Image quality and diagnostic accuracy of unenhanced SSFP MR angiography compared with conventional contrast-enhanced MR angiography for the assessment of thoracic aortic diseases

Dynamic MR angiography in acute aortic dissection

BACKGROUND

To evaluate the benefit (additional flow information), image quality, and diagnostic accuracy of a dynamic magnetic resonance angiography (MRA) combining high spatial and temporal resolution for the preinterventional assessment of acute aortic dissection.

METHODS

Nineteen patients (12 men, 7 women; aged 32-78 years) with acute aortic dissection underwent contrast-enhanced four-dimensional (4D) MRA and 3D conventional high-resolution MRA (3D MRA) within one examination on a 1.5 Tesla MR system. Both MRA datasets for each patient were evaluated and compared for image quality and visualization of vascular details on a 5-point scale (5 = excellent image quality, 1 = nondiagnostic image quality). In addition, presence and relevance of additional hemodynamic information (flow direction and organ perfusion delay) gained by dynamic MRA were assessed.

RESULTS

Conventional 3D MRA provided significantly higher values for image quality of the aorta and aortic side branches compared with dynamic MRA (aorta: 4.3 versus 3.3; P = 0.006 side branches: 4.2 versus 3.3; P = 0.02). However, in 10 of the 19 patients (53%) the additionally available information on flow dynamics due to dynamic MRA (e.g., delayed perfusion of parenchymal organs) led to a change in therapy planning and realization.

CONCLUSION

Dynamic MRA is a technique that combines functional flow and morphological information. Thus, the combination of 3D and dynamic MRA provides all requested information for treatment planning in patients suffering from acute aortic dissection.

3D morphometry using automated aortic segmentation in native MR angiography: an alternative to contrast enhanced MRA?

INTRODUCTION

Native-MR angiography (N-MRA) is considered an imaging alternative to contrast enhanced MR angiography (CE-MRA) for patients with renal insufficiency. Lower intraluminal contrast in N-MRA often leads to failure of the segmentation process in commercial algorithms. This study introduces an in-house 3D model-based segmentation approach used to compare both sequences by automatic 3D lumen segmentation, allowing for evaluation of differences of aortic lumen diameters as well as differences in length comparing both acquisition techniques at every possible location.

METHODS AND MATERIALS

Sixteen healthy volunteers underwent 1.5-T-MR Angiography (MRA). For each volunteer, two different MR sequences were performed, CE-MRA: gradient echo Turbo FLASH sequence and N-MRA: respiratory-and-cardiac-gated, T2-weighted 3D SSFP. Datasets were segmented using a 3D model-based ellipse-fitting approach with a single seed point placed manually above the celiac trunk. The segmented volumes were manually cropped from left subclavian artery to celiac trunk to avoid error due to side branches. Diameters, volumes and centerline length were computed for intraindividual comparison. For statistical analysis the Wilcoxon-Signed-Ranked-Test was used.

RESULTS

Average centerline length obtained based on N-MRA was 239.0±23.4 mm compared to 238.6±23.5 mm for CE-MRA without significant difference (P=0.877). Average maximum diameter obtained based on N-MRA was 25.7±3.3 mm compared to 24.1±3.2 mm for CE-MRA (P<0.001). In agreement with the difference in diameters, volumes obtained based on N-MRA (100.1±35.4 cm(3)) were consistently and significantly larger compared to CE-MRA (89.2±30.0 cm(3)) (P<0.001).

CONCLUSIONS

3D morphometry shows highly similar centerline lengths for N-MRA and CE-MRA, but systematically higher diameters and volumes for N-MRA.

CT and MRI in the Evaluation of Thoracic Aortic Diseases

Computed tomography (CT) and magnetic resonance imaging (MRI) are the most commonly used imaging examinations to evaluate thoracic aortic diseases because of their high spatial and temporal resolutions, large fields of view, and multiplanar imaging reconstruction capabilities. CT and MRI play an important role not only in the diagnosis of thoracic aortic disease but also in the preoperative assessment and followup after treatment. In this review, the CT and MRI appearances of various acquired thoracic aortic conditions are described and illustrated.

