Quantification of flow dynamics in congenital heart disease: applications of velocity-encoded cine MR imaging.

Velocity-encoded cine (VEC) magnetic resonance (MR) imaging is a valuable technique for quantitative assessment of flow dynamics in congenital heart disease (CHD). VEC MR imaging has a variety of clinical applications, including the measurement of collateral flow and pressure gradients in coarctation of the aorta, differentiation of blood flow in the left and right pulmonary arteries, quantification of shunts, and evaluation of valvular regurgitation and stenosis. After surgical repair of CHD, VEC MR imaging can be used to monitor conduit blood flow, stenosis, and flow dynamics. There are some pitfalls that can occur in VEC MR imaging. These include potential underestimation of velocity and flow, aliasing, inadequate depiction of very small vessels, and possible errors in pressure gradient measurements. Nevertheless, VEC MR imaging is a valuable tool for preoperative planning and postoperative monitoring in patients with CHD.     

CTA and MRA in mesenteric ischemia: part 2, Normal findings and complications after surgical and endovascular treatment

OBJECTIVE: A number of surgical and endovascular options exist for the treatment of acute and chronic mesenteric ischemia. Both surgical and endovascular treatments necessitate close clinical and imaging follow-up because the consequences of acute occlusions can be catastrophic. MDCT angiography (CTA) and contrast-enhanced MR angiography (MRA) are the preferred imaging techniques in this setting. CONCLUSION: We review the appearance of the normal and complicated surgical and endovascular treatment on CTA and MRA.     

Noninvasive imaging of treated cerebral aneurysms, Part II: CT angiographic follow-up of surgically clipped aneurysms

Although not useful for the evaluation of coiled aneurysms, CT angiography (CTA) is far superior to MR angiography (MRA) for the evaluation of aneurysms after surgical clipping. Using the latest multidetector row scanners and optimized imaging parameters, CTA can often effectively depict and follow small aneurysm remnants; demonstrate patency, stenosis, or vasospasm in the adjacent parent vessels; and provide surveillance of the entire cerebrovasculature for de novo aneurysms after surgical clipping. Despite these advances, conventional angiography remains the gold standard for the evaluation of surgically treated aneurysms and should be liberally used to resolve any cases of diagnostic uncertainty on noninvasive imaging.     

Introduction to cardiac imaging in infants and children: techniques, potential, and role in the imaging work-up of various cardiac malformations and other pediatric heart conditions

The increasing prevalence of congenital heart disease (CHD) can be attributed to major improvements in diagnosis and treatment. Although echocardiography is the most commonly used imaging modality for diagnosis and follow-up of subjects with CHD, the evolution of cardiovascular magnetic resonance (MR) imaging and increasingly computed tomography (CT) does offer new ways to visualize the heart and the great vessels. The development of cardiovascular MR techniques allows for a comprehensive assessment of cardiac anatomy and function. This provides information about the long-term sequlae of the underlying complex anatomy, hemodynamic assessment of residual post-operative lesions and complications of surgery. As much of the functional data in CHD patients is usually acquired with invasive X-ray angiography, non-invasive alternatives such as cardiovascular MR (and CT) are desirable. This review evaluates the role of MR imaging in the management of subjects with CHD, particularly detailing recent developments in imaging techniques as they relate to the various CHD diagnoses we commonly encounter in our practice.     

Relative charaterisitics of MR angiography and competing vascular imaging modalities

This article reviews the general characteristics of several vascular imaging modalities with the purpose of identifying the distinguishing features of magnetic resonance (MR) angiography. Brief discussions of conventional x-ray film angiography, intravenous and intraarterial digital subtraction angiography (DSA), duplex and color Doppler flow ultrasound (US), computed tomographic (CT) angiography, transesophageal and intravascular US, angioscopy, and MR angiography are presented. The advantages and disadvantages of each are discussed. The general attributes and image quality features of MR angiography, intraarterial DSA, CT angiography, and US are compared. It is concluded that no single imaging modality will presently suffice for all purposes. Because of its noninvasiveness, rapidly improving image quality, and ability to directly provide velocity information, MR angiography is likely to play a role in an increasing number of clinical applications.     

MR angiography after renal revascularization: spectrum of expected anatomic results and postintervention complications.

