Technical developments in MR angiography

MR Angiography: Contrast-Enhanced Acquisition Techniques

Contrast-enhanced MR angiography (CE-MRA) is a frequently used MR imaging technique for evaluating cardiovascular structures. In many ways, it is similar to contrast-enhanced computed tomography (CT) angiography, except a gadolinium-based contrast agent (instead of iodinated contrast) is injected. Although the physiological principles of contrast injection overlap, the technical factors behind enhancement and image acquisition are different. CE-MRA provides an excellent alternative to CT for vascular evaluation and follow-up without requiring nephrotoxic contrast and ionizing radiation. This review describes the physical principles, limitations, and technical applications of CE-MRA techniques.

MR Imaging of the Mesenteric Vasculature

MR angiography is a flexible imaging technique enabling morphologic assessment of mesenteric arterial and venous vasculature. Conventional gadolinium-based contrast media and ferumoxytol are used as contrast agents. Ferumoxytol, an intravenous iron replacement therapy approved by the US Food and Drug Administration for iron deficiency anemia, is an effective and well tolerated blood pool contrast agent. The addition of 4D flow MR imaging enables a functional assessment of the arterial and venous vasculature; when coupled with a meal challenge, the severity of mesenteric arterial stenosis is well appreciated. Noncontrast MR angiographic techniques are useful for evaluating suspected mesenteric ischemia.

Dealing with vascular conundrums with MR imaging

Magnetic resonance (MR) imaging is a robust imaging modality for evaluation of vascular diseases. Technological advances have made MR imaging widely available for accurate and time-efficient vascular assessment. In this article the clinical usefulness of MR imaging techniques and their application are reviewed, using examples of vascular abnormalities commonly encountered in clinical practice, including abdominal, pelvic, and thoracic vessels. Common pitfalls and problem solving in interpretation of vascular findings in body MR imaging are also discussed.

Arteriovenous shunt visualization in arteriovenous malformations with arterial spin-labeling MR imaging

BACKGROUND AND PURPOSE

A reliable quantitative technique for measuring arteriovenous (AV) shunt in vascular malformations is not currently available. Here, we evaluated the hypothesis that continuous arterial spin-labeled (CASL) perfusion MR imaging can be used to detect and measure AV shunt in patients with arteriovenous malformations (AVMs).

MATERIALS AND METHODS

CASL perfusion MR imaging was performed in 7 patients with AVMs. Semiquantitative AV shunt estimates were generated based on a thresholding strategy by using signal-intensity difference (DeltaM) images to avoid potential errors in cerebral blood flow (CBF) calculation related to abnormal transit times and nonphysiologic blood-tissue water exchange in and around the AVMs. The potential for measuring CBF in regions distant from and near the AVM was explored, as was the relationship of CBF changes related to the size of the shunt.

RESULTS

In all 7 cases, striking increased intensity was seen on CASL perfusion DeltaM maps in the nidus and venous structures draining the AVM. Shunt estimates ranged from 30% to 0.6%. Mean CBF measurements in structures near the AVMs were not significantly different from the contralateral measurements. However, CBF in adjacent ipsilateral white matter increased relative to the contralateral side as the percent shunt increased (P = .02). Cortical gray matter CBF Delta (contralateral-ipsilateral) values demonstrated the same effect, but the correlation was weak and not significant. Thalamic CBF decreased ipsilaterally with increasing percent AV shunt (P = .01), indicating a possible steal effect. Basal ganglia Delta values showed little change in CBF with the size of the AV shunt.

CONCLUSION

CASL perfusion MR imaging can demonstrate AV shunting, providing high lesion conspicuity and a novel means for evaluating AVM physiology.

Highly accelerated cardiovascular MR imaging using many channel technology: concepts and clinical applications

Cardiovascular magnetic resonance imaging (CVMRI) is of proven clinical value in the non-invasive imaging of cardiovascular diseases. CVMRI requires rapid image acquisition, but acquisition speed is fundamentally limited in conventional MRI. Parallel imaging provides a means for increasing acquisition speed and efficiency. However, signal-to-noise (SNR) limitations and the limited number of receiver channels available on most MR systems have in the past imposed practical constraints, which dictated the use of moderate accelerations in CVMRI. High levels of acceleration, which were unattainable previously, have become possible with many-receiver MR systems and many-element, cardiac-optimized RF-coil arrays. The resulting imaging speed improvements can be exploited in a number of ways, ranging from enhancement of spatial and temporal resolution to efficient whole heart coverage to streamlining of CVMRI work flow. In this review, examples of these strategies are provided, following an outline of the fundamentals of the highly accelerated imaging approaches employed in CVMRI. Topics discussed include basic principles of parallel imaging; key requirements for MR systems and RF-coil design; practical considerations of SNR management, supported by multi-dimensional accelerations, 3D noise averaging and high field imaging; highly accelerated clinical state-of-the art cardiovascular imaging applications spanning the range from SNR-rich to SNR-limited; and current trends and future directions.

