Intravenous, contrast-enhanced MR colonography using air as endoluminal contrast agent: impact on colorectal polyp detection

Purpose

To compare diagnostic accuracy and patient tolerance of MR colonography with intravenous contrast and luminal air (MRC) to conventional colonoscopy (CC).

Materials and methods

IRB approval and written informed consent were obtained. Forty-six patients, both screening and symptomatic, underwent MRC followed by CC. The MRC technique employed 3D T1W spoiled gradient echo sequences performed after the administration of gadopenetate dimeglumine, with parallel imaging. The diagnostic accuracy and tolerance of patients for MRC was compared to CC.

Results

Twenty-four polyps were detected in eighteen patients with CC (5 polyps ≥10 mm, 4 polyps 6–9 mm, 15 polyps ≤5 mm). MRC was 66.7% (12/18) sensitive and 96.4% (27/28) specific for polyp detection on a per-patient basis. When analyzed by polyp size, sensitivity and specificity of MRC was 100% (5/5) and 100% (19/19), respectively, for lesions greater than 10 mm, 100% (4/4) and 100% (20/20) for lesions 6–9 mm, and sensitivity of 20% (3/15) lesions less than 5 mm. The sensitivity and specificity of MRC for detecting significant lesions (>6 mm) was 100% (9/9) and 100% (15/15), respectively. Regarding tolerance of the exams, there were no significant differences between MRC and CC. Thirty-five percent (n = 16) of patients preferred MRC as a future screening test compared to 33% (n = 15) for CC.

Conclusion

MRC using air as an intraluminal contrast agent is a feasible and well-tolerated technique for detecting colonic polyps ≥6 mm in size. Further studies are warranted.     

Parallel imaging enhanced MR colonography using a phantom model

PURPOSE: To compare various Array Spatial and Sensitivity Encoding Technique (ASSET)-enhanced T2W SSFSE (single shot fast spin echo) and T1-weighted (T1W) 3D SPGR (spoiled gradient recalled echo) sequences for polyp detection and image quality at MR colonography (MRC) in a phantom model. Limitations of MRC using standard 3D SPGR T1W imaging include the long breath-hold required to cover the entire colon within one acquisition and the relatively low spatial resolution due to the long acquisition time. Parallel imaging using ASSET-enhanced T2W SSFSE and 3D T1W SPGR imaging results in much shorter imaging times, which allows for increased spatial resolution. MATERIALS AND METHODS: Using two porcine colon phantoms each with eight simulated 3-10-mm "polyps," baseline reference sequences acquired without ASSET (6-mm slices and readout bandwidth [BW] 62 kHz) were compared with 11 SSFSE and 8 SPGR sequences acquired with 2-fold ASSET acceleration. ASSET-enhanced SSFSE and SPGR sequences comprised BW/matrix combinations ranging from 20-62 kHz/256-352x256, respectively, with slice thicknesses adjusted from 3.0 to 4.5 mm to maintain a 23-26-second acquisition time and 30 cm slab thickness. Two experienced radiologists viewed the datasets in a randomized, blinded fashion. RESULTS: Compared to reference sequences, ASSET-enhanced SSFSE and SPGR sequences facilitated better polyp detection and had similar overall image quality and per-phantom specificity. The two best ASSET-enhanced SSFSE (3 and 4.5 mm slices, each with BW of 62.5 kHz and 352x256 matrices) and three best ASSET-enhanced SPGR BW/slice thickness/matrix combinations of 31 kHz/4.4 msec/192x256; 62/3.4/192x256; and 62/4.0/192x256, respectively, permitted detection of all polyps>or=5 mm. CONCLUSION: Parallel imaging using ASSET-enhanced T2W SSFSE and T1W 3D SPGR improves the ability to detect significant colon polyps in an MRC phantom model.     