A new perceptual difference model for diagnostically relevant quantitative image quality evaluation: a preliminary study

PURPOSE

Most objective image quality metrics average over a wide range of image degradations. However, human clinicians demonstrate bias toward different types of artifacts. Here, we aim to create a perceptual difference model based on Case-PDM that mimics the preference of human observers toward different artifacts.

METHOD

We measured artifact disturbance to observers and calibrated the novel perceptual difference model (PDM). To tune the new model, which we call Artifact-PDM, degradations were synthetically added to three healthy brain MR data sets. Four types of artifacts (noise, blur, aliasing or "oil painting" which shows up as flattened, over-smoothened regions) of standard compressed sensing (CS) reconstruction, within a reasonable range of artifact severity, as measured by both PDM and visual inspection, were considered. After the model parameters were tuned by each synthetic image, we used a functional measurement theory pair-comparison experiment to measure the disturbance of each artifact to human observers and determine the weights of each artifact's PDM score. To validate Artifact-PDM, human ratings obtained from a Double Stimulus Continuous Quality Scale experiment were compared to the model for noise, blur, aliasing, oil painting and overall qualities using a large set of CS-reconstructed MR images of varying quality. Finally, we used this new approach to compare CS to GRAPPA, a parallel MRI reconstruction algorithm.

RESULTS

We found that, for the same Artifact-PDM score, the human observer found incoherent aliasing to be the most disturbing and noise the least. Artifact-PDM results were highly correlated to human observers in both experiments. Optimized CS reconstruction quality compared favorably to GRAPPA's for the same sampling ratio.

CONCLUSIONS

We conclude our novel metric can faithfully represent human observer artifact evaluation and can be useful in evaluating CS and GRAPPA reconstruction algorithms, especially in studying artifact trade-offs.

MRI for acute chest pain: current state of the art

This article reviews the magnetic resonance imaging (MRI) and angiography (MRA) techniques, imaging findings, and evidence for evaluating patients with acute chest pain due to acute pulmonary embolus (PE), aortic dissection (AD), and myocardial infarction (MI). When computed tomographic angiography (CTA) is contraindicated, MRI and MRA are important alternative imaging modalities for diagnosis and management of patients with acute PE, AD, and MI. Familiarity with the techniques, imaging findings, and evidence is critical to safely and appropriately managing patients presenting with acute chest pain.

Comparison between proximal thoracic vascular measurements obtained by contrast-enhanced magnetic resonance angiography and by transthoracic echocardiography in infants and children with congenital heart disease

Accurate assessment of the proximal thoracic vasculature in infants and children with congenital heart disease (CHD) is vital for deciding the appropriate surgical or interventional procedure and predicting outcomes. This information usually is obtained by transthoracic echocardiography (TTE). Contrast-enhanced magnetic resonance angiography (CE-MRA) frequently is used to obtain diagnostic data when the image quality by TTE is limited. Calculation of z-scores for measurements obtained by CE-MRA in this population currently is not possible due to the lack of normative data. A reasonable agreement between vessel dimensions by CE-MRA and TTE will allow the use of TTE-based z-scores on measurements from CE-MRA. This study examines the accuracy and agreement of proximal thoracic vascular measurements obtained by CE-MRA versus TTE. Infants and children younger than 3 years with CHD who had a CE-MRA between August 2006 and May 2011 were retrospectively identified. Main and branch pulmonary arteries, ascending aorta, distal transverse arch, and aortic isthmus were measured from CE-MRA and TTE in analogous imaging planes and locations by two investigators blinded to each other. The study enrolled 35 subjects with CHD. The median age was 129 days (range, 0-1077 days), and the median weight was 5.8 kg (range, 2.16-17 kg). The median interval between the two imaging methods was 9 days (range, 0-60 days). Data analysis was performed with 129 of the 210 possible paired measurements. The remaining 81 paired measurements could not be performed due to inaccurate visualization of vessel borders or an unavailable imaging plane from TTE, CE-MRA, or both. The range of vessel sizes measured from 2.8 to 23.4 mm. There was excellent correlation between CE-MRA and TTE (r = 0.94, p < 0.001). The mean difference between the measurements was -0.1 ± 1.2 mm, and the limits of agreement were -2.5 to 2.3 mm. Proximal thoracic vascular measurements obtained by CE-MRA and TTE in infants and children with CHD have a strong correlation. The agreement between these two imaging methods is adequate. Until normative data for vessel size measurements obtained from CE-MRA are available for this population, TTE-based z-scores can be applied to the measurements obtained by CE-MRA.

