Voxel sensitivity function description of flow-induced signal loss in MR imaging: implications for black-blood MR angiography with turbo spin-echo sequences.

The conditions in which the image intensity of vessels transporting laminar flow is attenuated in black-blood MR angiography (BB-MRA) with turbo spin-echo (TSE) and conventional spin-echo (CSE) pulse sequences are investigated experimentally with a flow phantom, studied theoretically by means of a Bloch equation-voxel sensitivity function (VSF) formalism, and computer modeled. The experiments studied the effects of: a) flow velocity, b) imaging axes orientation relative to the flow direction, and c) phase encoding order of the TSE train. The formulated Bloch equation-VSF theory describes flow effects in two-dimensional (2D)- and 3D-Fourier transform magnetic resonance imaging. In this theoretical framework, the main attenuation mechanism instrumental to BB-MRA, i.e., transverse magnetization dephasing caused by flow in the presence of the imaging gradients, is described in terms of flow-induced distortions of the individual voxel sensitivity functions. The computer simulations predict that the intraluminal homogeneity and extent of flow-induced image intensity attenuation increase as a function of decreasing vessel diameter, in support of the superior image quality achieved with TSE-based BB-MRA in the brain.     

Hemispherical gradient coils for magnetic resonance imaging

Hemispherical gradient coils offer an open geometry that is well suited to imaging of the human brain. The windings of a hemispherical gradient coil are on average closer to the target region than those of a comparable cylindrical coil, and consequently hemispherical coils can produce higher efficiency at fixed inductance. The mathematical formalism needed for the design of hemispherical gradient coils is described here, including expressions relating the current distribution on the hemisphere to the magnetic field generated, as well as the stored energy and power dissipation. In addition, expressions for the torque experienced by the current distribution in the presence of the main magnetic field have been derived and used to develop an approach allowing the design of torque‐balanced, hemispherical transverse gradient coils. Hemispherical coil designs suitable for brain imaging are presented and shown to have improved performance compared with their cylindrical counterparts. Small, prototype, hemispherical z‐ and x‐gradient coils have been constructed and tested in phase‐mapping experiments at 3 T. The experimental results show good agreement with theoretical predictions, validating the mathematical expressions used in the coil design process. The formalism also allows the design of coils wound on a complete spherical surface, and the performance of such coils is additionally described here. Magn Reson Med, 2005. © 2005 Wiley‐Liss, Inc.     

Simultaneous imaging of myocardial motion and chamber blood flow with SPAMM n' EGGS (Spatial Modulation of Magnetization With Encoded Gradients for Gauging Speed)

PURPOSE: To provide simultaneous measurements of one-dimensional (1-D) myocardial displacement and 1-D chamber blood flow in a single breath-held acquisition using an MR imaging technique, SPAMM n' EGGS (Spatial Modulation of Magnetization With Encoded Gradients for Gauging Speed). MATERIALS AND METHODS: Velocity encoding bipolar gradients sensitive to chamber blood flow were played out before the readout gradient in a 1-1 SPAMM-tagged MR imaging pulse sequence. For any given motion-flow encoded direction, the acquired image sequence was later postprocessed to separate the tag motion and blood flow terms. Experiments were performed on seven normal volunteers, and two pigs with moderate ischemic mitral regurgitation. Left-ventricular motion and trans-valvular flow obtained using the SPAMM n' EGGS pulse sequence was compared against measurements obtained using standard tagging and phase-contrast pulse sequences, respectively. RESULTS: Results in normal volunteers and diseased pigs demonstrate multiphase correlated measurements of myocardial motion and chamber blood flow using SPAMM n' EGGS. A close correspondence in these measurements to conventional tagging and phase-contrast sequences is confirmed. CONCLUSION: We have demonstrated that simultaneous acquisition of myocardial motion and chamber blood flow is possible within a single breath-hold. The data obtained using the SPAMM n' EGGS pulse sequence may be useful in the planning and evaluation of mitral-valve repair procedures.     

