Magnetic resonance imaging: Review of imaging techniques and overview of liver imaging

Magnetic resonance imaging (MRI) of the liver is slowly transitioning from a problem solving imaging modality to a first line imaging modality for many diseases of the liver. The well established advantages of MRI over other cross sectional imaging modalities may be the basis for this transition. Technological advancements in MRI that focus on producing high quality images and fast imaging, increasing diagnostic accuracy and developing newer function-specific contrast agents are essential in ensuring that MRI succeeds as a first line imaging modality. Newer imaging techniques, such as parallel imaging, are widely utilized to shorten scanning time. Diffusion weighted echo planar imaging, an adaptation from neuroimaging, is fast becoming a routine part of the MRI liver protocol to improve lesion detection and characterization of focal liver lesions. Contrast enhanced dynamic T1 weighted imaging is crucial in complete evaluation of diseases and the merit of this dynamic imaging relies heavily on the appropriate timing of the contrast injection. Newer techniques that include fluoro-triggered contrast enhanced MRI, an adaptation from 3D MRA imaging, are utilized to achieve good bolus timing that will allow for optimum scanning. For accurate interpretation of liver diseases, good understanding of the newer imaging techniques and familiarity with typical imaging features of liver diseases are essential. In this review, MR sequences for a time efficient liver MRI protocol utilizing newer imaging techniques are discussed and an overview of imaging features of selected common focal and diffuse liver diseases are presented.     

Advanced magnetic resonance imaging findings in salivary gland tumors

Salivary gland tumors (SGTs) make up a small portion (approximately 5%) of all head and neck tumors. Most of them are located in the parotid glands, while they are less frequently located in the submandibular glands, minor salivary glands or sublingual gland. The incidence of malignant or benign tumors (BTs) in the salivary glands varies according to the salivary gland from which they originate. While most of those detected in the parotid gland tend to be benign, the incidence of malignancy increases in other glands. The use of magnetic resonance imaging (MRI) in the diagnosis of SGTs is increasing every day. While conventional sequences provide sufficient data on the presence, localization, extent and number of the tumor, they are insufficient for tumor specification. With the widespread use of advanced techniques such as diffusion-weighted imaging, semi-quantitative and quantitative perfusion MRI, studies and data have been published on the differentiation of malignant or BTs and the specificity of their subtypes. With diffusion MRI, differentiation can be made by utilizing the cellularity and microstructural properties of tumors. For example, SGTs such as high cellular Warthin’s tumor (WT) or lymphoma on diffusion MRI have been reported to have significantly lower apparent diffusion values than other tumors. Contrast agent uptake and wash-out levels of tumors can be detected with semi-quantitative perfusion MRI. For example, it is reported that almost all of the pleomorphic adenomas show an increasing enhancement time intensity curve and do not wash-out. On quantitative perfusion MRI studies using perfusion parameters such as Ktrans, Kep, and Ve, it is reported that WTs can show higher Kep and lower Ve values than other tumors. In this study, the contribution of advanced MRI to the diagnosis and differential diagnosis of SGTs will be reviewed.     

肝脏局灶性病变MR动态增强扫描及临床意义

目的探讨肝脏局灶性病变ＭＲ动态增强扫描方法及临床意义。方法作者前瞻性研究了１３６例肝脏局灶性病变，包括肝细胞性肝癌、周围型肝内胆管细胞性肝癌、转移瘤及海绵状血管瘤。采用平静呼吸状态下梯度回波Ｋ空间中心部分采集技术、７个连续层面８个时相动态增强扫描。结果各例均动态增强扫描成功。动态扫描显示时间信号强度曲线在肝细胞性肝癌呈速升速降型；胆管细胞癌呈渐升型；转移瘤呈环形强化，缓慢升高型；海绵状血管瘤呈速升平台型。结论平静呼吸下Ｋ空间中心部分采集肝脏动态扫描，可以显示肝脏局灶病变的血供状态，而且不同病变具有不同的强化特征。     

Perfusion maps of the whole liver based on high temporal and spatial resolution contrast-enhanced MRI (4D THRIVE): Feasibility and initial results in focal liver lesions

Purpose

To prospectively evaluate a new imaging sequence (4D THRIVE) for whole liver perfusion in high temporal and spatial resolution. Feasibility of parametric mapping and its potential for characterizing focal liver lesions (FLLs) are investigated.