Clinical validation of high-resolution fast spin-echo MR colonography after colon distention with air

To evaluate the feasibility of MR colonography (MRC) with air using two-dimensional (2D) T1-weighted fast spin-echo (T1wFSE) in patients scheduled for conventional colonoscopy (CC) after classic bowel preparation, and assess the ability of the technique to detect colonic lesions. The distention was sufficient for diagnosis, and the technique provided adequate delineation of the wall in the majority of segments. Residual fluid obscured the wall in different segments, especially in the ascending and descending colon (supine position) and in the cecum, transverse, and sigmoid colon (prone position). These findings were consistent with CT colonography. MRC visualized three lesions, missed one lesion >10 mm, visualized none of four lesions <5 mm, and yielded one false-positive lesion (5-10 mm). Missed lesions can be due to inconsistency in the slice positions between consecutive breath-holds, which is inherent to the multishot technique. Residual fluid may have obscured the smaller lesions. The shortcomings of the technique are limited coverage and signal drop-off at the borders of the field of view (FOV). Before multishot 2D T1wFSE colonography can become a valid screening method, improved patient preparation and a more practical technique are needed.     

MRI动态增强扫描在乳腺良恶性病变中的诊断价值

目的:评价MRI动态增强扫描在乳腺良恶性病变中的诊断价值。方法:回顾性分析58例经病理证实的乳腺病变的MRI动态增强扫描强化形态及时间-信号强度曲线图表现。结果:良性病变共22例(纤维腺瘤8例,导管内乳头状瘤5例,乳腺囊肿5例,导管囊状扩张伴慢性炎症1例,囊性乳腺病3例);恶性病变共36例(浸润性导管癌28例,浸润性小叶癌3例,导管原位癌2例,浸润性乳头状癌2例,髓样癌1例)。恶性病变多表现为边缘毛刺、分叶征或边缘模糊,环状强化或不规则强化。良性病变多表现为边缘光滑整齐的均匀强化,其中5例囊肿无强化。其中形态学阳性预测值为89.47%,阴性预测值为90.00%,准确率为89.66%。强化表现阳性预测值为89.74%,阴性预测值为94.74%,准确率为91.38%。时间-信号强度曲线图:22例良性病变,13例(59.09%)表现为Ⅰ型,8例(36.36%)表现为Ⅱ型,1例(4.55%)表现为Ⅲ型;36例恶性病变,1例(2.78%)表现为Ⅰ型,9例(25.00%)表现为Ⅱ型,26例(72.22%)表现为Ⅲ型,阳性预测值为79.55%,阴性预测值为92.86%,准确率为82.76%。结论:乳腺MRI动态增强的形态学表现与时间-信号强度曲线图相结合,在良恶性病变的诊断与鉴别诊断中具有较高的价值。     

Microbubbles as novel pressure-sensitive MR contrast agents

Magnetic resonance imaging contrast agents that are sensitive to pressure would be useful for evaluating cardiovascular function. One such potential contrast agent conslsts of gas-filtod liposome microbubbles. The magnetic susceptibility of the microbubbles locally perturb the statte magnetic field, which influences the transverse-relaxation properties of the surrounding medium. Changes in the pressure atter the bubble dimensions, which affects the magnetic field perturbations and, hence, the transverserelaxation. The effect of these microbubbles on the T₂ relaxation times of a water-based medium was measured for liposomes filled with different gases—nitrogen, argon, air, oxygen, xenon, neon, perflu-oropentane, perfluorobutane, and sulfur hexafluoride. The air-filled, perfluoropentane-fllled and the oxygen-filted liposomes demonstrated the largest effect on transverse-relaxation. The influence of pressure on both gradient-echo and spin-echo signal intenstties for air-filled microbubbles was also evaluated. Pressure-induced changes in signal intensity were consistently observed for both the spin-echo and gradient-echo pulses sequences.     

静脉注射钆类对比剂对乳腺正常腺体及肿瘤组织MR扩散加权成像的影响

目的 探讨乳腺MR DWI质量及正常腺体与癌组织的ADC值是否受到钆类对比剂增强扫描的影响.方法 对25例已经针吸穿刺或粗针活检病理证实为乳腺癌的患者在静脉注射Gd-DTPA前、后行DWI扣描,将R01分别置于癌灶及正常腺体,应用配对t检验及非参数检验比较b值=0及1000 s/mm2时增强前后图像中癌灶和正常腺体的信噪比(SNR)、对比噪声比(CNR)及ADC值的变化.结果 增强前、后癌灶的SNR(34.56±11.34和33.60±14.34)与正常腺体的SNR(9.88±3.16和10.42±4.18)以及图像CNR(24.16±9.05和22.26±10.05)的差异无统计学意义(P癌灶SNR=0.70,P正常腺体SNR=0.11;PCNR=0.17);增强前后癌灶的ADC值分别为(0.96±0.13)×10-3和(0.95±0.14)×10-3mm2/s,正常腺体分别为(1.90±0.47)×10-3 和(1.91±0.61)×10-3mm2/s,两者在增强后ADC值与其增强前差异均无统计学意义(P=0.20和0.97).结论 乳腺痛组织和正常腺体DWI的质量及ADC测量值不会受到Gd-DTPA的影响,故在乳腺MR扫描中,DWI可以在增强扫描后进行.     