Imaging of the renal arteries: value of MR angiography

We compared the efficacy of MR angiography with that of conventional angiography for visualizing the renal arteries and detecting renovascular disease. Thirty-three MR angiographic studies, consisting of axial two-dimensional (2-D) phase-contrast, coronal 2-D phase-contrast, and coronal 2-D time-of-flight acquisitions, were performed within 48 hr of conventional arteriography. The studies were done to evaluate possible renovascular hypertension (n = 25) or potential donor nephrectomy (n = 8). The three MR image sets were interpreted independently, in random order by three observers, with regard to the number of renal arteries, degree of vessel visualization, arteriovenous overlap, and presence of renovascular disease. A fourth interpretation was based on the combined axial and coronal phase-contrast image sets. Evaluation was limited to the proximal 35 mm of each renal artery. Renal artery visualization and detection of renovascular disease were more complete with coronal phase-contrast (80% sensitivity, 91% specificity) than with time-of-flight (53% sensitivity, 97% specificity) images. Combined axial and coronal phase-contrast images permitted visualization of the proximal 35 mm of all dominant renal arteries and detection of 13 of 15 stenoses (87% sensitivity, 97% specificity). Our data suggest that biplanar MR angiography has considerable potential as a noninvasive screening technique for the evaluation of renovascular disease.     

Chemical shift imaging of bone marrow: preliminary experience

A phase-contrast method of chemical shift imaging was used to evaluate bone marrow in normal volunteers and in patients with metabolic, inflammatory, traumatic, and neoplastic disorders. Five normal volunteers were examined in order to obtain preliminary data on normal patterns of signal intensity in hematopoietic and fatty marrow using both conventional magnetic resonance imaging and proton chemical shift imaging. Normally, hematopoietic marrow yields low signal intensity on phase-contrast images; pathologic conditions affecting hematopoietic marrow typically result in increased signal intensity due to either accumulated lipid or water. Because of its high fat content, yellow marrow normally yields high signal intensity on phase-contrast images, whereas abnormal conditions usually result in decreased phase-contrast signal intensity due to increased tissue water. Proton chemical shift imaging is likely to be a valuable supplement to standard magnetic resonance imaging techniques in the study of bone marrow in vivo.     

Noise reduction in three-dimensional phase-contrast MR velocity measurementsl

The authors have developed a method to reduce noise in three-dimensional (3D) phase-contrast magnetic resonance (MR) velocity measurements by exploiting the property that blood is incompressible and, therefore, the velocity field describing its flow must be divergence-free. The divergence-free condition is incorporated by a projection operation in Hilbert space. The velocity field obtained with 3D phase-contrast MR imaging is projected onto the space of divergence-free velocity fields. The reduction of noise is achieved because the projection operation eliminates the noise component that is not divergence-free. Signal-to-noise ratio (S/N) gains on the order of 15%-25% were observed. The immediate effect of this noise reduction manifests itself in higher-quality phase-contrast MR angiograms. Alternatively, the S/N gain can be traded for a reduction in imaging time and/or improved spatial resolution.     

Comparison between the physiological response to roller skiing and in-line skating in biathletes

PURPOSE: Roller skiing is frequently used in Nordic disciplines during the off-season periods. Recently, in-line skating has become a potential alternative. In the present study, the responses of heart rate, oxygen uptake, respiratory exchange ratio, and lactic acid concentration to roller skiing and in-line skating were compared in competitive biathletes. METHODS: Eight male subjects performed three tests with both devices on a hilly outdoor track. They were requested to adjust their speed in such a way that the following criteria were met: intensity 1, lactate concentration about 2 mmol x L(-1); intensity 2, lactate concentration about 4 mmol x L(-1); AND intensity 3, maximal speed. RESULTS: Though the subjects were not experienced in-line skaters, all managed to adjust the required intensities. This was achieved through increased velocities during in-line skating. Independent of the exercise intensity the differences in speed ranged between 1.0 and 1.4 m x s(-1). The relationships between lactic acid concentration, oxygen uptake, respiratory exchange ratio, and heart rate were not influenced by the test device. The respiratory exchange ratio amounted to 0.88, 0.95, and 1.02 for intensities 1 to 3, respectively. CONCLUSIONS: These results show that in-line skating can be regarded as an alternative to roller skiing for off-seasonal training in Nordic disciplines. A potential advantage of in-line skating is that aerobic training intensities can be obtained at competitive velocities.     