CMR in inflammatory vasculitis

Vasculitis, the inflammation of blood vessels, can produce devastating complications such as blindness, renal failure, aortic rupture and heart failure through a variety of end-organ effects. Noninvasive imaging with cardiovascular magnetic resonance (CMR) has contributed to improved and earlier diagnosis. CMR may also be used in serial evaluation of such patients as a marker of treatment response and as an indicator of subsequent complications. Unique strengths of CMR favoring its use in such conditions are its abilities to noninvasively visualize both lumen and vessel wall with high resolution. This case-based review focuses on the large- and medium-vessel vasculitides where MR angiography has the greatest utility. Because of increasing recognition of cardiac involvement in small-vessel vasculitides, this review also presents evidence supporting greater consideration of CMR to detect and quantify myocardial microvascular disease. CMR's complementary role amidst traditional clinical, serological and other diagnostic techniques in personalized care for patients with vasculitis is emphasized. Specifically, the CMR laboratory can address questions related to extent and severity of vascular involvement. As ongoing basic and translational studies better elucidate poorly-defined underlying molecular mechanisms, this review concludes with a discussion of potential directions for the development of more targeted imaging approaches.

Evaluation by MRA of aortic dilation late after repair of tetralogy of Fallot

OBJECTIVES

This study evaluated predictors for aortic dilation (AD) in patients with repaired tetralogy of Fallot (rTOF) using magnetic resonance angiography (MRA).

BACKGROUND

AD is common in patients with rTOF and may result in increased morbidity and mortality. There are no guidelines for evaluation of AD for rTOF patients.

METHODS

All adults with rTOF who previously underwent MRA had retrospective aortic measurements at the sinuses of Valsalva (SoV) and ascending aorta (AsAo). Rate of change in diameter was determined in patients with multiple MRAs. Chart review identified risk factors for AD. Univariate and multivariate analyses tested predictors of AD.

RESULTS

Of the 87 patients who met the inclusion criteria, 12 (14%) had AD. At baseline, mean diameter was 3.6 ± 0.6 cm and 3.1 ± 0.6 cm at the SoV and AsAo, respectively. The AsAo was larger than the SoV in 17%. Predictors of AD included male gender, age, right aortic arch, pregnancy, older age at complete repair, smoking, and systemic hypertension. Serial studies were available in 55 patients; the rate of growth was slow: 0.4 ± 0.9 mm/year (SoV) and 0.1 ± 0.8mm/year (AsAo).

CONCLUSIONS

AD is common in rTOF at the SoV and AsAo. Transthoracic echocardiography, which does not always image the AsAo as well as MRA, may not image AD in rTOF in cases in which the AsAo is dilated. Although several risk factors correlate with AD in rTOF, the rate of aortic growth is slow, suggesting that rTOF patients may not require frequent aortic imaging.

Imaging of thoracic aortic disease

Aortic pathology can be more complex to understand on imaging than is initially appreciated. There are a number of imaging modalities that provide excellent assessment of aortic pathology and enable the accurate monitoring of disease. This review discusses the imaging of the most common disease processes that affect the aorta in adults, with the primary focus being on CT and MRI.

