Eiko Ueno

Unenhanced MR Angiography: Techniques and Clinical Applications in Patients with Chronic Kidney Disease

Alongside the two conventional unenhanced magnetic resonance (MR) angiographic techniques, namely time-of-flight and phase-contrast MR angiography, several novel techniques have since been developed, including electrocardiograph (ECG)-gated fast spin echo (FSE), steady-state free precession (SSFP), and arterial spin labeling. These techniques are increasingly being used to avoid severe complications caused by contrast materials, such as iodinated contrast material-induced nephropathy and gadolinium-induced nephrogenic systemic fibrosis. However, image acquisition and interpretation with these techniques are more complicated than with contrast-enhanced MR angiography because of numerous types of artifacts. Appropriate use of these techniques can allow diagnosis of vascular diseases in patients with chronic kidney disease without using contrast materials. For example, time-of-flight angiography is the main technique for evaluating intracranial arteries. Phase-contrast imaging is increasingly being used for physiologic evaluation rather than morphologic evaluation. Meanwhile, ECG-gated FSE MR angiography can show peripheral arteries in more detail. SSFP MR angiography with or without arterial spin labeling can provide high-resolution images of blood vessels including renal arteries, the aorta, and coronary arteries. Black-blood imaging is also used to evaluate vessel walls and intravascular abnormalities including plaque, dissection, and thrombi. The authors review the principles of the currently available unenhanced MR angiographic techniques, along with their advantages and limitations, and describe their clinical applications. This article should help readers select the most appropriate unenhanced MR angiographic technique to assess vascular diseases in patients with chronic kidney disease. Supplemental material available at http://radiographics.rsna.org/lookup/suppl/doi:10.1148/rg.312105075/-/DC1.

Optimizing parameters for flat-panel detector digital tomosynthesis.

Digital tomosynthesis is a novel technique that allows easy and swift volume data acquisition in selected regions of the body. However, many radiologists and technologists are unfamiliar with this technique and the potential artifacts related to data acquisition. Digital tomosynthesis requires a single linear sweep of the x-ray tube assembly with corresponding tomographic reconstruction of large-area flat-panel detector radiographic data. Standard acquisition parameters include sweep angle, sweep direction, patient barrier-object distance, number of projections, and total radiation dose. Potential acquisition-related artifacts include blurring-ripple, ghost artifact-distortion, poor spatial resolution, image noise, and metallic artifact. A comprehensive understanding of the relationships between acquisition parameters and potential associated artifacts is critical to optimizing acquisition technique and avoiding misinterpretation of artifacts. Sweep direction should be chosen on the basis of the anatomy of interest and the purpose of the examination so as to reduce the influence of blurring-ripple, ghost artifact-distortion, and metallic artifact. Adjusting the sweep angle, number of projections, and radiation dose will optimize depth resolution, avoid ripple in the sections of interest, and reduce unnecessary radiation exposure without compromising image quality. Thus, it is important that the radiologist and technologist establish appropriate protocols for different examination types to allow optimal utilization of this novel imaging technique.

Navigator‐triggered prospective acquisition correction (PACE) technique vs. conventional respiratory‐triggered technique for free‐breathing 3D MRCP: An initial prospective comparative study using healthy volunteers

To confirm the superiority of the navigator‐triggered prospective acquisition correction (PACE) technique over the conventional respiratory‐triggered (RESP) technique, something that has been perceived experimentally but without definite evidence, for free‐breathing three‐dimensional (3D) magnetic resonance cholangiopancreatography (MRCP) using healthy volunteers.

Feasibility of diffusion-weighted MRI for defining placental invasion.

The purpose of this clinical note is to describe the feasibility of using diffusion‐weighted imaging for diagnosing placental invasion with a case of placenta increta and six cases without it. Diffusion‐weighted imaging (DWI) at a b‐value of 1000 sec/mm2 can clearly define the border between the placenta and myometrium because only the placenta shows very high signal intensity. The corresponding image at a b‐value of 0 sec/mm2 shows the myometrium with high signal intensity compared with the surrounding fat. Therefore, fusion of the two images can be used additionally to visualize thickness of the myometrium. As a result, DWI can be used to visualize the focal thinning of the myometrium caused by placenta increta, which has been difficult to diagnose on conventional magnetic resonance imaging sequences without contrast enhancement. However, the use of DWI for placental invasion should be determined following careful consideration of its risks and benefits, as fetus safety has not been established. J. Magn. Reson. Imaging 2009;30:666–671.

Improved delineation of the anterior spinal artery with model-based iterative reconstruction in CT angiography: a clinical pilot study.

