomen in noncooperative patients at 1.5 Tesla and 3.0 Tesla MRI

PURPOSE

To evaluate the diagnostic image quality of postgadolinium water excitation-magnetization-prepared rapid gradient-echo (WE-MPRAGE) sequence in abdominal examinations of noncooperative patients at 1.5 Tesla (T) and 3.0T MRI.

MATERIALS AND METHODS

Eighty-nine consecutive patients (48 males and 41 females; mean age +/- standard deviation, 54.6 +/- 16.6 years) who had MRI examinations including postgadolinium WE-MPRAGE were included in the study. Of 89 patients, 33 underwent noncooperative protocol at 1.5T, 10 underwent noncooperative protocol at 3.0T, and 46 underwent cooperative protocol at 3.0T. Postgadolinium WE-MPRAGE, MPRAGE, and three-dimensional gradient-echo sequences of these three different groups were qualitatively evaluated for image quality, extent of artifacts, lesion conspicuity, and homogeneity of fat-attenuation by two reviewers retrospectively, independently, and blindly. The results were compared using Wilcoxon signed rank and Mann-Whitney U tests. Kappa statistics were used to measure the extent of agreement between the reviewers.

RESULTS

The average scores indicated that the images were diagnostic for WE-MPRAGE at 1.5T and 3.0T in noncooperative patients. WE-MPRAGE achieved homogenous fat-attenuation in 31/33 (94%) of noncooperative patients at 1.5T and 10/10 (100%) of noncooperative patients at 3.0T. WE-MPRAGE at 3.0T had better results for image quality, extent of artifacts, lesion conspicuity and homogeneity of fat-attenuation compared with WE-MPRAGE at 1.5T, in noncooperative patients (P = 0.0008, 0.0006, 0.0024, and 0.0042; respectively). Kappa statistics varied between 0.76 and 1.00, representing good to excellent agreement.

CONCLUSION

WE-MPRAGE may be used as a T1-weighted postgadolinium fat-attenuated sequence in noncooperative patients, particularly at 3.0T MRI.

Magnetic resonance imaging of the liver: a review

In this review article, the authors discuss the essential aspects of liver magnetic resonance imaging (MRI), including protocol, intravenous contrast use and disease entities. At present, liver MRI uses fast scanning techniques, allowing the maximization of the principles of image quality, reproducibility of image quality and good conspicuity of disease. MRI is the most accurate imaging modality for the detection and characterization of diffuse and focal liver disease. In the expert opinion section, the authors refer to the advantages and challenges of 3.0T liver imaging.

Liver MR Imaging: 1.5T versus 3T

This article focuses on technical challenges in transferring 1.5T liver protocols to 3T systems and the overall comparison of MR sequences, highlighting the advantages and disadvantages of imaging at the higher field strength. An important benefit is the capacity of acquiring high-quality, thin-section postgadolinium T1-weighted three-dimensional gradientecho sequences, most clinically relevant for the detection and characterization of small hypervascular malignant diseases. Further research and development is necessary to overcome disadvantages, such as with in- and out-of phase T1-weighted gradient-echo sequences, and to minimize artifacts that appear at 3T.

Computed tomography and magnetic resonance imaging of hepatic metastases

The detection and characterization of liver metastases is well performed with either computed tomography or magnetic resonance imaging. The administration of intravenous contrast is essential for almost all indications, with multiphasic imaging aiding in lesion characterization and detection. The use of multidetected CT (MDCT) provides the ability for optimized vascular and multiplanar imaging, but has also resulted in increased examination complexity. Tissue-specific MR contrast agents can yield the highest rate of lesion detection and thus may be useful in presurgical planning.

Chronic hepatitis: correlation of early patchy and late linear enhancement patterns on gadolinium-enhanced MR images with histopathology initial experience