The use of magnetic resonance (MR) angiography in screening for renal artery stenosis has been extensively evaluated. However, the MR angiographic findings after renal artery revascularization are not as well characterized. The renal artery and parenchyma can be evaluated after revascularization with a comprehensive MR imaging protocol that includes T1- and T2-weighted spin-echo sequences, three-dimensional (3D) gadolinium-enhanced MR angiography, and 3D phase-contrast MR angiography. Because surgical techniques for revascularization vary, knowledge of the surgical procedure is necessary to ensure inclusion of the pertinent anatomy at 3D gadolinium-enhanced MR angiography and to define appropriate 3D phase-contrast MR angiography volumes. The 3D gadolinium-enhanced MR angiography volume can be manipulated to view relevant vascular anatomy at the optimal obliquity and section thickness. This protocol allows robust, noninvasive evaluation of the expected arterial anatomy after revascularization, including renal artery endarterectomy, aortorenal bypass grafts, and extraanatomic reconstructions. In cases of suspected postrevascularization complications, gadolinium-enhanced MR angiography is useful because of its lack of nephrotoxicity and radiation exposure. Immediate complications of renal revascularization include renal artery thrombosis, renal infarction, and perinephric hemorrhage. Long-term complications include aneurysms of bypass grafts and recurrent stenosis of the renal artery.     

3.0-Tesla MR angiography of intracranial aneurysms: comparison of time-of-flight and contrast-enhanced techniques

PURPOSE: To determine whether 3.0-T elliptical-centric contrast-enhanced (CE) magnetic resonance (MR) angiography is superior to 3.0-T elliptical-centric time-of-flight (TOF) MR angiography in the detection and characterization of intracranial aneurysms, and to determine whether increasing the acquisition matrix size in 3.0-T CE MR angiography improves image quality. MATERIALS AND METHODS: A total of 50 consecutive patients referred for MR angiographic evaluation of a known or suspected intracranial aneurysm underwent MR angiography, including three-dimensional TOF and elliptical-centric CE techniques at 3.0 T. The 3.0-T three-dimensional TOF and 3.0-T CE examinations were graded for image quality. A blind review identified the presence and location of aneurysms. RESULTS: A total of 28 aneurysms were identified in 23 of the 50 patients. The 3.0-T TOF MR angiography had a higher mean score for image quality than the 3.0-T elliptical-centric CE MR angiography (P < 0.0001). A total of 14 patients with aneurysms had conventional angiography for comparison. The 3.0-T TOF showed all the aneurysms, whereas 3.0-T CE MR angiography did not show 1 of 19 aneurysms when conventional angiography was the reference standard. CONCLUSION: For imaging intracranial aneurysms, 3.0-T TOF MR angiography offers better image quality than 3.0-T CE MR angiography using the elliptical-centric technique.     

MR-Tomographie des Herzens

The introduction of magnetic resonance (MR) tomography has fundamentally changed radiological diagnosis for many diseases. Invasive digital subtraction angiography has already been widely replaced by noninvasive MR angiography for most of the vascular diseases. The rapid technical development of MR imaging in recent years has opened new functional imaging techniques. MR imaging of the heart allows simultaneous measurement of morphological and functional parameters in a single noninvasive examination without any radiation exposure. Because of the high spatial resolution and the reproducibility cine MR imaging is now the gold standard for functional analysis. With the improvement of myocardial perfusion and viability studies many diseases of the heart can be diagnosed in a single examination. MR spectroscopy is the only method which allows a view of the metabolism of the heart. New examinations for vascular imaging and flow quantification complete the goal of "one-stop-shop" imaging of the heart. MR imaging is the only diagnostic modality which allows a complete evaluation of many diseases of the heart with one technique, basic examination as well as follow-up studies. The very rapid improvement in MRI will overcome most of the limitations in the near future, especially concerning MR coronary angiography.     

Intracranial vascular abnormalities: value of MR phase imaging to distinguish thrombus from flowing blood