MR angiography of the head and neck: value of two-dimensional phase-contrast projection technique.

MR angiography is commonly performed by using two- and three-dimensional time-of-flight and three-dimensional phase-contrast techniques. These procedures require long examination times and processing of imaging data by computing maximum intensity projections. Two-dimensional phase-contrast projection angiography has neither of these disadvantages. We analyzed the value of this technique for head and neck MR angiography in 84 patients and 15 control subjects. Patients were examined to resolve specific clinical questions such as the presence of arteriovenous malformations (20 cases), patency of carotid and vertebral arteries (35 cases), patency of the superior sagittal sinus (14 cases), patency of saphenous vein bypass grafts (11 cases), and vascularity of masses (four cases). Conventional angiograms were available for correlation in 22 patients. Two-dimensional phase-contrast projection angiograms were generated by using a gradient-recalled-echo sequence sensitized to flow with the use of flow-encoding gradients. Projection MR angiograms were obtained in approximately 3.5 min by combining images obtained with flow-encoding gradients applied along the axes defining the image plane. MR angiograms were subjectively evaluated by three observers without reference to routine MR images or conventional angiograms. High-quality studies were obtained in 93% of control subjects and 90% of patients examined. The findings based on phase-contrast angiography were confirmed with conventional angiography in 21 of 22 patients for whom conventional angiograms were available. Evaluation of vascular grafts and of the patency of major cranial vessels was easily done with two-dimensional phase-contrast angiography and was useful in postoperative follow-up examinations. We also advocate its use for superior sagittal sinus thrombosis and follow-up examinations in patients with arteriovenous malformations.     

MRI和心导管检查同步测定肺动脉舒张压的相关性研究

目的　探索MRI无创性测量肺动脉舒张压 (PADP)的新途径。材料与方法　应用MR相位速度图测量 40例具有肺动脉瓣返流 (PR)患者的最大返流速度并计算出舒张末期肺动脉与右心室间的压差 (PVDG) ,同时 ,与右心导管检查结果进行比较 ,通过直线相关与回归分析MRI测量PADP的敏感性和特异性。结果　MRI测定的PADP为 4.82± 1.2 4mmHg(1mmHg =0 .133kPa) ,右心导管的测值为 4.5 2± 2 .0 0mmHg ,二者之间无明显差异 (t=1.45 ,P >0 .0 5 ) ,但具有高度相关性 (r =0 .94,P <0 .0 1)。回归方程y =0 .82x + 4.5 8,其敏感性和特异性高达 90 %。结论　MRI是评价PR患者PADP的较可靠的新技术。     

A framework for generalized reference image reconstruction methods including HYPR-LR, PR-FOCUSS, and k-t FOCUSS

Purpose:

To investigate the relationships among highly constrained back projection (HYPR)‐LR, projection reconstruction focal underdetermined system solver (PR‐FOCUSS), and k‐t FOCUSS by showing how each method relates to a generalized reference image reconstruction method. That is, the generalized series model employs a fixed reference image and multiplicative corrections—that model is extended here to consider reference images more broadly, both in image space and in transform spaces (x‐t and x‐f spaces), and that can evolve with iteration.

Materials and Methods:

Theoretical relationships between the methods were derived. Computer simulations were done to compare HYPR‐LR to one iteration of PR‐FOCUSS. The generalized reference approaches applied in the x‐t or x‐f domain were compared using computer simulation, five cardiac cine imaging datasets, and six myocardial perfusion datasets.