The purposes of our study were to describe the early and late enhancement patterns of the liver on gadolinium-enhanced dynamic magnetic resonance (MR) images in patients with chronic hepatitis and to correlate these findings with histopathology. Patients were entered into the study based on the presence of chronic hepatitis, imaging evaluation with MR imaging (MRI), including early and late postgadolinium images, and histopathologic correlation. Early and late dynamic postgadolinium MR images of 29 consecutive patients with a pathologically proven diagnosis of chronic hepatitis were retrospectively evaluated for the presence of three types of enhancement, i.e., homogeneous, linear, and patchy. Correlation was made between the enhancement patterns on MR images and blinded retrospective interpretation of the histopathologic specimens, which were obtained within 3 months of the MR examination. Of the 29 patients, 16 (55.2%) patients showed patchy enhancement on the early gadolinium-enhanced MR images. In 11 (68.8%) of these 16 patients, histopathology demonstrated numerous macrophages, variable hepatocyte necrosis, and increased steatosis. The remaining 13 (44.8%) patients showed homogeneous enhancement on the early gadolinium-enhanced MR images. In 11 (84.6%) of these 13 patients, histopathology demonstrated few or no macrophages, little or no hepatocellular necrosis, and little or no steatosis. The correlation between patchy enhancement and acute liver inflammation was significant (P = 0.005). On the late gadolinium-enhanced MR images, 20 (69.0%) of 29 patients showed prominent linear enhancement. In 19 (95.0%) of these 20 patients, histopathology revealed hepatic fibrosis. We concluded that in patients with chronic hepatitis, the presence of early patchy enhancement indicates either concurrent or recent hepatocellular damage, whereas the presence of late linear enhancement indicates the presence of fibrosis, with a high degree of correlation with histopathologic findings.

Role of magnetic resonance imaging in the staging of gastrointestinal neoplasms

A concise review is presented of the current applications and techniques of magnetic resonance imaging (MRI) in the field of diagnostic imaging of oncologic disease of the gastrointestinal tract, with a summary of the diagnostic possibilities of MRI in the various types of tumor pathology of the gastrointestinal tract. We conclude with a review of the specific situations in which MRI presents distinct advantages over other diagnostic imaging methods (such as computed tomography and ultrasound).

Biliary hamartomas: solitary and multiple lesions shown on current MR techniques including gadolinium enhancement

The purpose of this study was to describe the magnetic resonance (MR) imaging features of biliary hamartomas on T1- and T2-weighted and gadolinium-enhanced sequences, and to correlate these findings with histopathology. MR imaging findings in four patients with pathologically proved biliary hamartomas are described. In all patients, MR imaging sequences, including T1- and T2-weighted and early and late gadolinium-enhanced images, were retrospectively evaluated for the size, morphology, signal intensity, and enhancement pattern of the lesions. Correlation was made between the MR imaging findings and histopathology. Biliary hamartomas ranged in diameter from 0.5 to 1.5 cm. Lesions were solitary in one patient and numerous in three patients. In all patients, the lesions were low signal on T1-weighted images and high signal and well-defined on T2-weighted images and demonstrated thin rim enhancement on early post-gadolinium images that persisted on late post-gadolinium images. No appreciable central enhancement of the lesions was observed. At histopathology, the lesions were composed of cystic spaces and fibrous stroma. Lesions showed compressed liver parenchyma surrounding the lesions (three cases) and inflammatory cell infiltrate (one case), which correlated with the rim enhancement on the gadolinium-enhanced MR images. Most of the biliary hamartomas in our small series were less than 1 cm in diameter and of high signal intensity on T2-weighted images, and had a thin rim of enhancement on early and late post-gadolinium images. The imaging features were explainable by the underlying histopathology. In patients with known malignancy, caution should be exercised not to misinterpret these lesions as metastases due to the presence of thin rim enhancement. J. Magn. Reson Imaging 1999;10:196-201, 1999.

MRI of liver metastases: limitation of spleen-liver model in optimizing pulse sequences

The spleen-liver model, as a predictor for contrast-to-noise ratio (C/N) in liver metastases, was verified for seven sequences in 22 patients with 70 colorectal metastases. Optimization of conventional spin-echo, T1-magnetization-prepared gradient-echo and fat frequency-selective presaturation inversion-recovery fast spin echo can be done using the spleen-liver model. C/N of liver-spleen and liver-metastases, however, differed significantly on our T1 gradient-echo and T2-weighted fast spin-echo images, with and without fat-selective saturation.

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.

CT and MR to assess the response of liver tumors to hepatic perfusion

Assessment of the response of liver tumors to hepatic perfusion is strongly based on cross-sectional imaging. CT and MRI have gained considerably in diagnostic accuracy with the introduction of new, fast acquisition techniques such as spiral CT or breath-hold techniques in MRI. In addition, the administration of contrast agents has improved and has led to new injection protocols in spiral CT and the development of liver-specific contrast agents in MRI. Imaging techniques, however, strongly rely on changes in morphology such as size, vascularization and signs of necrosis. Functional signs of tumor metabolism cannot be visualized directly. While most functional imaging techniques at present are based on nuclear medicine, MR spectroscopy (MRS) offers the potential to assess tumor metabolism. Its promise is a direct match between the morphologic information of MRI and the metabolic information provided by MRS. The application of MRS to the liver, however, is still in this infancy. This article will give an overview of the multitude of CT and MR techniques that can be used to monitor tumor response. It will discuss the various signs of tumor regression and some typical complications of hepatic perfusion therapy. In particular, the influence of tumor characteristics-tics on the optimum choice of imaging technique will be demonstrated.