The interpretation of conventional spin-echo and gradient-echo MR images of intracranial vascular lesions can be complex and ambiguous owing to variable effects on image intensity caused by flowing blood or thrombus. MR phase images, obtained simultaneously with conventional-magnitude images, are useful for evaluating proton motion (i.e., blood flow), and therefore can simplify the diagnosis of the presence or absence of thrombosis within a vascular structure or lesion. Fourteen patients with a variety of intracranial vascular abnormalities (aneurysms, superior sagittal sinus thrombosis, neoplasms adjacent to venous sinuses, and vascular malformations) were evaluated with conventional MR and phase imaging for the presence of blood flow. The phase images correlated with angiography in all cases. Phase imaging was not necessarily better than conventional spin-echo imaging in all cases, but it simplified the evaluation of thrombus vs blood flow in many. In three of five aneurysms, the phase images were diagnostic for evaluating lumen patency whereas the conventional images were ambiguous. Phase imaging was advantageous for detecting tumor invasion of the venous sinus when venous blood was enhanced by gadopentetate dimeglumine. A laminar flow phantom experiment determined the lower limits of sensitivity of phase imaging to be 0.5 cm/sec in the slice-select and 2.5 cm/sec in the read gradient directions. Phase imaging is a simple, reliable technique that can distinguish thrombosis from flowing blood within intracranial lesions. It is easily performed and adds no additional time to the MR examination.     

Spinal cord feeding arteries at MR angiography for thoracoscopic spinal surgery: feasibility study and implications for surgical approach

PURPOSE: To prospectively investigate the feasibility of contrast material-enhanced magnetic resonance (MR) angiography for visualization of the spinal vasculature in patients referred for video-assisted thoracoscopic surgical treatment of a thoracic herniated disk and to prospectively assess the influence of preoperative imaging of the spinal vasculature on the choice of surgical approach. MATERIALS AND METHODS: Eight patients (three men and five women; mean age, 58 years; range, 42-83 years) with a thoracic herniated disk underwent contrast-enhanced MR angiography of the thoracoabdominal aorta and posterior branches. Imaging was performed with three-dimensional first-pass contrast-enhanced MR angiographic technique and a triple dose of gadolinium-based contrast agent. Images were analyzed by two observers in consensus to localize the Adamkiewicz artery (AKA) and its connections to the aorta and the anterior spinal artery (ASA). This information was used to determine any change in surgical approach. RESULTS: In all eight patients, the AKA, the ASA, and the connections with the aorta were identified. The AKA originated between T9 and L2 in all patients and derived from the left side of the aorta in 75% (six of eight) of the patients. In three patients in whom the AKA was observed on the left side, the surgical approach was changed to the right side to preserve spinal cord integrity. CONCLUSION: Preoperative imaging of the AKA is feasible with contrast-enhanced MR angiography. Contrast-enhanced MR angiography can be used to image the main feeding arteries of the spinal cord in patients undergoing thoracoscopic spinal surgery, and results can be used to change the side of surgical approach.     

Spinal vascular malformations: MR angiography after treatment

PURPOSE: To evaluate the role of magnetic resonance (MR) angiography in the assessment of spinal vascular malformation therapy. MATERIALS AND METHODS: Thirty-four patients with spinal vascular malformations (30 dural arteriovenous fistulas, two perimedullary arteriovenous fistulas, and two intramedullary arteriovenous malformations) underwent MR angiography and MR imaging before and after endovascular or surgical treatment. RESULTS: MR angiography showed residual flow in perimedullary vessels in seven patients with dural fistula after embolization with liquid adhesive. In all seven, treatment failure was confirmed with arteriography. Long-lasting disappearance of flow in perimedullary vessels was demonstrated at MR angiography in 22 patients with dural fistula. MR imaging demonstrated normalization of spinal cord volume in 16 of 22 patients and signal intensity on T2-weighted images in three patients. Disappearance of cord enhancement was observed in five of 21 patients and of perimedullary enhanced vessels in six of 13 patients. In one additional patient with dural fistula treated with embolization, early posttreatment MR angiography showed disappearance of flow in perimedullary vessels, which reappeared at follow-up and was consistent with reopening of a small residual fistula. Posttreatment MR angiography demonstrated transient reduction of flow in the nidus in two patients with intramedullary malformations treated with embolization. Permanent disappearance of flow in the perimedullary vessel was seen after endovascular treatment in two patients with perimedullary fistula. CONCLUSION: MR angiography is more sensitive than MR imaging in depicting residual or recurrent flow in peri- or intramedullary vessels, which indicates patency of the vascular malformation.     

