Generalized MR interferography

Generalized MR interferography allows the measurement of parameters which affect the phase of the MR signal. Interference is generated by the simultaneous generation of two echoes in one excitation cycle. Local variations of the signal phase are transformed into displacement of the stripe pattern generated by interference. A number of possible experiments based on this principle are presented. The resulting interferographic images allow the fast determination of parameters such as field inhomogeneity, susceptibility, flow, and motion.     

Verification and evaluation of internal flow and motion. True magnetic resonance imaging by the phase gradient modulation method

We report qualitative and quantitative evaluation and verification studies of the bipolar phase gradient modulation method for true MR imaging of internal flow and motion velocities. Velocity encoding modulations provide speed-of-motion and direction-sensitive images using special phase-sensitive reconstructions. True motion MR imaging does not depend upon subject parameters, T1 or T2, nor upon selective active-volume time-of-flight calculations, nor is it limited strictly to fluid-flow velocities. Conventional MR sequences often induce strong accidental phase gradient modulations that can cause severe artifacts in conventional MR scans and limit the useful sensitivities of true motion MR. Multiple steps of velocity encoding allow resolution of separate elements of the velocity spectrum, and enable suppression of all such phase-artifact difficulties. Some view-to-view phase inconsistencies are intrinsic to the subject being scanned, e.g., strong motion variations during the heart cycle; limitations due to such effects require external modifications in the scanning, such as cardiac gating. Since conventional density information remains in the data, independent of velocity encoding modulations, we suggest a multiple encoding sequence and saving the MR raw data. These evaluations and verifications demonstrate exciting potential in clinical application for the phase gradient modulation method of true flow and motion MR imaging.     

Dyke award. Harmonic modulation of proton MR precessional phase by pulsatile motion: origin of spinal CSF flow phenomena

The effects of pulsatile motion on MR imaging of spinal CSF were quantitatively evaluated with a spine phantom that simulated spinal CSF pulsation. Two fundamental interdependent pulsation flow phenomena were observed: variable reductions in signal intensity of pulsatile CSF (signal loss) and spatial mismapping of this signal beyond the confines of the subarachnoid space (phase-shift images). Phase-shift images were observed as multiple regions of signal intensity conforming morphologically to the subarachnoid space but displaced symmetrically from it along the phase-encoding axis, either added to or subtracted from stationary signal intensity. Both CSF pulsation flow phenomena occurred secondary to harmonic modulation of proton precessional phase (temporal phase shift) by the unique pulsatile motion of spinal CSF when the repetition time was not an integral multiple of the pulsation period. Each flow phenomenon was analyzed with the spine phantom independently to control individual imaging and physiologic parameters including imaging plane, repetition time, echo time, slice thickness, number of echoes, number of excitations, CSF pulsation amplitude, and CSF pulsation period. In the axial plane, signal loss was present on both first- and second-echo images and was more pronounced with larger pulsation amplitudes and smaller slice thicknesses. A quantitative relationship between these two parameters allowed the prediction of CSF pulsation amplitude when the slice thickness was known and the CSF signal intensity was measured. In the sagittal plane, signal loss was present on first-echo images, was more pronounced with larger pulsation amplitudes, and underwent incomplete even-echo rephasing on second-echo images. Phase-shift images were influenced by the relationship between repetition time and CSF pulsation period. They were partly eliminated on sagittal but not on axial second-echo images because of incomplete even-echo rephasing. Both signal loss and phase-shift images were completely eliminated with CSF gating or pseudogating, indicating the rationale for gating during clinical spinal MR. The clinical significance of these findings is that awareness of the existence of spinal CSF pulsation flow phenomena avoids diagnostic confusion, whereas understanding their etiology provides a rational approach, such as CSF gating, to eliminate them.     