Materials and methods

Fifteen patients suspected for colorectal liver metastases (LMs) were included. Parametric maps were evaluated qualitatively (ring-enhancement and lesion heterogeneity) and compared to three-phased contrast-enhanced MRI. Quantitative analysis was based on average perfusion values of entire FLLs. Reference standard comprised surgery with histopathology or follow-up imaging. Fisher's exact test was used for qualitative and Kruskal-Wallis test for quantitative analysis.

Results

In total 29 LMs, 17 hemangiomas and 4 focal nodular hyperplasias were evaluated. FLLs could be differentiated by qualitative assessment of parametric maps respectively three-phased contrast-enhanced MRI (Fisher's p < 0.001 for comparisons between LMs and hemangiomas and LMs and FNHs for both ring-enhancement and lesion heterogeneity) rather than by quantitative analysis of parametric maps (Chi-square for Kep = 0.33 (p = 0.847) and Chi-square for Kel = 1.35 (p = 0.509)).

Conclusion

This preliminary study shows potential of 4D THRIVE for whole liver imaging enabling calculation of parametric maps. Qualitative rather than quantitative analysis was accurate for differentiating malignant and benign FLLs.     

肝脏局灶性病变MRI表现与病理学的相关性研究

肝脏局灶性病变的MRI信号强度受许多病理因素的影响。病变的组织学特征:细胞构成、血液供应、间质构成和肿瘤内的坏死或者出血等,都会使病变的T1及T2弛豫时间发生显著变化。细胞内的某些内容物:糖原、脂肪、黑色素、铁和铜,在决定MRI信号表现方面起着重要的作用。MRI与病理学之间联系的研究对于诊断局灶性病变非常有帮助。     

Detection of liver metastases: Gadoxetic acid-enhanced three-dimensional MR imaging versus ferucarbotran-enhanced MR imaging

Purpose

To compare the diagnostic performance of gadoxetic acid-enhanced MRI with ferucarbotran-enhanced MRI for the detection of liver metastases.

Materials and methods

Thirty-six patients with 80 liver metastases who underwent gadoxetic acid-enhanced MRI using a three-dimensional volumetric interpolated technique and ferucarbotran-enhanced MRI with a mean interval of 7 days (range, 5-10 days) were included in this study. Two observers independently interpreted the two sets of images - the gadoxetic acid set (unenhanced, early dynamic and 20 min delayed phase images) and the ferucarbotran set (unenhanced and ferucarbotran-enhanced T2*-weighted-gradient echo and T2-weighted turbo spin echo images). Diagnostic accuracy was evaluated using the alternative-free response receiver operator characteristic (ROC) method. Sensitivity and positive predictive value were also evaluated.

Results

There was a trend toward increased areas under the ROC curve (Az values) for the gadoxetic acid set (0.950, 0.948) as compared with the ferucarbotran set (0.941 and 0.939) of images, but no significant difference was found for both observers (p < 0.05). Sensitivities of the gadoxetic acid set (93.8% and 92.5%) were also slightly better than those of the ferucarbotran set (88.8% and 87.5%) with no significant difference (p = 0.13). The two image sets showed similar positive predictive values (98.7% and 98.6%, respectively).

Conclusions

Gadoxetic acid-enhanced MRI showed comparable diagnostic performance to ferucarbotran-enhanced MRI for the detection of liver metastases.     