Accuracy and precision of vessel area assessment: manual versus automatic lumen delineation based on full-width at half-maximum

PURPOSE

To evaluate the accuracy and precision of manual and automatic blood vessel diameter measurements, a quantitative comparison was conducted, using both phantom and clinical 3D magnetic resonance angiography (MRA) data. Since diameters are often manually measured, which likely is influenced by operator dependency, automatic lumen delineation, based on the full-width at half-maximum (FWHM), could improve these measurements.

MATERIALS AND METHODS

Manual and automatic diameter assessments were compared, using MRA data from a vascular phantom (geometry obtained with μCT) and clinical MRA data. The diameters were manually assessed by 15 MRA experts, using both caliper and contour tools. To translate the experimental results to clinical practice, the precision obtained using phantom data was compared to the precision obtained with clinical data.

RESULTS

A diameter error <10% was obtained with resolutions above 2, 3, and 5 pixels/diameter for the automatic FWHM, contour, and caliper methods, respectively. Using phantom data, precision of the manual methods was low (error >20%), even at high resolutions, while precision for the automatic method was high (error <3%) when using more than 2 pixels/diameter. A similar trend was found with clinical data.

CONCLUSION

The results obtained clearly demonstrate improvement in the accuracy and precision of vessel diameter measurements with use of the automatic FWHM-based method.

Noncontrast MRA for the diagnosis of vascular diseases

The association between gadolinium-based contrast agents and neprogenic systemic fibrosis has helped propel noncontrast angiography techniques to center stage in the MR evaluation of vascular disease, especially in individuals with intrinsic renal diseases. Although balanced steady-state free precession, phase contrast, and time-of-flight sequences are currently being revisited and improved, new noncontrast angiographic methods have been created and are under development: ECG-gated 3D partial-Fourier fast spin echo (FSE) and 3D variable flip angle FSE (SPACE). All of these are attempts to develop noncontrast methods that offer equal or superior vascular diagnosis as compared with contrast-enhanced MR angiography.

Single breathhold noncontrast thoracic MRA using highly accelerated parallel imaging with a 32-element coil array

PURPOSE

To evaluate the feasibility of performing single breathhold three-dimensional (3D) thoracic noncontrast MR angiography (NC-MRA) using highly accelerated parallel imaging.

MATERIALS AND METHODS

We developed a single breathhold NC MRA pulse sequence using balanced steady state free precession (SSFP) readout and highly accelerated parallel imaging. In 17 subjects, highly accelerated noncontrast MRA was compared against electrocardiogram-triggered contrast-enhanced MRA. Anonymized images were randomized for blinded review by two independent readers for image quality, artifact severity in eight defined vessel segments and aortic dimensions in six standard sites. NC-MRA and CE-MRA were compared in terms of these measures using paired sample t- and Wilcoxon tests.

RESULTS

The overall image quality (3.21 ± 0.68 for NC-MRA versus 3.12 ± 0.71 for CE-MRA) and artifact (2.87 ± 1.01 for NC-MRA versus 2.92 ± 0.87 for CE-MRA) scores were not significantly different, but there were significant differences for the great vessel and coronary artery origins. NC-MRA demonstrated significantly lower aortic diameter measurements compared with CE-MRA; however, this difference was not considered clinically relevant (>3 mm difference) for less than 12% of segments, most commonly at the sinotubular junction. Mean total scan time was significantly lower for NC-MRA compared with CE-MRA (18.2 ± 6.0 s versus 28.1 ± 5.4 s, respectively; P < 0.05).

CONCLUSION

Single breathhold NC-MRA is feasible and can be a useful alternative for evaluation and follow-up of thoracic aortic diseases.

3D whole-heart coronary MR angiography at 1.5T in healthy volunteers: comparison between unenhanced SSFP and Gd-enhanced FLASH sequences

Objective

To validate the optimal cardiac phase and appropriate acquisition window for three-dimensional (3D) whole-heart coronary magnetic resonance angiography (MRA) with a steady-state free precession (SSFP) sequence, and to compare image quality between SSFP and Gd-enhanced fast low-angle shot (FLASH) MR techniques at 1.5 Tesla (T).