Phased array breath-hold versus non-breath-hold MR imaging of focal liver lesions: a prospective comparative study

This study was undertaken to determine whether phased array breath-hold T1- and T2-weighted sequences can replace non-breath-hold spin echo (SE) sequences in the imaging of focal liver lesions by comparing overall image quality, liver-lesion contrast, and artifact. Both breath-hold and non-breath-hold T1-weighted and T2-weighted imagings of focal liver lesions were prospectively compared in 120 patients with suspected focal liver lesions imaged at 1.5 T with use of a body phased array multicoil. Breath-hold images were acquired with T1-weighted fast low-angle shot (FLASH) and T2-weighted turbo spin echo (TSE) sequences, and non-breath-hold images were made with conventional T1- and T2-weighted SE sequences. Qualitative image analysis was done by three blinded readers, and quantitative analysis was done. The highest signal-to-noise ratios were obtained with breath-hold T1-weighted FLASH sequence. The signal-to-noise ratios of breath-hold T2-weighted TSE sequence were slightly inferior to those of non-breath-hold SE sequence. Both T1-weighted and T2-weighted breath-hold sequences had less image artifact. Overall image quality of breath-hold sequences was better than that of non-breath-hold sequences for both T1- and T2-weighted sequences (P < .01). The tissue contrast of T1-weighted FLASH sequence was superior to that of SE sequence (P < .01). On T2-weighted imaging, tissue contrast of solid lesions was better on conventional SE sequence than that on breath-hold TSE sequence (P < .01). Respiratory ghost artifact was less prominent on T1-weighted FLASH sequence, although this artifact was occasionally seen on breath-hold T2-weighted TSE sequence. In a state-of-art MR unit with use of a phased array multicoil, conventional T1-weighted can be replaced by breath-hold sequences. On T2-weighted imaging, because solid tumor-liver contrast on breath-hold TSE imaging is inferior to that on non-breath-hold SE image, breath-hold imaging may not replace conventional non-breath-hold T2-weighted SE sequence.

Accuracy and significance of pretransplant liver volume measured by magnetic resonance imaging

Measurement of liver volume in patients with advanced liver disease is used to gauge the appropriate size of donor organs and may have prognostic value. We sought to determine the accuracy of magnetic resonance imaging (MRI) in measuring liver volume in 19 adult patients under consideration for liver transplantation. We also correlated the liver volume determination to the clinical severity of disease. Liver volume was measured at MRI by averaging the calculated volumes from coronal and transverse breath-hold T1-weighted images. These results were compared to the explanted liver volume measured by fluid displacement and the explant mass. The correlation coefficient for MRI liver volume and the explant displacement volume was 0.90. The mean liver volume for Child-Pugh class AB by MRI was 1986 +/- 568 mL (1002-2470 mL) compared to 1433 +/- 379 mL (540-1889 mL) in Child-Pugh class C patients (p = .02). We conclude that MRI offers an anatomically accurate means of determining adult liver volume in vivo. Lower mean liver volumes were observed in Child-Pugh class C patients. In addition to its ability to provide tumor screening and vascular assessment, MRI is able to provide accurate determinations of liver volume in patients undergoing liver transplant evaluations.

Comparison of in-phase and opposed-phase GRE and conventional SE MR pulse sequences in T1-weighted imaging of liver lesions

PURPOSE

Our goal was to compare in-phase (IP) and opposed-phase (OP) GRE and conventional SE sequences in T1-weighted (T1-W) imaging of the liver and to evaluate chemical shift GRE imaging in characterizing liver/lesions for fat content.

METHOD

IP and OP T1-W GRE with fast low angle shot (FLASH) technique and T1-W SE sequences were compared in 162 patients at 1.0 T. Chemical shift GRE imaging was used to characterize lesions with fat content. Two hundred sixteen lesions were analyzed in three groups of liver: (a) "normal" liver (n = 74 with 110 lesions); (b) cirrhotic liver (n = 76 with 85 lesions); and (c) fatty liver (n = 12 with 21 lesions). Liver/lesion contrast and liver/lesion contrast-to-noise ratio were assessed for lesion detectability. The percentage of signal intensity variation (SIV) between IP and OP images was used to characterize lesions for fat content.