Motion artifacts in brain and spine MR

As a result of the effects of cerebrospinal fluid and blood motion, motion artifacts are common in brain and spine MR imaging. These artifacts, which are most common on T2-weighted spin-echo sequences, show up as streaking and ghosting in the phase-encoding (PE) direction (PE artifacts) and as loss of signal from flowing material (flow void). A review of the physical and physiologic causes of these artifacts and a review and explanation of several techniques that are useful in reducing or eliminating them are presented in this article.     

The application of phase shifts in NMR for flow measurement

A brief overview of the history of the application of phase shifts in NMR, and in particular NMR imaging, is presented. The imaging methods include direct phase mapping, Fourier flow imaging (where the flow data are Fourier transformed into one dimension of an image), and alternative methods, where flow-related phase shifts are utilized for flow measurement from the magnitude of the signal. A discussion then follows of the principal errors that can affect the accuracy of the various flow imaging techniques, with particular reference to the phase mapping methods that have been used extensively in our institution. The results from a number of experiments are included to illustrate the extent of the errors and methods of removing or minimizing these effects are suggested.     

Comparison of phantom and computer-simulated MR images of flow in a convergent geometry: Implications for improved two-dimensional MR angiography

The signal loss that occurs in regions of disturbed flow significantly decreases the clinical usefulness of MR angiography in the imaging of diseased arteries. This signal loss is most often attributed to turbulent flow; but on a typical MR angiogram, the signal is lost in the nonturbulent upstream region of the stenosis as well as in the turbulent downstream region. In the current study we used a flow phantom with a forward-facing step geometry to model the upstream region. The flow upstream of the step was convergent, which created high levels of convective acceleration. This region of the flow field contributes to signal loss at the constriction, leading to overestimation of the area of stenosis reduction. A computer program was designed to simulate the image artifacts that would be caused by this geometry in two-dimensional time-of-flight MR angiography. Simulated images were compared with actual phantom images and the flow artifacts were highly correlated. The computer simulation was then used to test the effects of different orders of motion compensation and of fewer pixels per diameter, as would be present in MR angiograms of small arteries. The results indicated that the computational simulation of flow artifacts upstream of the stenosis provides an important tool in the design of optimal imaging sequences for the reduction of signal loss.     

Cavernous sinus and inferior petrosal sinus flow signal on three-dimensional time-of-flight MR angiography.

BACKGROUND AND PURPOSE: Venous flow signal in the cavernous sinus and inferior petrosal sinus has been shown on MR angiograms in patients with carotid cavernous fistula (CCF). We, however, identified flow signal in some patients without symptoms and signs of CCF. This review was performed to determine the frequency of such normal venous flow depiction at MR angiography. METHODS: Twenty-five 3D time-of-flight (TOF) MR angiograms obtained on two different imaging units (scanners A and B) were reviewed with attention to presence of venous flow signal in the cavernous sinus or inferior petrosal sinus or both. Twenty-five additional MR angiograms were reviewed in patients who had also had cerebral arteriography to document absence of CCF where venous MR angiographic signal was detected, as well as to gain insight into venous flow patterns that might contribute to MR angiographic venous flow signal. Differences in scanning technique parameters were reviewed. RESULTS: Nine (36%) of the 25 MR angiograms obtained on scanner A but only one (4%) of the 25 obtained on scanner B showed flow signal in the cavernous or inferior petrosal sinus or both in the absence of signs of CCF. On review of 25 patients who had both MR angiography and arteriography, three patients with venous signal at MR angiography failed to exhibit CCF at arteriography. CONCLUSION: Identification of normal cavernous sinus or inferior petrosal sinus venous signal on 3D TOF MR angiograms may occur frequently, and is probably dependent on technical factors that vary among scanners. The exact factors most responsible, however, were not elucidated by this preliminary review.     