Optimized visualization of vessels in contrast enhanced intracranial MR angiography

In this study, the problem of small vessel visualization in magnetic resonance angiography is addressed. The loss of vessel contrast due to slow flow-related signal saturation can be compensated by the T₁ reduction obtained from the use of an MR contrast agent, such as Gd-DTPA. The vesselfbackground signal-difference-to-noise ratio (SDNR) is shown to strongly depend on the imaging parameters, as well as on the time course of the blood T₁ values obtained from the contrast injection. Specifically, it was found that vessel SDNR increases almost linearly with TR, if the sampling bandwidth is reduced proportionately.     

Assessment of myocardial infarction in pigs using a rapid clearance blood pool contrast medium.

Delayed enhancement MRI using extracellular contrast media allows reliable detection of myocardial infarction. If blood pool contrast media like P792 (Vistarem, Guerbet, France), in addition to improving coronary MR angiography, can be shown to also produce delayed enhancement in myocardial infarction they could improve the prerequisites for a comprehensive cardiac MR examination. In this study reperfused myocardial infarction in five minipigs was imaged with an inversion-recovery fast low-angle shot sequence using P792 (0.013 mmol Gd/kg) and the extracellular contrast medium Gd-DOTA (Dotarem, 0.1 mmol Gd/kg, Guerbet). The infarction size determined on MRI using P792 (7.55 +/- 2.31 cm(2)) highly correlated both with histomorphometry (7.81 +/- 2.18 cm(2), r = 0.991, P < 0.002) and with MRI using Gd-DOTA (7.85 +/- 2.35 cm(2), r = 0.978, P < 0.005). Bland-Altman analysis showed that the limit of agreement of MRI using P792 compared to histomorphometry was 3.3 +/- 7.6% of the infarction size. The contrast-to-noise ratio between infarcted and remote myocardium was not significantly different between Gd-DOTA (5.9 +/- 2.4) and P792 (4.4 +/- 1.1, P = 0.5). The blood pool contrast medium P792 allows reliable assessment of viability with good contrast and accuracy.     

Dynamic MR mammography: three-dimensional real-time visualization of contrast enhancement in virtual reality

RATIONALE AND OBJECTIVES: In view of the increasing use of breast magnetic resonance (MR) imaging to supplement x-ray mammography. the authors developed a method for fast and efficient analysis of dynamic MR images of the female breast. MATERIALS AND METHODS: The MR image data sets were acquired with a saturation-recovery turbo fast low-angle shot sequence to detect the kinetics of the contrast agent concentration in the whole breast at a high temporal and spatial resolution. A morphologic three-dimensional fast low-angle shot data set was also acquired. The dynamic image data sets were analyzed with tracer kinetic modeling to describe the physiologic processes underlying the contrast enhancement in mathematical terms and enable the estimation of functional tissue-specific parameters, which reflect the status of microcirculation. To display morphologic and functional tissue information simultaneously, the authors developed a multidimensional real-time visualization system (with three-dimensional texture mapping), which enables a practical and intuitive human computer interface in virtual reality. RESULTS: The spatially differentiated representation of the computed functional tissue parameters superimposed on the anatomic information offers several possibilities: (a) more discernible contrast enhancement, (b) inspection of the data volume in three-dimensional space by means of rotation and transparency variation, (c) location of lesions in space and thus faster and more natural recognition of topologic coherencies, and (d) fast and efficient overview in compressed form. CONCLUSION: A feasibility study demonstrated that multidimensional visualization of contrast enhancement in virtual reality is a practicable idea. Detection and location of multiple breast lesions may be an important application.     