RESULTS

The OP GRE sequence had significantly higher contrast for normal and cirrhotic livers (p < 0.001), and the IP GRE sequence had significantly higher contrast and contrast-to-noise ratio for fatty liver (p < 0.001). There were no significant differences between OP, IP, and T1-W SE imaging in cirrhotic cases for contrast-to-noise ratio (p < 0.28). Chemical shift imaging detected fat in 21 lesions (9.7%, mean SIV, 191.1%) (sensitivity and specificity 100% when compared with fine needle aspiration cytology).

CONCLUSION

OP GRE sequences could replace conventional SE sequences in T1-W imaging in nonfatty livers, whereas in fatty livers, T1-W SE sequences could be obviated, but both OP and IP sequences are necessary. Chemical shift imaging (OP and IP) can be used to accurately characterize lesions for fat content.

Recent technical advances in MR imaging of the abdomen

MRI of the abdomen has been under development for well over a decade. In the past, considerable work was directed toward identification and suppression of the artifacts caused by motion. However, within the last several years, image quality has further improved, particularly as various fast-scan techniques have been adapted for abdominal imaging. The purpose of this work is to review these technical developments. Specific methods include adaptation of the acquisition time to breath-holding, acquisition over multiple respiratory cycles, adjustment of the contrast of various sequences, and development of more sensitive receiver coils and faster gradient systems. Opportunities for future development are also identified, including improved slice sampling, increased in-plane spatial resolution, real-time means for monitoring respiration, and expanded applications. As these technical advances are implemented, it is expected that the overall sensitivity and specificity of abdominal MRI will further improve.

MRI in characterization of focal liver lesions: comparison of T2 weighting by conventional spin-echo and turbo spin-echo sequences

Forty-one patients with 61 proved focal liver lesions underwent MRI of the liver at 1.0 T, with the aim of evaluating the usefulness of turbo spin-echo (TSE) sequences in characterizing focal liver lesions, by comparing them with conventional spin-echo (CSE) sequences. Two different TSE protocols were employed. with constant echo time and varying repetition time: TSE-S (3000 msec) and TSE-L (5100 msec). All images were evaluated quantitatively (signal-to-noise ration 'SNR') and qualitatively: because benign lesions were all liquid (12 cysts and 10 hemangiomas), they were well characterized morphologically on the basis of signal intensity. Mean SNR was significantly different between metastases and benign lesions (P < .0001) with all T2 sequences. Among the single T2 sequences tested, logistic regression analysis showed TSE-L to have the best predictive ability of the nature of focal lesions, with a G value of 42.02, compared to 29.87 of TSE-S and 25.55 of CSE second echo (SE II). The combination of TSE-L with TSE-S did not modify these results, whereas the combination of TSE-L with CSE only resulted in slight improvement (G = 46.95). Comparison of the receiver operating characteristic (ROC) curves showed only SE II (area under the ROC curve of .8312) to be significantly inferior to the best single sequence, or TSE-L (area under the ROC curve of .9176; P = .027). All sequences were equivalent in qualitative evaluation with good reproducibility, sensitivity ranging from .94 to 1.0 and specificity ranging from .86 to .93. This study confirms the value of TSE sequences in characterization of focal liver lesions. Time of acquisition is strongly reduced with these sequences, whereas results are fairly similar to those obtained with CSE. TSE sequences could therefore replace CSE for the study of focal liver lesions.

Breathhold imaging of the upper abdomen using a circular polarized-array coil: comparison with standard body coil imaging

Current studies emphasize the use of array coils to decrease noise and increase the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). We applied T1-weighted and T2-weighted standard nonbreathhold spin echo (SE) sequences and T1-weighted FLASH, TurboFLASH, T2-weighted spin-echo time (TSE), and heavily T2-weighted half Fourier acquisition single-shot TSE (HASTE) sequences during breathhold for abdominal imaging in 15 normal volunteers. The breathhold scans were performed using both a standard coil and a circular polarized array coil. We analyzed the signal intensity (SI), SNR, and CNR of abdominal organs in all sequences. SNRs increased in all cases by an overall factor of approximately 3 due to an 8% increase in overall SIs and a 50% decrease in noise when applying the array coil. Although the array-coil FLASH sequence performed at least as well as the respective SE sequence, the SNRs of the array-coil TurboFLASH, TSE breathhold, and HASTE sequences were generally lower. We conclude that array-coil imaging significantly improves fast imaging of the abdomen.