Contrast agents for MR imaging of the liver: a clinical overview

Different contrast agents have been clinically used in MR imaging of the liver including extracellular gadolinium chelates, contrast agents targeted to the macrophage-monocytic phagocytic system (MMPS), hepatobiliary contrast agents, and blood-pool contrast agents. Extracellular gadolinium chelates are optimally used for characterization of focal hepatic lesions, whereas hepatobiliary and MMPS-targeted contrast agents are optimally used for detection and preoperative evaluation. The present review portrays these contrast agents and discusses their advantages and shortcomings. Received 14 June 1996; Revision received 14 August 1996; Accepted 16 August 1996     

Advanced magnetic resonance imaging techniques in the evaluation of musculoskeletal tumors

Magnetic resonance (MR) imaging is an important modality for the preoperative and posttreatment evaluation of musculoskeletal tumors. In some cases, MR imaging is unable to offer information about the extension of tumoral necrosis and the presence of viable cells. Advanced MR imaging techniques are now used in association with conventional MR imaging to improve the diagnostic accuracy and the evaluation of treatment response. This article discusses each advanced MR imaging technique with regard to the clinical applications of tumor detection and characterization, differentiation of benign from malignant tumors and tumor tissue from nontumor tissue, and assessment of treatment response.     

Respiratory gated diffusion-weighted imaging of the liver: value of apparent diffusion coefficient measurements in the differentiation between most commonly encountered benign and malignant focal liver lesions

The purpose of this study was to measure apparent diffusion coefficient values of normal liver parenchyma and focal liver lesions utilizing a respiratory gated diffusion sequence with multiple b-values and to investigate whether apparent diffusion coefficient (ADC) measurements may be utilized to characterize and differentiate between malignant and benign focal hepatic lesions. Thirty-eight consecutive patients underwent MRI of the liver including diffusion-weighted imaging (DWI). A single-shot echo planar imaging sequence was applied in coronal orientation with multiple b-values (0, 50, 500, 1,000 s/mm²) and respiratory gating. ADC values were recorded on corresponding maps utilizing region of interest measurements in patients with benign (group A), malignant (group B) focal lesions and liver parenchyma (group C). Statistical analysis was applied to check whether differences in mean ADC values were significant (p<0.05). No focal lesions were detected in 11 patients, with a mean ADC value (CI 95%) of liver parenchyma 1.25×10⁻³ mm²/s (1.21×10⁻³ mm²/s−1.29×10⁻³ mm²/s). Differences in mean ADC of liver parenchyma between group A and B were not significant (p=0.054, 1.30×10⁻³ mm²/s and 1.31×10⁻³ mm²/s, respectively). Mean ADC value (95% CI) of 22 benign lesions found in 18 patients was 2.55×10⁻³ mm²/s (2.35×10⁻³ mm²/s−2.74×10⁻³ mm²/s), while the mean ADC value (95% CI) of 16 malignant lesions recorded in 9 patients was 1.04×10⁻³ mm²/s (0.9×10⁻³ mm²/s−1.17×10⁻³ mm²/s). The difference between mean ADC values of benign and malignant focal lesions was statistically significant (p<0.0001). Respiratory gated diffusion-weighted imaging in the liver is technically feasible. Apparent diffusion coefficient measurements can be useful in differentiating malignant from benign focal liver lesions.     

肝脏血管平滑肌脂肪瘤MRI表现

目的：总结分析肝脏血管平滑肌脂肪瘤（AML）的MRI表现,比较上皮样与非上皮样肝脏AML的MRI表现差异。方法：搜集经手术病理证实的肝脏AML患者15例,1l例行MRI检查,其中上皮样AML5例,非上皮样AML6例,1例非上皮样AML仅行MRI平扫,其余10例均行MRI平扫及动态增强扫描。结果：化学位移成像栓出7例肿瘤的脂肪成分,而频率饱和法仅检出3例肿瘤的脂肪成像。无脂肪型中上皮样与非上皮样AML各2例;少脂肪型中上皮样AML1例,非上皮样AML3例;富脂肪型中上皮样AML2例,非上皮样AML1例;上皮样与非上皮样AML脂肪含量相仿。10例行增强扫描的病例中动脉期强化9例,门脉期强化6例,延迟期强化5例,延迟期环形包膜样强化9例,动态增强曲线8例呈动脉期速升,门脉期及延迟期强化迅速减低。结论：上皮样与非上皮样AML影像学表现差异较小,MRI检出脂肪含量多寡难以作为肿瘤是否为上皮样AML的诊断标准。在MRI动态增强扫描中,上皮样AML与富血管平滑肌的经典型AML强化形式相似。     

Characterization of focal liver lesions by ADC measurements using a respiratory triggered diffusion-weighted single-shot echo-planar MR imaging technique