Parallel imaging in cardiovascular MRI: methods and applications

Cardiovascular MR imaging (CVMR) has become a valuable modality for the non-invasive detection and characterization of cardiovascular diseases. CVMR requires high imaging speed and efficiency, which is fundamentally limited in conventional cardiovascular MRI studies. With the introduction of parallel imaging, alternative means for increasing acquisition speed beyond these limits have become available. In parallel imaging some image data are acquired simultaneously, using RF detector coil sensitivities to encode simultaneous spatial information that complements the information gleaned from sequential application of magnetic field gradients. The resulting improvements in imaging speed can be used in various ways, including shortening long examinations, improving spatial resolution and/or anatomic coverage, improving temporal resolution, enhancing image quality, overcoming physiological constraints, detecting and correcting for physiologic motion, and streamlining work flow. Examples of each of these strategies will be provided in this review. First, basic principles and key concepts of parallel MR are described. Second, practical considerations such as coil array design, coil sensitivity calibrations, customized pulse sequences and tailored imaging parameters are outlined. Next, cardiovascular applications of parallel MR are reviewed, ranging from cardiac anatomical and functional assessment to myocardial perfusion and viability to MR angiography of the coronary arteries and the large vessels. Finally, current trends and future directions in parallel CVMR are considered.

Sensitivity encoding (SENSE) for contrast-enhanced 3D MR angiography of the abdominal arteries

PURPOSE

To assess sensitivity encoding (SENSE) for contrast-enhanced MR angiography (CE-MRA) of the abdominal arteries in comparison with standard MRA protocols.

MATERIALS AND METHODS

In 22 patients MRA of the abdominal arteries was performed twice (once using a standard protocol, and once with the additional use of SENSE). In 10 patients all examination parameters were kept constant (TR/TE/FA = 3.8 msec/1.3 msec/30 degrees ), and a reduction in scan time from 22 to 11 seconds was realized with the use of SENSE. In 12 patients, using SENSE the acquisition matrix was increased from 208 to 416, keeping the scan time constant. Image quality was scored on a five-point scale by three radiologists. Additionally, ROI-based measurements of CNR were performed.

RESULTS

For both protocols, image quality was significantly improved using SENSE. The time-reducing SENSE protocol yielded an average score of 4.2 points vs. 3.1 for the standard protocol. Using SENSE to increase the acquisition matrix, an average score of 4.3 was reached vs. 3.2 for the standard protocol (P < 0.05). The number of depictable small vessels and their bifurcations was significantly increased by either of the two SENSE protocols as compared to the standard imaging procedure.

CONCLUSION

SENSE for MRA of the abdominal arteries significantly increases image quality and permits a substantial reduction in breath-hold time or a significantly improved spatial resolution.

Magnetic resonance angiography in children: technique, indications, and imaging findings

Non-invasive imaging of cardiovascular disease in pediatric subjects has been an elusive and much anticipated development for many years. Magnetic resonance angiography (MRA) has now been established in many institutions as an important diagnostic method for evaluating vascular disease in adults. However, MRA techniques have disseminated more slowly in children owing to significant technical challenges in the pediatric population, including motion, low signal-to-noise ratio, and suboptimal temporal or spatial resolution. Recent technical developments in MRA have addressed many of these issues, and the MRA acquisition methods are far more robust than previously used techniques. The objective of this manuscript is to discuss the indications for MR imaging, the techniques employed, and the imaging findings expected on MRA of children with vascular disease.

Magnetic resonance imaging of the vascular system: a practical approach for the radiologist

Contrast-enhanced magnetic resonance angiography (CE-MRA) has benefited from rapid technologic developments, including specific hardware and pulse sequence design. This article provides a brief practical overview of technique together with clinical examples of utility in daily application, from the view of an interventional radiologist. CE-MRA is rapidly replacing catheter-based diagnostic angiography for examination of the carotid arteries, aorta, renal arteries, and lower extremity.

CTA and MRA: visualization without catheterization

The ideal modality for vascular imaging would be noninvasive and inexpensive. A volumetric acquisition would permit visualization of vessels from arbitrary angles. High contrast between the vessel lumen and background tissue would be coupled with excellent spatial resolution allowing accurate depiction of small vessels. Characterization of the constituent components of the vessel wall would be possible. High temporal resolution would both freeze the motion of fast moving vessels and show the direction and speed of blood flow. Finally, the modality would expose the patient to a minimal amount of ionizing radiation or potentially toxic contrast agents. Diagnostic conventional catheter angiography offers unsurpassed spatial and temporal resolution. However, catheter angiography is an interventional procedure, exposes the patient to both ionizing radiation and iodinated contrast, and does not depict the vessel wall. Additionally, view angles are chosen before the administration of contrast and may not demonstrate certain lesions. These limitations have driven the development of both computed tomography angiography (CTA) and magnetic resonance angiography (MRA). Both of these modalities rapidly acquire volumetric data sets, which can then be evaluated slice by slice or by more advanced volumetric rendering techniques. CTA and MRA are minimally invasive and less costly than angiography. While CTA and MRA cannot compete with the spatial or temporal resolution of conventional angiography, present technology has proven clinical efficacy in a wide range of applications. The principles behind CTA and MRA and their comparative strengths and weaknesses will be discussed. The different volumetric rendering techniques will be reviewed. Finally, recent advances that will likely further improve these modalities will be summarized.

Healthcare technology, economics, and policy: an evolving balance

Economic and policy issues increasingly influence healthcare technology solutions as we wrestle with keeping healthcare quality high while keeping costs under control. The healthcare system is also undergoing rapid change brought about by managed care, shifting care to team providers in an outpatient or home environment. The development of health technologies under new market arrangements needs to be examined so that quality of care and cost are not at cross roads. As these issues continue to evolve in the public forum, it is critical for engineers and contributors to technology in healthcare to understand the dynamics that influence technology need and and adoption in the healthcare market. We hope that this special issue will provide a basis for initial understanding of these issues and stimulate further involvement by the bioengineering community at large, for the improvement of healthcare across all of society.