Detection and characterization of primary liver cancer in rats by MS-264-enhanced MRI

A new MR contrast agent, MS-264 (Gd(1RS)-1-(p-butylbenzyl)-DTPA), was developed to achieve hepatobiliary specificity and its potential evaluated for detecting and characterizing liver tumors in rats with chemically induced hepatocellular carcinoma (HCC). In seven rats with 66 HCC lesions, enhancements of different abdominal organs and tumors were compared on T1-weighted images after intravenous administration of Gd-DTPA (0.3 mmol/kg) and MS-264 (0.05 mmol/kg). MR images were correlated with postmortem microangiographic and histological findings. An overall enhancement of different organs, which normalized within 24 h, was observed after Gd-DTPA and MS-264 injection. MS-264 caused a higher relative enhancement (RE) in liver (60%), compared with that of Gd-DTPA (40%), which resulted in a prompt negative contrast enhancement in 59 of 66 HCCs. All were moderately to poorly differentiated (Grades II-IV) tumors. Six of these 59 negative contrast-enhancing lesions showed a positively enhanced peritumoral rim, which corresponded histologically to malignant infiltration (n = 2) or compression (n = 4). On the other hand, six well differentiated HCCs showed prolonged positive enhancement. However, one well differentiated HCC was not positively enhanced by MS-264, probably due to poor access of the agent to the lesion. In comparison to that of the precontrast images, enhancement with Gd-DTPA and MS-264 increased the number of detected lesions by 22 and 42%, respectively. In this animal study, MS-264 proved to be useful in detection and characterization of primary liver cancers.

Spoiling of transverse magnetization in gradient-echo (GRE) imaging during the approach to steady state

The signal evolution behaviors and corresponding image appearances for different methods of spoiling or refocusing the transverse magnetization in short TR gradient-echo imaging during the approach to steady state were investigated experimentally and using computer simulations based on the Bloch equations. Specifically, ideally spoiled, gradient-spoiled, gradient-refocused, and RF-spoiled pulse sequence configurations were studied. This study showed that, for the gradient-spoiled configuration, the signal evolution is position and phase-encoding order-dependent and, under typical imaging conditions, can deviate substantially from the ideally spoiled signal evolution at some spatial positions, resulting in intensity banding image artifacts. For the gradient-refocused configuration, the signal evolution oscillates toward the steady state and, generally, does not closely approximate that of ideal spoiling, resulting in different image contrast or image blurring. Using RF spoiling, the signal evolution closely approximates the ideally spoiled case for flip angles less than approximately 20 degrees and T2 values of less than approximately 200 ms and results in relatively artifact-free images. Also, this study showed that, for RF spoiling, an RF-pulse phase-difference increment other than 117 degrees, such as 84 degrees may be optimal for gradient-echo imaging during the approach to steady state.

Cross-sectional imaging of the liver

The liver can be affected by a number of conditions that can involve the liver focally or diffusely. CT and MRI are reliable techniques for demonstrating these abnormalities, and both imaging tools play an important role in the clinical evaluation of the patients. However, each technique provides different information, and the most appropriate cross-sectional technique used should be tailored in each instance to the individual patient situation.

Comparison of breath-hold T1-weighted MR sequences for imaging of the liver

Three rapid T1-weighted gradient-echo techniques for imaging of the liver were compared: fast low-angle shot (FLASH) and section-selective (SSTF) and non-section-selective (NSTF) inversion-recovery TurboFLASH. Ten healthy volunteers were imaged at 1.5 T, with breath-hold images acquired in the transaxial and coronal planes and non-breath-hold images in the transaxial plane. Breath-hold images were evaluated quantitatively and qualitatively, and non-breath-hold images were evaluated qualitatively. FLASH images had significantly higher (P < .001) spleen-liver signal difference-to-noise ratios (SD/Ns) than NSTF and SSTF images. Liver signal-to-noise ratios (S/Ns) were significantly higher (P < .001) on FLASH images than on NSTF and SSTF images. With breath hold, FLASH images were rated as having the highest quality in the axial plane, followed by NSTF and SSTF images. In the coronal plane, NSTF images were rated as having the highest quality. For images acquired during patient respiration, NSTF images had the highest quality and showed the least degradation. The results suggest that FLASH images have the highest SD/N and S/N for liver imaging and have the highest quality in the axial plane. In patients who cannot suspend respiration. NSTF images may be least affected by breathing artifact and provide reasonable image quality.