The aim of this study was to determine apparent diffusion coefficients (ADCs) of focal liver lesions on the basis of a respiratory triggered diffusion-weighted single-shot echo-planar MR imaging sequence (DW-SS-EPI) and to evaluate whether ADC measurements can be used to characterize lesions. One hundred and two patients with focal liver lesions [11 hepatocellular carcinomas (HCC), 82 metastases, 4 focal nodular hyperplasias (FNH), 56 hemangiomas and 51 cysts; mean size, 16.6 mm; range 5–92 mm] were examined on a 1.5-T system using respiratory triggered DW-SS-EPI (b-values: 50, 300, 600 s/mm²). Results were correlated with histopathologic data and follow-up imaging. The ADCs of different lesion types were compared, and lesion discrimination using optimal thresholds for ADCs was evaluated. Mean ADCs (×10⁻³mm²/s) were 1.24 and 1.04 for normal and cirrhotic liver parenchyma and 1.05, 1.22, 1.40, 1.92 and 3.02 for HCCs, metastases, FNHs, hemangiomas and cysts, respectively. Mean ADCs differed significantly for all lesion types except for comparison of metastases with HCCs and FNHs. Overall, 88% of lesions were correctly classified as benign or malignant using a threshold value of 1.63 × 10⁻³mm²/s. Measurements of the ADCs of focal liver lesions on the basis of a respiratory triggered DW-SS-EPI sequence may constitute a useful supplementary method for lesion characterization.     

Detection of liver metastases: gadobenate dimeglumine-enhanced three-dimensional dynamic phases and one-hour delayed phase MR imaging versus superparamagnetic iron oxide-enhanced MR imaging

The aim of this study was to compare the diagnostic performance of gadobenate dimeglumine (Gd-BOPTA)-enhanced MR imaging, including dynamic phases and one-hour delayed phase, versus superparamagnetic iron oxide (SPIO)-enhanced imaging for detection of liver metastases. Twenty-three patients with 59 liver metastases underwent Gd-BOPTA-enhanced MR imaging (unenhanced, arterial, portal, equilibrium and one-hour delayed phase) using three-dimensional volumetric interpolated imaging and SPIO-enhanced T2-weighted turbo spin–echo and T2*-weighted gradient-echo sequences on a 1.5-T unit. Three observers independently interpreted the three sets of images, i.e. Gd-BOPTA-enhanced dynamic MRI (set 1), delayed phase imaging (set 2) and SPIO-enhanced MRI (set 3). Diagnostic accuracy was evaluated using the alternative-free response receiver operating chracteristic (ROC) analysis. Sensitivity and positive predictive value were also evaluated. The mean accuracy (Az values) and sensitivity of Gd-BOPTA-enhanced delayed phase imaging (0.982, 95.5%) were comparable to those of SPIO-enhanced imaging (0.984, 97.2%). In addition, Az values and sensitivities of both imaging sets were significantly higher than those of Gd-BOPTA-enhanced dynamic images (0.826, 77.4%: p<0.05). There was no significant difference in the positive predictive value among the three image sets. Gd-BOPTA-enhanced delayed phase imaging showed comparable diagnostic performance to SPIO-enhanced imaging for the detection of liver metastases, and had a better diagnostic performance than Gd-BOPTA-enhanced dynamic images.     

常规SE序列和动态增强MRI诊断肝局灶性病变的比较

目的比较常规SE序列与动态Gd-U．----x增强扫描对肝局灶病变的诊断价值。方法对34例肝局灶病变做了常规SE平扫和动态Gd-U．---A增强及延迟万WI增强扫描；就各序列对肝局灶病变的检出率、病变的信噪比（C／N）值和图像质展进行定员和定性分析。结果36例共142个病灶，动态Gd-lyl？l？A检出率（138／142，958％）明st高于IFZWI和延迟TW［增强（128／14，叨％；119／142，838％）（P＜005）：动态Gd-IJ：1713A增强的C／N值高于TZWI和TIWI延迟增强（P＜0．05）；动态增强的伪影较L们少（P＜0．01），而病灶清晰度各序列之间无明显差别（P＞0．历）。结论动态u-ly：1717A增强扫描在病灶检出率、图像质显反C／N值方面均优于IWI，对肝局灶病变的诊断是一种有价值的方法，     