Troubleshooting Arterial-Phase MR Images of Gadoxetate Disodium-Enhanced Liver

Gadoxetate disodium is a widely used magnetic resonance (MR) contrast agent for liver MR imaging, and it provides both dynamic and hepatobiliary phase images. However, acquiring optimal arterial phase images at liver MR using gadoxetate disodium is more challenging than using conventional extracellular MR contrast agent because of the small volume administered, the gadolinium content of the agent, and the common occurrence of transient severe motion. In this article, we identify the challenges in obtaining high-quality arterial-phase images of gadoxetate disodium-enhanced liver MR imaging and present strategies for optimizing arterial-phase imaging based on the thorough review of recent research in this field.     

Technologic advances in abdominal MR imaging

Magnetic resonance (MR) imaging is finding an ever-growing role in the evaluation of a wide range of conditions in the abdomen. No longer confined to problem solving regarding abnormalities in solid organs, such as the liver and kidneys, MR imaging is increasingly being applied to the evaluation of the pancreatic and biliary ductal systems and even the bowel. Recent technical advances in hardware and software have allowed the acquisition of MR images that are largely free of artifact secondary to bowel peristalsis or respiratory motion; images providing excellent anatomic detail can now be obtained routinely. Faster sequences have reduced image acquisition time, thereby improving patient acceptance and allowing more efficient utilization of machine time. New three-dimensional sequences allow rapid image acquisition, reducing section misregistration and motion artifact while improving multiplanar reformations. The potential of MR imaging to provide functional and anatomic information is intriguing, and new techniques, including diffusion and perfusion imaging, are being evaluated. This review considers the advances in imaging hardware and pulse sequence design that underlie the increasing role of MR imaging in evaluation of the abdomen and discusses evolving clinical applications.     

Optimization of encoding gradients for MR‐ARFI

MR acoustic radiation force imaging provides a promising method to monitor therapeutic ultrasound treatments. By measuring the displacement induced by the acoustic radiation force, MR acoustic radiation force imaging can locate the focal spot, without a significant temperature rise. In this work, the encoding gradient for MR acoustic radiation force imaging is optimized to achieve an enhanced accuracy and precision of the displacement measurement. By analyzing the sources of artifacts, bulk motion and eddy currents are shown to introduce errors to the measurement, and heavy diffusion‐weighting is shown to result in noisy displacement maps. To eliminate these problems, a new encoding scheme is proposed, which utilizes a pair of bipolar gradients. Improved precision is achieved with robustness against bulk motion and background phase distortion, and improved accuracy is achieved with reduced diffusion‐weighting and optimized encoding pulse width. The experiment result shows that the signal‐to‐noise ratio can be enhanced by more than 2‐fold. These significant improvements are obtained at no cost of scan time or encoding sensitivity, enabling the detection of a displacement less than 0.l μm in a gel phantom with MR acoustic radiation force imaging. Magn Reson Med 63:1050–1058, 2010. © 2010 Wiley‐Liss, Inc.     

磁共振快速扫描技术在脊柱扫描中的应用（附304例分析）

磁共振成像（ＭＲＩ）是一新型、非损伤性检查方法。笔者采用Ｐｈｉｌｉｐｓ０．５Ｔ超导ＭＲ机的快速扫描技术，即本机型称为快速场回波序列（ＦＦＥ），对３０４例脊柱疾患扫描。提出了ＦＦＥ序列的优点，即可以增强影像的对比度，产生脊髓造影的效果；克服了ＭＲ检查时间长的缺点；提高了图像的信噪比；减少由于运动和血流产生的伪影，为脊柱ＭＲ检查的最佳序列。     

Variability of myocardial signal on magnetic resonance images

The diagnosis of myocardial disease by magnetic resonance (MR) imaging depends on accurate measurement of myocardial signal intensity. The authors performed 15 experiments in four rabbits at 1.9 T with spin-echo MR imaging to study the variability of myocardial signal intensity throughout the cardiac cycle and to measure myocardial T2 values. Variability in signal from the myocardium throughout the cardiac cycle was observed in all experiments. During systole, a significant increase in myocardial signal was noted, when data acquisition was performed with electrocardiogram (ECG)-gating and controlled ventilation (P = .02). An inverse relationship between myocardial signal and phase noise was found, indicating the motion-related nature of the variation of myocardial signal. A similar inverse relationship was observed in images obtained from a normal human volunteer. Ex vivo myocardial T2 values of rabbit myocardial tissue were significantly higher than the in vivo values (P = .003), reflecting residual motion despite cardiac gating and controlled ventilation.     