OBJECTIVE The purpose of this study was to investigate the utility of model-based iterative reconstruction (MBIR) for improving delineation of the anterior spinal artery (ASA) during routine-dose CT angiography. MATERIALS AND METHODS For imaging of 10 patients (six men, four women; mean age, 73.9 ± 7.5 years) consecutively undergoing CT angiography of the whole aorta with a 12-HU noise index, we used filtered back projection with a standard kernel, adaptive statistical iterative reconstruction of 40% with a detail kernel, and MBIR to reconstruct axial and oblique coronal multiplanar reformation images to delineate the ASA. We measured objective noise in the spinal cord and contrast-to-noise ratio (CNR) between the aorta and spinal cord on axial images at the T12 level. Two radiologists independently graded subjective noise and ASA delineation on the multiplanar reformation images from 1 (poor) to 4 (excellent). We compared results among the three reconstructions using one-way analysis of variance and Tukey-Kramer significance tests. RESULTS Objective noise, CNR, and subjective image noise and ASA delineation improved significantly with MBIR. Image noise was 18.4 ± 3.6 HU and CNR, 23.4 ± 8.6 (reader 1 scores, 3.9 ± 0.3 and 3.7 ± 0.5; reader 2, 3.9 ± 0.3 and 3.5 ± 0.7). With filtered back projection, image noise was 34.7 ± 8.3 HU and CNR 12.1 ± 4.0 (reader 1 scores, 2.0 ± 0.0 and 2.2 ± 0.4; reader 2, 2.2 ± 0.4 and 2.5 ± 0.7), and with ASIR, 33.0 ± 8.1 HU and 12.7 ± 4.3 (reader 1 scores, 2.0 ± 0.0 and 2.2 ± 0.4; reader 2, 2.2 ± 0.4 and 2.5 ± 0.7) (p < 0.05). Results between filtered back projection and adaptive statistical iterative reconstruction were comparable. CONCLUSION Use of MBIR can improve delineation of the ASA during CT angiography.

Current and Novel Imaging Techniques in Coronary CT

Multidetector coronary computed tomography (CT), which is widely performed to assess coronary artery disease noninvasively and accurately, provides excellent image quality. Use of electrocardiography (ECG)-controlled tube current modulation and low tube voltage can reduce patient exposure to nephrotoxic contrast media and carcinogenic radiation when using standard coronary CT with a retrospective ECG-gated helical scan. Various imaging techniques are expected to overcome the limitations of standard coronary CT, which also include insufficient spatial and temporal resolution, beam-hardening artifacts, limited coronary plaque characterization, and an inability to allow functional assessment of coronary stenosis. Use of a step-and-shoot scan, iterative reconstruction, and a high-pitch dual-source helical scan can further reduce radiation dose. Dual-energy CT can improve contrast medium enhancement and reasonably reduce the contrast dose when combined with noise reduction with the use of iterative reconstruction. High-definition CT can improve spatial resolution and diagnostic evaluation of small or peripheral coronary vessels and coronary stents. Dual-source CT and a motion correction algorithm can improve temporal resolution and reduce coronary motion artifacts. Whole-heart coverage with 320-detector CT and an intelligent boundary registration algorithm can eliminate stair-step artifacts. By decreasing beam hardening and enabling material decomposition, dual-energy CT is expected to remove or reduce the depiction of coronary calcification to improve intraluminal evaluation of calcified vessels and to provide detailed analysis of coronary plaque components and accurate qualitative and quantitative assessment of myocardial perfusion. Fractional flow reserve derived from coronary CT is a state-of-the-art noninvasive technique for accurately identifying myocardial ischemia beyond coronary CT. Understanding these techniques is important to enhance the value of coronary CT for assessment of coronary artery disease.

Measurement of vascular wall attenuation: Comparison of CT angiography using model-based iterative reconstruction with standard filtered back-projection algorithm CT in vitro

OBJECTIVES To compare the performance of model-based iterative reconstruction (MBIR) with that of standard filtered back projection (FBP) for measuring vascular wall attenuation. STUDY DESIGN After subjecting 9 vascular models (actual attenuation value of wall, 89 HU) with wall thickness of 0.5, 1.0, or 1.5 mm that we filled with contrast material of 275, 396, or 542 HU to scanning using 64-detector computed tomography (CT), we reconstructed images using MBIR and FBP (Bone, Detail kernels) and measured wall attenuation at the center of the wall for each model. We performed attenuation measurements for each model and additional supportive measurements by a differentiation curve. We analyzed statistics using analyzes of variance with repeated measures. RESULTS Using the Bone kernel, standard deviation of the measurement exceeded 30 HU in most conditions. In measurements at the wall center, the attenuation values obtained using MBIR were comparable to or significantly closer to the actual wall attenuation than those acquired using Detail kernel. Using differentiation curves, we could measure attenuation for models with walls of 1.0- or 1.5-mm thickness using MBIR but only those of 1.5-mm thickness using Detail kernel. We detected no significant differences among the attenuation values of the vascular walls of either thickness (MBIR, P=0.1606) or among the 3 densities of intravascular contrast material (MBIR, P=0.8185; Detail kernel, P=0.0802). CONCLUSIONS Compared with FBP, MBIR reduces both reconstruction blur and image noise simultaneously, facilitates recognition of vascular wall boundaries, and can improve accuracy in measuring wall attenuation.