Results:

PR‐FOCUSS and HYPR‐LR gave comparable errors, with PR‐FOCUSS slightly outperforming HYPR‐LR. The baseline image is important to the performance of k‐t FOCUSS and x‐t FOCUSS, as demonstrated by results from cardiac cine imaging. For cardiac perfusion reconstructions with the use of a temporal average image as the baseline image, k‐t FOCUSS gave lower errors than x‐t FOCUSS.

Conclusion:

HYPR‐LR and PR‐FOCUSS are closely related: both work for radial sampling and use reference images in the x‐t domain; HYPR‐LR is an approximate implementation of the generalized reference framework, while PR‐FOCUSS is a conjugate gradient implementation of the generalized reference framework. The superiority of generalized reference approaches applied in the x‐t or x‐f domain was sensitive to the characteristics of the acquired data and to the baseline image used. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Preferential arterial imaging using gated thick-slice gadolinium-enhanced phase-contrast acquisition in peripheral MRA

PURPOSE: To investigate the feasibility of preferential arterial imaging using gadolinium-enhanced thick-slice phase-contrast imaging. METHODS: Six healthy volunteers were studied using a peripheral-gated segmented k-space CINE phase-contrast pulse sequence using four views per RR interval with flow encoding in the superior-inferior direction. Images at the level of the popiteal trifurcation were acquired postcontrast with different section thicknesses (4-8 cm) and VENC values (20-150 cm/sec), and phase-difference processing. RESULTS: The post-gadolinium contrast-enhanced thick-slice phase-contrast acquisitions demonstrated the ability to visualize the tibio-peroneal (trifurcation) arteries, especially in systole. With MR contrast agents, the signal from blood is raised significantly above that of stationary tissue from T(1) shortening such that the partial volume artifact is reduced in thick-slice acquisitions. Furthermore, by selecting the VENC value as a function of the cardiac cycle, the noise floor can be raised to selectively suppress flow values less than that of the noise threshold, allowing better accentuation of arterial structures at systole. CONCLUSIONS: Thick-slice phase-contrast acquisition with phase-difference processing has been observed to reduce partial volume artifacts when an MR contrast agent substantially increases signal in the vasculature over that of normal background tissue. Preferential arterial images can be obtained by either increasing the VENC value to selectively suppress signal from slow flow in the veins or by subtracting the diastolic phase image from the peak systolic phase image. J. Magn. Reson. Imaging 2001;13:714-721.     

Plow quantitation with echo-planar phase-contrast velocity mapping: In vitro and in vivo evaluation

We present a multishot echo-planar imaging (EPI) phase-contrast implementation for flow quantitation. The measurement accuracy of this technique was evaluated in vitro and in vivo. A gated eight-shot EPI phase-contrast sequence (TR/TE = 16/7.4, 45° flip angle), with a flow-phase interval of 32 msec and an in-plane resolution of 2× 2 mm was initially evaluated in a pulsatile flow phantom. Subsequently, EPI phase-contrast flow measurements of the ascending and descending aorta, obtained in 10 volunteers, were compared with flow volume data acquired with a conventional cine phase-contrast sequence (TR/TE = 24/7, 45° flip angle, 48-msec flow-phase interval, 2 × 1 mm in-plane resolution). Comparisons between flow measurements were made using data obtained with the flow probe and cine phase contrast as the standard of reference for in vitro and in vivo measurements, respectively. EPI phase-contrast sequences reduced data acquisition times tenfold compared with cine phase-contrast sequences. EPI phase-contrast flow measurements correlated were with phantom flow (r = 0.98, slope = 1.1) as well as with aortic cine phase-contrast flow volume determinations (r = 0.98). A 95% confidence interval of measurement differences between echo-planar and cine phase-contrast imaging, ranging from 2.0 to - 1058 mL/min was computed. Ultrafast phase-contrast flow measurements are possible. Multishot EPI phase-contrast imaging provides high measurement accuracy in pulsatile vessels while keeping the image acquisition interval short enough to be accomplished in a comfortable breath-hold.     