Time-resolved 3D MR angiography with parallel imaging for evaluation of hemodialysis fistulas and grafts: initial experience

OBJECTIVE: We optimized a time-resolved 3D contrast-enhanced MR angiography sequence with integrated parallel imaging technique that can provide a large field of view with high temporal and spatial resolution, by which the hemodialysis access and the entire course of the inflow and outflow vessels can be imaged at a single anatomic station. Our objective was to evaluate the feasibility and usefulness of this method in the evaluation of patients referred for possible abnormalities in hemodialysis access. CONCLUSION: Time-resolved contrast-enhanced 3D MR angiography with parallel imaging has the potential to provide a rapid and comprehensive evaluation for the surveillance and diagnosis of hemodialysis access malfunctions. This technique may function as an important complement to conventional digital subtraction angiography and may be able to help guide medical management. The MR angiography protocol we present is a noninvasive, versatile, and time-efficient technique, without the need of direct graft puncture or flow interruption, and can be performed using a single injection of contrast material at a single station.     

Pelvic arteriovenous malformations: gadolinium-enhanced three-dimensional MR angiography findings

Pelvic arteriovenous malformations (PAVMs) are rare disorders traditionally diagnosed by conventional angiography. Breath-hold three-dimensional gadolinium-enhanced MR angiography (3D-Gd-MRA) is a state-of-the-art alternative for vascular imaging. We describe the 3D-Gd-MRA findings in two patients with PAVMs. The 3D-Gd-MRA approach provides a noninvasive and versatile method for evaluation of PAVMs that enables both angiographic assessment of the malformations and evaluation of visceral involvement, which can preclude surgical intervention. Received: 15 September 1999; Revised: 27 December 1999; Accepted: 27 December 1999     

Evaluation of two-dimensional thick-slice MR DSA: preliminary study]

We have used two-dimensional contrast-enhanced MR angiography for a single thick slice and called it MR DSA. This technique realizes nearly subsecond scanning per image and is therefore superior to other types of contrast-enhanced MR angiography in temporal resolution. To determine the optimal parameters of this technique, we calculated SSR (the signal of brain to the signal of contrast agent ratio) in various settings. We also obtained MR DSA images of 26 cases. We believe that MR DSA may play a supportive role in conventional MR imaging or angiography, since its high temporal resolution is of value in the evaluation of cerebrovascular diseases.     

Evaluation of surgical procedures for cyanotic congenital heart disease by using MR imaging

ECG-gated MR imaging has been shown to be effective for the diagnosis of congenital heart disease. In this study, we assessed its role in the postoperative evaluation of surgical procedures in patients with complex congenital heart disease. MR images of 26 patients with Rastelli (five), Fontan (three), Senning (three), Damus (one), Jatene (eight), Waterston (four), and Potts (two) procedures were evaluated retrospectively. The accuracy of MR imaging was compared with that of angiography in 20 patients. The surgical anastomoses were identified in all patients. Patency, atresia, or hypoplasia of central pulmonary arteries and postoperative complications (focal stenoses of pulmonary arteries, thrombosed conduit, peri-conduit abscess) were shown. Narrowing of the right ventricular outflow tract and focal compression of the proximal pulmonary arteries were recognized as specific complications of the Jatene procedure. MR imaging appears to be effective in the postoperative evaluation of surgical procedures used for congenital heart disease. It should be considered as an alternative to repeated catheterization and angiography for the postoperative examination of children with complex congenital heart disease.     

Intraindividual comparison of 3D coronary MR angiography and coronary CT angiography

RATIONALE AND OBJECTIVES: To compare the diagnostic value of magnetic resonance (MR) and computed tomography (CT) for the detection of coronary artery disease (CAD) with special regard to calcifications. MATERIALS AND METHODS: Twenty-seven patients with known CAD were examined with a targeted, navigator-gated, free-breathing, steady-state free precession MR angiography sequence (repetition time = 5.6 milliseconds, echo time = 2.8 milliseconds, flip angle 110 degrees ) and 16-slice coronary CT angiography. Segment-based sensitivity, specificity, and accuracy for the detection of stenoses larger than 50% were determined as defined by the gold standard catheter coronary angiography along with the subjective image quality (Grade 1-4). The degree of calcifications in each segment was quantified using a standard calcium scoring tool. RESULTS: Of 115 possible segments, 7% had to be excluded in MR imaging because of poor image quality. In CT, 3% were nondiagnostic because of image quality and 15% were not evaluable because of calcifications. Values for the detection of relevant coronary artery stenoses in the evaluated segments were: sensitivity: MR imaging 85% versus CT 96%; specificity: 88% versus 96%; accuracy: 87% versus. 96%. Average subjective image quality was 1.8 for MR imaging and 1.6 for CT. Of the 15% of segments that had to be excluded from CT evaluation because of calcifications, MR imaging provided the correct diagnosis segments in 67%. CONCLUSIONS: CT provided a better image quality with superior accuracy for the detection of CAD. Despite its overall inferiority, MR imaging proved to be helpful method in interpreting coronary stenosis in severely calcified segments.     