Optimal use of contrast medium in contrast enhanced MR imaging of the heart

With the recent development of fast MR imaging techniques, the diagnostic value of contrast enhanced MR imaging of the heart has been substantially improved. Since no tissue-specific contrast medium is available for clinical cardiac MR imaging at this point, both the early and late dynamics of extracellular MR contrast medium should be carefully evaluated for assessing the multiple aspects of cardiac function, including myocardial blood flow, myocardial, viability, and left ventricular function. Myocardial blood flow can be assessed by monitoring the first-pass passage of MR contrast medium. Quantitative assessments of arterial input function and output function in the regional myocardium can provide more accurate detection of altered myocardial blood flow in patients with coronary artery disease. Excellent contrast between infarcted myocardium and normal tissue can be obtained with delayed contrast enhanced MR imaging. Myocardial infarction, including small subendocardial infarction and chronic scar, is demonstrated as an area of "hyperenhancement" on delayed enhanced MR images, while the signal from normal myocardium is nearly null. This review paper describes the optimal dose and injection rate of MR contrast material for functional cardiac MR imaging studies. In addition, practical suggestions for obtaining good cardiac MR images and interpreting contrast enhanced MR images are given and are explained in detail.     

Ultrasound and MR imaging of diabetic mastopathy

AIM: To review the imaging findings of diabetic mastopathy, and document the colour flow ultrasound and MR imaging features in this benign condition. MATERIALS AND METHODS: Diabetic mastopathy was clinically and histologically diagnosed in eight lesions in six women. All six women underwent conventional mammography and high frequency grey-scale ultrasound. Colour flow ultrasound was performed additionally in six lesions in four women and MR imaging in four lesions in three women before biopsy. The imaging findings were reviewed and correlated with final histological diagnosis. RESULTS: Mammography showed regional asymmetric increased opacity with ill-defined margins in all lesions. A heterogeneously hypoechoic mass with ill-defined margins was identified on high frequency grey-scale ultrasound in all lesions. Marked posterior acoustic shadowing was present in seven of eight (88%) lesions. Six lesions interrogated with colour flow ultrasound showed absence of Doppler signal. MR imaging in three women revealed non-specific stromal enhancement. CONCLUSION: Diabetic mastopathy shows absence of Doppler signal on colour flow ultrasound and non-specific stromal enhancement on MR imaging.     

ECG-optimized phase contrast line-scanned MR angiography

We describe a rapid phase contrast line scan MR angiographic imaging technique. A projection angiogram is obtained by sequentially imaging a series of thin slices oriented perpendicular to the primary flow direction. Bipolar gradient subtraction is employed to suppress signal from static tissues, which in turn allows elimination of phase encoding in the depth dimension. The sequence is cardiac gated to improve image quality and to allow observation of hemodynamics. To further improve image quality, the amplitude of the bipolar gradient is altered throughout the cardiac cycle to provide maximum vessel signal at all cardiac phases. The ECG-gated phase contrast line scan sequence has been used to image regions where cardiac pulsatility and respiratory motion compromise the quality of images obtained using standard spin warp angiographic methods. © 1992 Academic Press, Inc.     

Detection and quantification of valvular heart disease with dynamic cardiac MR imaging.