Aorta wall motion monitoring by 1-d MRI of perpendicular diameters.

A method for monitoring the cross-sectional size of blood vessels rapidly is introduced. The method creates a one-dimensional (1-D) profile of a strip along the diameter of a vessel using magnetic resonance imaging (MRI). The strips can be much wider than pixels in a typical two-dimensional (2-D) image to increase the signal-to-noise ratio. A second strip perpendicular to the first is also imaged sequentially to allow the detection, estimation, and correction of errors in diameter measurements resulting from the strip inadvertently covering a chord, rather than a diameter. Diameter measurements derived from 1-D profiles agree with measurements derived from 2-D images. This method is nearly an order of magnitude faster than 2-D MRI and has the potential for real-time implementation. J. Magn. Reson. Imaging 1999;10:833-840.     

Normal-incidence multilayer mirrors for the 120-450 å wavelength region

An electron-beam deposition method is used to fabricate spherical and flat mirrors with a peak reflectivity of 20% for wavelengths between 120 and 450 ?. It is experimentally shown that the fabricated mirrors can be employed to form an image with a resolution of 1 μm for ultrasoft x rays. The possibility of producing intense directional high-temperature plasma radiation by spherical multilayer mirrors is demonstrated. A radiation intensity of 107 W/cm2 around 182 ? has been experimentally obtained in the laser plasma image plane.     

Multistep phase difference phase contrast imaging

A new technique for multistep phase-contrast image processing is presented. The N-step method consists of simply forming the linear average of the N — 1 adjacent phase-difference signals. It has similar noise reduction properties as other multistep techniques, but the simplicity of the noise variance of the N-step technique allows intuitive insight into phase-difference phase-contrast processing and noise reduction, which can aid in the design of efficient and improved phase-contrast imaging sequences. As well, the computational simplicity of the N-step phase-difference technique compared with any other known multistep technique is advantageous. Like other multistep techniques, it has far more efficient noise reduction properties than simple two-step, multiple average phase-contrast imaging, even when normalized for total scan time. A three-step phase-difference velocity image has 50% less variance than an image acquired with two steps and two scans averaged but is obtained in 25% less scan time. Given its advantages, it should now be the chosen technique for increasing velocity-to-noise and contrast-to-noise ratios in all phase-difference phase-contrast clinical applications.     

Optimizing spatiotemporal sampling for k-t BLAST and k-t SENSE: application to high-resolution real-time cardiac steady-state free precession.

In k-t BLAST and k-t SENSE, data acquisition is accelerated by sparsely sampling k-space over time. This undersampling in k-t space causes the object signals to be convolved with a point spread function in x-f space (x = spatial position, f = temporal frequency). The resulting aliasing is resolved by exploiting spatiotemporal correlations within the data. In general, reconstruction accuracy can be improved by controlling the k-t sampling pattern to minimize signal overlap in x-f space. In this work, we describe an approach to obtain generally favorable patterns for typical image series without specific knowledge of the image series itself. These optimized sampling patterns were applied to free-breathing, untriggered (i.e., real-time) cardiac imaging with steady-state free precession (SSFP). Eddy-current artifacts, which are otherwise increased drastically in SSFP by the undersampling, were minimized using alternating k-space sweeps. With the synergistic combination of the k-t approach with optimized sampling and SSFP with alternating k-space sweeps, it was possible to achieve a high signal-to-noise ratio, high contrast, and high spatiotemporal resolutions, while achieving substantial immunity against eddy currents. Cardiac images are shown, demonstrating excellent image quality and an in-plane resolution of approximately 2.0 mm at >25 frames/s, using one or more receiver coils.     