Combined dynamic contrast enhanced breast MR and proton spectroscopic imaging: a feasibility study

PURPOSE: To investigate the feasibility of combined dynamic contrast enhanced (DCE) and magnetic resonance spectroscopy (MRS) in evaluating breast lesions. METHODS: Nine patients with positive mammograms scheduled for either biopsy or mastectomy were examined on a 1.5-T MR scanner. DCE was performed with administration of gadolinium-DTPA contrast using a two-dimensional spoiled gradient recall echo (SPGR) sequence. Proton spectroscopy (TR/TE = 2000/272 msec) was performed using PRESS single slice (10 mm). Lesion time intensity curves were classified as persistent (type 1), plateau (type 2), or washout (type 3) pattern enhancement. Choline (Cho) signal-to-noise ratios (SNRs) and enhancement patterns were compared between benign and malignant lesions as determined by histopathology. RESULTS: Five patients had breast carcinoma and four had benign lesions. Type 1 enhancement was found in two benign cases, type 2 enhancement in two of four benign and four of five malignant lesions, and one malignant case exhibited a type 3 pattern. Choline SNR was significantly different (P < 0.003) between benign and malignant lesions (2.0 +/- 0.3 vs. 5.7 +/- 1.4; P < 0.003). Choline SNR was less than 4.0 in all of the benign lesions, including the two lesions with type 2 enhancement. CONCLUSION: Proton MRS appears to be a promising technique for classification of breast lesions when DCE results are equivocal. A combination of DCE and MRS is feasible, and may have improved specificity compared to either modality alone.     

Gadolinium-DTPA-dextran: a macromolecular MR blood pool contrast agent

RATIONALE AND OBJECTIVES: Magnetic resonance (MR) imaging blood pool agents offer numerous advantages for vascular and tumor imaging. The purpose of this study was to test gadolinium-diethylenetriaminepentaacetate-dextran ([Gd]DTPA-dextran) as a new water soluble macromolecular blood pool agent for MR imaging. MATERIALS AND METHODS: [Gd]DTPA-dextran (187 gadolinium atoms per dextran, molecular weight 165 kD, diameter 17.6 nm) was synthesized. Fifteen anesthetized New Zealand White rabbits with thigh VX2 tumors were scanned in a knee coil at 1.5T. Coronal 3D MR angiographic sequences were obtained before and at several time points up to 72 hours after the intravenous bolus injection of [Gd]DTPA-dextran providing gadolinium at either 0.05 (n = 4) or 0.1 mmol/kg (n = 8) or [Gd]DTPA-bismethylamide (BMA) providing gadolinium at 0.1 mmol/kg (n = 3). Time enhancement curves for aorta, cava, and tumor rim were compared by univariate General Linear Model. RESULTS: Contrast enhancement of cava and aorta relative to a water phantom were significantly greater at all time points after either dose of [Gd]DTPA-dextran than after [Gd]DTPA-BMA (P < 0.01). Tumor rim enhancement was less intense for either dose of [Gd]DTPA-dextran at peak than for [Gd]DTPA-BMA (P < 0.05). Tumor rim enhancement with both doses of [Gd]DTPA-dextran became equivalent to that of [Gd]DTPA-BMA at one hour and was greater at 24 hours (P < 0.05). CONCLUSION: [Gd]DTPA-dextran is a new macromolecular MR contrast agent that can be synthesized to carry a high density of gadolinium atoms without intra-molecular cross-linking. It provides significantly greater vascular residence time than a conventional gadolinium chelate and shows promise for MR blood pool imaging.     

Dark-lumen magnetic resonance colonography in patients with suspected sigmoid diverticulitis: a feasibility study

To assess dark-lumen magnetic resonance colonography (MRC) for the evaluation of patients with suspected sigmoid diverticulitis. Forty patients with suspected sigmoid diverticulitis underwent MRC within 72 h prior to conventional colonoscopy (CC). A three-dimensional T1-weighted volumetric interpolated breath-hold examination sequence was acquired after an aqueous enema and intravenous administration of gadolinium-based contrast agents. All MRC data were evaluated by two radiologists. Based on wall thickness and focal uptake of contrast material and pericolic reaction including mesenteric infiltration on T1-weighted sequence the sigmoid colon was assessed for the presence of diverticulitis. MRC classified 17 of the 40 patients as normal with regard to sigmoid diverticulitis. However, CC confirmed the presence of light inflammatory signs in four patients which were missed in MRC. MRC correctly identified wall thickness and contrast uptake of the sigmoid colon in the other 23 patients. In three of these patients false-positive findings were observed, and MRC classified the inflammation of the sigmoid colon as diverticulitis whereas CC and histopathology confirmed invasive carcinoma. MRC detected additionally relevant pathologies of the entire colon and could be performed in cases where CC was incomplete. MRC may be considered a promising alternative to CC for the detection of sigmoid diverticulitis.     