Materials and Methods

Thirty healthy volunteers (M:F = 25:5; mean age, 35 years; range, 24-54 years) underwent a coronary MRA at 1.5T. 3D whole-heart coronary MRA with an SSFP was performed at three different times: 1) at end-systole with a narrow (120-msec) acquisition window (ESN), 2) mid-diastole with narrow acquisition (MDN); and 3) mid-diastole with wide (170-msec) acquisition (MDW). All volunteers underwent a contrast enhanced coronary MRA after undergoing an unenhanced 3D true fast imaging with steady-state precession (FISP) MRA three times. A contrast enhanced coronary MRA with FLASH was performed during MDN. Visibility of the coronary artery and image quality were evaluated for 11 segments, as suggested by the American Heart Association. Image quality was scored by a five-point scale (1 = not visible to 5 = excellent). The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were evaluated at the proximal coronary arteries.

Results

The SSFP sequence rendered higher visibility coronary segments, higher image quality, as well as higher SNR and CNR than the Gd-enhanced FLASH technique at 1.5T (p < 0.05). The visibility of coronary segments, image quality, SNR and CNR in the ESN, MDN and MDW with SSFP sequence did not differ significantly.

Conclusion

An SSFP sequence provides an excellent method for the 3D whole-heart coronary MRA at 1.5T. Contrast enhanced coronary MRA using the FLASH sequence does not help improve the visibility of coronary segments, image quality, SNR or CNR on the 3D whole-heart coronary MRA.

Acute chest pain: the role of MR imaging and MR angiography

MR imaging (MRI) and MR angiography (MRA) have gained a high level of diagnostic accuracy in cardiovascular disease. MRI in cardiac disease has been established as the non-invasive standard of reference in many pathologies. However, in acute chest pain the situation is somewhat special since many of the patients presenting in the emergency department suffer from potentially life-threatening disease including acute coronary syndrome, pulmonary embolism, and acute aortic syndrome. Those patients need a fast and definitive evaluation under continuous monitoring of vital parameters. Due to those requirements MRI seems to be less suitable compared to X-ray coronary angiography and multislice computed tomography angiography (CTA). However, MRI allows for a comprehensive assessment of all clinically stable patients providing unique information on the cardiovascular system including ischemia, inflammation and function. Furthermore, MRI and MRA are considered the method of choice in patients with contraindications to CTA and for regular follow-up in known aortic disease. This review addresses specific features of MRI and MRA for different cardiovascular conditions presenting with acute chest pain.

Use of respiratory biofeedback and CLAWS for increased navigator efficiency for imaging the thoracic aorta

A novel technique to guide a subjects' breathing pattern using a respiratory biofeedback (rBF) "game" to improve respiratory efficiency is presented. The continuously adaptive windowing strategy, a fully automatic and highly efficient free-breathing navigator gated technique, is used to acquire the data as it ensures that all potential navigator acceptance windows are possible. This enables the rBF to be fully adaptable to a subject's respiratory pattern. Images of the thoracic aorta acquired using balanced steady-state free precession with continuously adaptive windowing strategy respiratory motion control, with and without rBF, were compared in 10 healthy subjects. Total scan time was reduced by using rBF. The mean scan time was reduced from 7 min 44 s (463 cardiac cycles, ± 127 cc) without rBF to 5 min 43 s (380 cardiac cycles, ± 118 cc) with the use of rBF (P < 0.05). Respiratory efficiency was increased from 45% without rBF to 56% with rBF (P < 0.01). Image quality was the same for both techniques (P = ns). In conclusion, rBF significantly improved respiratory efficiency and reduced acquisition duration without affecting image quality.