X-ray phase-contrast imaging of the breast—advances towards clinical implementation

Breast cancer constitutes about one-quarter of all cancers and is the leading cause of cancer death in women. To reduce breast cancer mortality, mammographic screening programmes have been implemented in many Western countries. However, these programmes remain controversial because of the associated radiation exposure and the need for improvement in terms of diagnostic accuracy. Phase-contrast imaging is a new X-ray-based technology that has been shown to provide enhanced soft-tissue contrast and improved visualization of cancerous structures. Furthermore, there is some indication that these improvements of image quality can be maintained at reduced radiation doses. Thus, X-ray phase-contrast mammography may significantly contribute to advancements in early breast cancer diagnosis. Feasibility studies of X-ray phase-contrast breast CT have provided images that allow resolution of the fine structure of tissue that can otherwise only be obtained by histology. This implies that X-ray phase-contrast imaging may also lead to the development of entirely new (micro-) radiological applications. This review provides a brief overview of the physical characteristics of this new technology and describes recent developments towards clinical implementation of X-ray phase-contrast imaging of the breast.     

MR breast imaging : a comparative analysis of conventional and parallel imaging acquisition

PURPOSE: The objective of this study was to compare conventional breast magnetic resonance imaging (MRI) with breast MRI acquired with the sensitivity-encoding (SENSE) technique on a 1.5-T MRI scanner in the same patient, on the basis of image quality and kinetics analysis. MATERIALS AND METHODS: Thirty-one patients with suspicious mammography and US findings were included in the study. Conventional breast MRI consisted of the following sequences: T1 (matrix, 288 x 512); T2 (matrix 225 x 512); short tau inversion recovery (STIR) (matrix 320 x 224) and dynamic T1 [2D fast-field echo (FFE)] (matrix 256 x 512; temporal resolution相似文献   

General equations for optimal selection of diagnostic image acquisition parameters in clinical X-ray imaging

The purpose of this work was to examine the effects of relationship functions between diagnostic image quality and radiation dose on the governing equations for image acquisition parameter variations in X-ray imaging. Various equations were derived for the optimal selection of peak kilovoltage (kVp) and exposure parameter (milliAmpere second, mAs) in computed tomography (CT), computed radiography (CR), and direct digital radiography. Logistic, logarithmic, and linear functions were employed to establish the relationship between radiation dose and diagnostic image quality. The radiation dose to the patient, as a function of image acquisition parameters (kVp, mAs) and patient size (d), was used in radiation dose and image quality optimization. Both logistic and logarithmic functions resulted in the same governing equation for optimal selection of image acquisition parameters using a dose efficiency index. For image quality as a linear function of radiation dose, the same governing equation was derived from the linear relationship. The general equations should be used in guiding clinical X-ray imaging through optimal selection of image acquisition parameters. The radiation dose to the patient could be reduced from current levels in medical X-ray imaging.     

Image distortion correction in EPI: comparison of field mapping with point spread function mapping.

Echo-planar imaging (EPI) can provide rapid imaging by acquiring a complete k-space data set in a single acquisition. However, this approach suffers from distortion effects in geometry and intensity, resulting in poor image quality. The distortions, caused primarily by field inhomogeneities, lead to intensity loss and voxel shifts, the latter of which are particularly severe in the phase-encode direction. Two promising approaches to correct the distortion in EPI are field mapping and point spread function (PSF) mapping. The field mapping method measures the field distortions and translates these into voxel shifts, which can be used to assign image intensities to the correct voxel locations. The PSF approach uses acquisitions with additional phase-encoding gradients applied in the x, y, and/or z directions to map the 1D, 2D, or 3D PSF of each voxel. These PSFs encode the spatial information about the distortion and the overall distribution of intensities from a single voxel. The measured image is the convolution of the undistorted density and the PSF. Measuring the PSF allows the distortion in geometry and intensity to be corrected. This work compares the efficacy of these methods with equal time allowed for field mapping and PSF mapping.