Understanding interpretive errors in radiologists learning computed tomography colonography

RATIONALE AND OBJECTIVES: To determine if interpretive errors in the course of learning CT colonography are secondary to failures in detection or in characterization and determine the types of lesions frequently missed. MATERIALS AND METHODS: Fifteen radiologists completed an electronic CTC training module consisting of two parts: 1) a teaching file demonstrating the varied appearances of polyps, cancers, and pitfalls in interpreting exams; and 2) a test of 50 complete CTC datasets. Following review of each test case, radiologists were asked to indicate if and where a polyp was visualized. The module then showed each neoplasm (if any) located within the dataset. For false negative examinations, radiologists indicated if the lesion was not seen, was seen but interpreted as colonic wall or fold, or was seen but interpreted as stool or fluid. RESULTS: The average sensitivity for sessile, pedunculated, and flat polyps for these novice readers was 76%, 63%, and 32%, respectively. Average sensitivity for all morphologies of cancers (annular, polypoid, flat) was high (93%, 85%, 95%), with 8/11 missed cancers being secondary to failure in detection. The most frequently missed cancer was an annular constricting tumor (5/11). Overall, 55% (73/132) of errors were failures of detection and 45% (59/132) were errors in characterization. CONCLUSION: Radiologists learning CT colonography had slightly more errors of detection than characterization, but this difference was not statistically significant. Flat and pedunculated polyps and annular constricting cancers were the most frequently missed morphologies. Examples of these abnormalities should be emphasized in CTC training programs.     

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.     

Evaluation of [Gd(Bz-TTDA)]2- as a potential contrast agent in MR imaging of the hepatobiliary system: an animal study

PURPOSE: To evaluate the potential of a new lipophilic paramagnetic complex [Gd(Bz-TTDA)]2- [(4s)-4-benzyl-3,6,10-tri(carboxymethyl)-3,6,10-triazadodecandioic acid]2- designed for use as a hepatobiliary MR contrast agent. MATERIALS AND METHODS: MR imaging studies for normal and hepatocellular carcinoma (HCC) rat models were performed using a 1.5-T scanner. Sequential multislice T1-weighted turbo field echo (TFE) (TR/TE/flip angle: 15 msec/6.1 msec/25 degrees) coronal images of normal rats were obtained before and after intravenous injections of 0.1 mmol/kg [Gd(Bz-TTDA)]2- in study groups (N = 12) or 0.1 mmol/kg gadopentate dimeglumine (Gd-DTPA)2- in control groups (N = 12). Similar protocols of MR imaging with additional T2-weighted images were used for the rats with implanted HCC in both study and control groups (N = 12, in each group). MR images were analyzed to evaluate the time-enhancement change (% increase of signal-to-noise ratio [SI/N]) in normal liver, renal cortex, renal medulla, and tumors. The liver-lesion contrast-to-noise ratios (CNR) were also evaluated in study and control groups. The rats were killed immediately after the last MR scan to undergo autopsy and histopathologic observation. The acute toxicity test (medial lethal dose, LD50) in mice was also done. RESULTS: The liver enhancement in normal rats reached a plateau 5-50 minutes after injection of [Gd(Bz-TTDA)]2-, maintained for three hours, then gradually declined. Intensity of enhancement in liver, renal cortex, and medulla after injection of [Gd(Bz-TTDA)]2- was significantly higher than with Gd-DTPA. The efficacy of tumor characterization with injection of [Gd(Bz-TTDA)]2- was similar to that of Gd-DTPA at the early dynamic phase of the contrast study. However, the liver-lesion CNRs were significantly higher in the study group in the later phase, when tumor enhancement decreased and liver enhancement persisted. The dose of LD50 in acute toxicity test of [Gd(Bz-TTDA)]2- in mice was 7.5 mmol/kg. CONCLUSION: The preliminary results in this animal study indicated that [Gd(Bz-TTDA)]2- has the potential of becoming a reliable liver MR contrast agent.