Magnetic resonance imaging of atherosclerosis

Abundant data now link composition of the vascular wall, rather than the degree of luminal narrowing, with the risk for acute ischemic syndromes in the coronary, central nervous system, and peripheral arterial beds. Over the past few years, magnetic resonance angiography has evolved as a well-established method to determine the location and severity of advanced, lumen-encroaching atherosclerotic lesions. In addition, more recent studies have shown that high spatial resolution, multisequence MRI is also a promising tool for noninvasive, serial imaging of the aortic and carotid vessel wall, which potentially can be applied in the clinical setting. Because of the limited spatial resolution of current MRI techniques, characterization of coronary vessel wall atherosclerosis, however, is not yet possible and remains the holy grail of plaque imaging. Recent technical developments in MRI technology such as dedicated surface coils, the introduction of 3.0-T high-field systems and parallel imaging, as well as developments in the field of molecular imaging such as contrast agents targeted to specific plaque constituents, are likely to lead to the necessary improvements in signal to noise ratio, imaging speed, and specificity. These improvements will ultimately lead to more widespread application of this technology in clinical practice. In the present review, the current status and future role of MRI for plaque detection and characterization are summarized.     

磁共振弥散加权成像在食管癌放疗疗效评价中的应用价值探讨

目的 探讨磁共振弥散加权成像（DWI）在食管癌放疗疗效评价中的应用价值。方法 74例食管鳞癌患者接受三维适形放疗或调强放疗, 处方剂量56~61.2 Gy,单次1.8~2 Gy。放疗前、放疗末及放疗后1~3个月行DWI检查,用食管钡餐造影、DWI高信号消失情况、表面弥散系数（ADC）对放疗疗效进行评价,并与预后相结合进行生存分析。结果 中位随访时间22个月,随访率98.7%。放疗末DWI高信号消失组23例,1、2年生存率分别为82.6%、68.5%;放疗后1~3个月消失组40例,1、2年生存率分别为74.5%、49.2%;始终未消失组11例,1年生存率9.1%,生存期最长17个月（χ²=39.78,P<0.05）。造影评价完全缓解47例中,20例放疗末DWI高信号消失;23例放疗后1~3个月消失,其1、2年生存率分别为80%、64.3%和86.7%、66.1%;高信号始终未消失4例,生存期4~7个月（χ²=37.80,P<0.05）。造影评价为部分缓解的26例中,3例放疗末高信号消失;17例放疗后1~3个月高信号消失,1、2年生存率为58.8%、19.3%、高信号始终未消失6例,生存期4~17个月（χ²=11.72,P<0.05）。患者正常食管组织的ADC₀、食管肿瘤放疗前的ADC₁及放疗末的ADC₂值分别为（3.12±0.53）、（1.64±0.48）、（2.65±0.58）s/mm²,疗末ADC₂/ADC₀≤90%组（40例）与>90%组（34例）1、2年生存率分别为56.4%、28.2%和83.1%、75.7%（χ²=10.35,P<0.05）。放疗末肿瘤区域ADC值为独立预后影响因素（OR=0.060,P<0.05）。结论 利用磁共振弥散加权成像检查对食管癌放疗疗效进行评价可以较好地提示预后,弥补食管钡餐造影在疗效评价中的不足,疗后肿瘤区域ADC值大小为独立预后影响因素。     

消化道磁共振检查技术与诊断

随着MR技术的快速发展,胃肠道MR检查成为现实。本文将讨论最佳MR胃肠成像所需诊断技术要求及与已有的X线检查方法相比MR所处的位置。当前这一新的检查方法在食道、小肠、结肠中都有报道。这些发展显示胃肠道MR检查可能将由科学研究向常规的临床应用转变。     

Benign focal liver lesions: spectrum of findings on SonoVue-enhanced pulse-inversion ultrasonography