MR imaging of coronary artery flow in isolated and in vivo hearts.

Methods for imaging flow in coronary arteries with magnetic resonance (MR) imaging techniques are demonstrated in isolated heart preparations and live animal models. Coronary artery flow was first imaged with a flow-compensated gradient-echo pulse sequence in isovolumic and working perfused rat hearts and then in vivo. A bolus tracking technique was used to measure flow velocity in the coronary arteries. Ultrafast gradient-echo imaging techniques were then applied, with high resolution obtained by combining the information from several cardiac cycles. A stimulated-echo pulse sequence was demonstrated as a method for performing coronary angiography by flow tagging in isovolumic perfused hearts. This report describes the results of coronary flow MR imaging in isolated rat hearts and live mice and rats. The general approach has proved useful in evaluating new methods for coronary MR angiography and should permit well-controlled studies of pathologic conditions. This ability to image coronary flow in isolated hearts and in small animals should permit integrated MR studies of coronary flow, myocardial perfusion, myocardial metabolism, and cellular ionic status.     

Peripheral vascular disease: correlation of MR imaging and angiography

The capability of magnetic resonance (MR) imaging for detecting aortic, iliac, and femoral stenoses and occlusions was evaluated. Multisection spin-echo studies at 0.35 tesla were obtained of the infrarenal aorta to the femoral bifurcation in 24 patients, all of whom had undergone intraarterial angiography within 14 days of imaging. Transaxial MR images were compared with the angiograms. Arterial stenoses and occlusions in these vessels detected by MR imaging correlated with angiographic findings in 91% of the instances. Protrusional atherosclerotic plaques and occlusions and stenoses in the aortoiliac region were demonstrated accurately on MR images; complications of previous vascular surgery, such as aneurysms at sites of previous anastomoses or endarterectomy, were also identified. Due to the limited spatial resolution, MR images failed to demonstrate some femoral stenoses. MR imaging may be used for evaluation of aortoiliac vascular disease and for follow-up study after surgical revascularization. However, the limited spatial resolution, noncomposite display of the aortoiliofemoral circulation, and lack of evaluation of peripheral runoff provided by current MR imaging techniques militate against its replacing angiography prior to vascular intervention.     

Interventions in Infrainguinal Bypass Grafts

The interventional radiologist plays an important role in the detection and prevention of infrainguinal bypass failure. Early detection and evaluation of flow-limiting lesions effectively preserve graft (venous bypass and polyester or expanded polytetrafluoroethylene bypass) patency by identifying stenoses before occlusion occurs. Delay in treatment of the at-risk graft may result in graft failure and a reduced chance of successful revascularization. For this reason, surveillance protocols form an important part of follow-up after infrainguinal bypass surgery. As well as having an understanding of the application of imaging techniques including ultrasound, MR angiography, CT angiography and digital subtraction angiography, the interventional radiologist should have detailed knowledge of the minimally invasive therapeutic options. Percutaneous transluminal angioplasty (PTA), or alternatively cutting balloon angioplasty, is the interventional treatment of choice in prevention of graft failure and occlusion. Further alternatives include metallic stent placement, fibrinolysis, and mechanical thrombectomy. Primary assisted patency rates following PTA can be up to 65% at 5 years. When the endovascular approach is unsuccessful, these therapeutic options are complemented by surgical procedures including vein patch revision, jump grafting, or placement of a new graft.     

Approaches to MR angiography

One of the most revolutionary recent imaging advances is the use of magnetic resonance to study and produce morphologic representations of flowing blood vessels known as MR angiography. The ability to produce an image of even moderate spatial resolution of the three dimensional course of blood vessels with MR could have significant advantages over conventional invasive angiography which requires ionizing radiation and contrast material injection. By definition, MR angiography does not require the addition of any intravascular contrast agents and the images are produced entirely by the effect of the radio frequency pulses and magnetic field gradients on the spinning protons. Several researchers are already producing relatively high resolution MR angiograms using a variety of techniques. Essentially all techniques of MR angiography use variations of three steps to produce the image: (1) a projection image, (2) suppression of background static material, and (3) production of a flow sensitive image. This report will survey some of the more commonly used approaches to MR angiography that are currently under investigation.