Magnetic resonance (MR) imaging is rapidly gaining acceptance as an accurate, reproducible, noninvasive method for optimal assessment of structural and functional parameters in patients with valvular heart disease. The severity of valvular regurgitation can be evaluated with cine gradient-echo MR imaging, which allows measurement of the area of the signal void corresponding to the abnormal flow jet. Alternatively, this modality can be used to obtain ventricular volumetric measurements and calculate the regurgitant fraction, or velocity-encoded cine (VEC) MR imaging can be used to quantify regurgitant blood flow. The severity of valvular stenosis can be determined by evaluating the flow jet and associated findings with either modality or by using VEC MR imaging to calculate the transvalvular pressure gradient and valve area. Dynamic MR imaging allows accurate assessment of ventricular function and comprehensive evaluation of pathophysiologic changes. In addition, good interstudy reproducibility suggests a role for VEC MR imaging in assessing the effects of therapeutic intervention and monitoring regurgitant fraction, thereby helping in surgical planning and the prevention of ventricular dysfunction. With greater cost-effectiveness and the increasing availability of new hardware and more advanced techniques, MR imaging will become a routine procedure in valvular heart disease.     

Coronary flow and flow reserve in canines using MR phase difference and complex difference processing

Coronary artery disease continues to be the leading cause of death for adults in the United States. Magnetic resonance imaging (MR) has the potential to dramatically impact the diagnosis of heart disease by noninvasively providing a wide range of anatomic and physiologic information. Previous re search has shown that coronary flow, one component of a complete examination, can be accurately measured in the left anterior descending artery in viva The current work validates MR flow measurements in canine circumflex arteries using transit time ultrasound as a standard. The circumflex artery experiences greater in-plane motion and is a more stringent test for flow measurement accuracy. This work also com pares two methods of processing MR velocity data, phase difference and complex difference techniques, and examines the sources of error present in the animal validation model. Phase difference processing with a 30% magnitude threshold best matched the mean ultrasound flow values (30% PD = 1.04 × US + 1.49, r = 0.94), but it was very sensitive to vessel boundary identification. The complex difference process was less sensitive to vessel boundary identification and correlated well with the transit time ultrasound despite systematic un-derestimations. The reasons for the discrepancies are shown to stem from a number of possible sources including variabil ity of the ultrasound standard, low signal-to-noise ratios in the MR images, sensitivity of the MR technique to vessel bound ary identification, and motion artifacts in the images.     

MRI measurement of hepatic magnetic susceptibility-phantom validation and normal subject studies.

A magnetic resonance (MR) imaging method with the potential for assessing hepatic iron overload from measurements of hepatic magnetic susceptibility in vivo is described. Using the blood in the portal and hepatic veins as an internal reference, this technique uses the orientation dependence of signal phase to measure the susceptibility of the liver parenchyma. Computer simulations were done to investigate the requirements on spatial resolution and contrast ratio between the vessels and the background liver tissue for data acquisition. Validation studies were conducted using tube-embedded gel phantoms doped with iron-dextran from 0 to 10 mg Fe/mL to mimic healthy and iron-overloaded livers. The phantom measurements were conducted without motion and flow, under respiration-like oscillatory motion, and with flow. Studies on six normal human subjects demonstrated excellent reproducibility and precision. All images were collected at 1.5 T using a 3D T(1)-weighted turbo field echo sequence for inflow MR angiographies with full flow compensation and capable of cardiac synchronization, navigator gating, and motion correction.     

Status and development of new clinical contrast media for MR diagnosis of liver diseases]

MR contrast agents are developed for pharmaceutical manipulation of tissue signal intensities. Today it is widely recognized that MR contrast agents will play an increasingly important role in MR imaging of the liver. Contrast-enhanced MR-imaging allows to obtain simultaneously dynamic physiologic information and high anatomic detail. Up to now three major classes of MR contrast agents are available for clinical MR-imaging of the liver. These include paramagnetic perfusion agents, hepatobiliary agents, and super-paramagnetic RES-specific iron oxide particles. A fourth class of contrast agents now in use for animal experiments includes ultra-small super-paramagnetic particles which can be targeted to extra-reticuloendothelial structures such as asialoglycoprotein receptors of hepatocytes. In this article, we review recent advances in the development of MR contrast media and the clinical status of contrast-enhanced MR imaging of the liver.