The prevalence of benign focal liver lesions (BFLL) is high both in the general population and in patients with known malignancies. The gray-scale ultrasound (US) technique is usually the first-line imaging modality used in the radiological workup of such lesions, but unfortunately it lacks specificity. Furthermore, Doppler examination may often be unsatisfactory owing to motion artefacts, or when small or deeply located lesions are evaluated. Recently, microbubble-based contrast agents used in combination with gray-scale US techniques, which are very sensitive to nonlinear behavior of microbubbles, have led to a better depiction of both microvasculature and macrovasculature of focal hepatic masses, thus improving the reliability of using US in the assessment of liver tumors. This review illustrates the spectrum of enhancement patterns of BFLL on contrast-enhanced ultrasonography with SonoVue, a second-generation microbubble-based contrast agent.This paper is based on a work accepted for presentation as a scientific paper at the Scientific Assembly and Annual Meeting of the ECR 2004.     

The role of dynamic contrast-enhanced and diffusion weighted magnetic resonance imaging in the female pelvis

Functional imaging by means of dynamic multiphase contrast-enhanced magnetic resonance imaging (DCE-MRI) and diffusion weighted magnetic resonance imaging (DW-MRI) is now part of the standard imaging protocols for evaluation of the female pelvis. DCE-MRI and DW-MRI are important MR imaging techniques which enable the radiologist to move from morphological to functional assessment of diseases of the female pelvis. This is mainly due to the limitations of morphologic imaging, particularly in lesion characterization, accurate lymph node staging, assessment of tumour response and inability to differentiate post-treatment changes from tumour recurrence.DCE-MRI improves the accuracy of T2WI in staging of endometrial cancer. It also helps differentiate tumour recurrence from radiation fibrosis in patients with cervical cancer. DCE-MRI improves characterization of cystic adnexal lesions and detection of small peritoneal implants in patients with ovarian cancer.DW-MRI is valuable in preoperative staging of patients with endometrial and cervical cancer, especially in detection of extra-uterine disease. It does increase reader's confidence for detection of recurrent disease in gynaecological malignancies and improves detection of small peritoneal implants in patients with ovarian cancer.In this review article we give an overview of both DCE-MRI and DW-MRI techniques, concentrating on their main clinical application in the female pelvis, and present a practical approach of the added value of these techniques according to the main pathological conditions, highlighting the pearls and pitfalls of each technique.     

磁共振动态增强、扩散加权成像联合应用对乳腺病变的诊断价值

目的：探讨单一磁共振动态增强扫描诊断试验、扩散加权成像诊断试验及其联合应用对乳腺病变定性诊断的敏感性、特异性和阳性似然比、阴性似然比,比较其诊断效能。方法：对临床拟诊肿块的患者37例,同时进行动态增强扫描和扩散加权成像检查,均获得手术和病理证实,其中良性病灶18个,恶性病灶19个。对病变的边缘、形态特征、动态增强表现及时间一信号强度曲线采用评分法对病变性质分恶性、可疑恶性及良性三组进行判断。参照动态增强病变位置确定扩散图像病变所在,描记扩散图像上病变的感兴趣区,由软件计算获得表观扩散系数（ADC）值。对获取数据进行统计分析,采用t检验统计学方法进行良性和恶性ADC值比较。联合动态增强扫描和ADC值,采用评分法根据积分情况进行综合定性诊断。比较动态增强扫描、DWI ADC值及联合应用对乳腺病变定性诊断效能。结果：动态增强扫描（病灶边缘、形态学表现结合时间-信号强度曲线）诊断乳腺病变的敏感性、特异性和阳性似然比、阴性似然比分别为89．5％、72．2％和3．221、0．146。良性病变组ADC值1．474±0．441（×100^-3mm^2／s）,恶性病变组ADC值1．082±0．160（×10^-3mm^2／s）,两者间有显著统计学差异（P=0．002,〈0．05）。ADC值诊断敏感性、特异性和阳性似然比、阴性似然比分别为94．7％、66．7％和2．842、0．079。动态增强扫描和DWI-ADC值联合诊断的敏感性、特异性和阳性似然比、阴性似然比分别为94．7％、83．3％和5．684、0．063。结论：磁共振动态增强、扩散加权成像联合应用对乳腺病变的定性诊断敏感性、特异性、阳性似然比、阴性似然比均较单一动态增强扫描或扩散成像诊断效能强。