Hyperattenuating Signs at Unenhanced CT Indicating Acute Vascular Disease

When a vascular disease is suspected, the focus is usually on morphologic features seen at contrast material-enhanced multidetector computed tomography (CT). However, unenhanced CT also plays an important role in revealing so-called hyperattenuating signs, which represent a slight increase in the focal attenuation of a vessel. Hyperattenuating signs are occasionally observed when an acute clot has formed in a vessel and can be seen in various vascular diseases, including acute arterial occlusion, acute arterial dissection, aneurysm rupture, and acute venous thrombosis. The attenuation of these signs tends to increase because the concentration of hemoglobin increases as water content decreases. Hyperattenuating signs are a transient phenomenon, as the attenuation gradually decreases. Therefore, they can serve as unique findings indicating an acute state. Although hyperattenuating signs are not well understood, recognition of these signs is important because they can help reveal serious acute vascular diseases even at unenhanced CT.

Vascular Diameter Measurement in CT Angiography: Comparison of Model-Based Iterative Reconstruction and Standard Filtered Back Projection Algorithms In Vitro

OBJECTIVE The purpose of this study was to evaluate the performance of model-based iterative reconstruction (MBIR) in measurement of the inner diameter of models of blood vessels and compare performance between MBIR and a standard filtered back projection (FBP) algorithm. MATERIALS AND METHODS Vascular models with wall thicknesses of 0.5, 1.0, and 1.5 mm were scanned with a 64-MDCT unit and densities of contrast material yielding 275, 396, and 542 HU. Images were reconstructed images by MBIR and FBP, and the mean diameter of each model vessel was measured by software automation. Twenty separate measurements were repeated for each vessel, and variance among the repeated measures was analyzed for determination of measurement error. For all nine model vessels, CT attenuation profiles were compared along a line passing through the luminal center on axial images reconstructed with FBP and MBIR, and the 10-90% edge rise distances at the boundary between the vascular wall and the lumen were evaluated. RESULTS For images reconstructed with FBP, measurement errors were smallest for models with 1.5-mm wall thickness, except those filled with 275-HU contrast material, and errors grew as the density of the contrast material decreased. Measurement errors with MBIR were comparable to or less than those with FBP. In CT attenuation profiles of images reconstructed with MBIR, the 10-90% edge rise distances at the boundary between the lumen and vascular wall were relatively short for each vascular model compared with those of the profile curves of FBP images. CONCLUSION MBIR is better than standard FBP for reducing reconstruction blur and improving the accuracy of diameter measurement at CT angiography.

Utility of CT perfusion with 64-row multi-detector CT for acute ischemic brain stroke

We investigated the utility of computed tomographic (CT) perfusion (CTP) with 64-row multi-detector row CT (MDCT) to diagnose acute infarction and ischemic penumbra. We reviewed 58 clinical cases with acute ischemic stroke with CTP, compared the size of the area with long mean transit time (MTT) to that with abnormal intensity in magnetic resonance (MR) diffusion-weighted imaging (DWI) to diagnose penumbra, and compared the size of the area with reduced cerebral blood volume (CBV) in CTP to that in MR DWI to evaluate sensitivity for infarction. The total sensitivity of MTT to acute ischemic lesions was 81% (47/58). Sensitivity of MTT to segmental lesions was 100% (42/42) and for spot and focal lesions, 31% (5/16). In 13 patients, penumbra was diagnosed as lesions mismatched between MTT in CTP and MR DWI. When we regarded a lesion with decreased CBV as infarction, the sensitivity of CBV to segmental lesions was 85% (11/13), and the sensitivity to small infarction was 14% (4/28). Use of 64-row MDCT improves coverage and radiation exposure in head CTP. The combination of plain CT, CT angiography, and CTP with MDCT can demonstrate all segmental ischemic lesions and most large segmental infarctions, and their combined application is useful in considering indication and contraindication for thrombolysis. The problem of low sensitivity for small lesions remains, and MR DWI may be required to assess small infarctions when findings from combined plain CT, CT angiography, and CTP are negative in patients with suspected acute brain stroke.