Aneurysms of the ascending aorta and arch: the role of imaging in diagnosis and surgical management

Thoracic aortic aneurysms tend to be asymptomatic and were previously often diagnosed only after a complication such as dissection or rupture occurred. Better imaging techniques and an increase in the use of cross-sectional imaging has led to an increase in the diagnosis of aortic aneurysms, which has allowed for elective treatment prior to the development of a complication. The location, size and etiology of an aneurysm all impact the clinical outcomes and these factors are used to determine the appropriate timing of surgical replacement. Surgeons often rely on the information obtained from preoperative imaging to determine when to intervene and what type of procedure will be necessary, making it important for the radiologist to understand these issues in order to provide the necessary information. Postoperative imaging after surgical replacement of the aorta is also important, as there are some common findings that occur in this patient population that can impact how they are treated. The purpose of this article is to review the etiology and associated findings of aneurysms of the ascending aorta and arch, with a focus on how computed tomography angiography and magnetic resonance angiography findings are used to determine the appropriate timing for elective replacement and the type of surgical procedure, as well as the role of follow-up imaging. This will include a review of the most commonly performed types of surgical procedures, to provide an understanding of how the findings of preoperative imaging studies impact what the surgeon does in the operating room, as well as the expected findings of postoperative imaging studies.

Magnetic resonance angiography: current status and future directions

With recent improvement in hardware and software techniques, magnetic resonance angiography (MRA) has undergone significant changes in technique and approach. The advent of 3.0 T magnets has allowed reduction in exogenous contrast dose without compromising overall image quality. The use of novel intravascular contrast agents substantially increases the image windows and decreases contrast dose. Additionally, the lower risk and cost in non-contrast enhanced (NCE) MRA has sparked renewed interest in these methods. This article discusses the current state of both contrast-enhanced (CE) and NCE-MRA. New CE-MRA methods take advantage of dose reduction at 3.0 T, novel contrast agents, and parallel imaging methods. The risks of gadolinium-based contrast media, and the NCE-MRA methods of time-of-flight, steady-state free precession, and phase contrast are discussed.

Signal quality of single dose gadobenate dimeglumine pulmonary MRA examinations exceeds quality of MRA performed with double dose gadopentetate dimeglumine

During a recent multi-center trial assessing gadolinium (Gd)-enhanced magnetic resonance angiography (MRA) for diagnosis of acute pulmonary embolism (PE), the Food and Drug Administration announced a risk of nephrogenic sclerosing fibrosis in patients with renal insufficiency who had received intravenous Gd-based MR contrast agents. Although no patients in this trial had renal insufficiency, in cautious response to this announcement, the trial protocol was changed from an intravenous administration of 0.2 mmol/Kg of a conventional Gd-based MR contrast agent to 0.1 mmol/Kg of gadobenate dimeglumine. The study described herein compares the signal quality of pulmonary MRA performed with double dose conventional agent to single dose gadobenate dimeglumine. This study is a retrospective analysis of data from a prospective, multicenter study in men and women ≥18 years with documented presence or absence of PE. The study was approved by the Institutional Review Board at all participating centers, and all patients provided written indication of informed consent. We performed both objective and subjective analysis of pulmonary artery image quality. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) in the main pulmonary artery were assessed in single and double dose protocols and compared. SNR and CNR of the main PA were correlated with subjective quality assessment of main/lobar, segmental and subsegmental pulmonary arteries. Although there were individual outliers, both SNR (P = 0.01) and CNR (P = 0.008) were higher in all quartiles for examinations using gadobenate dimeglumine than with gadopentetate dimeglumine. Subjective quality of vascular signal intensity at each vessel order was significantly better for gadobenate dimeglumine (P < 0.0001), and correlated well with SNR and CNR at each order (<0.001). Because of agent high relaxivity, a single dose of gadobenate dimeglumine provides better pulmonary MRA signal quality than double dose of a conventional Gd-based MR contrast agent.

[MRI for therapy control in patients with aortic isthmus stenosis]

Aortic isthmus stenosis is the most common congenital aortic anomaly and is often a problem for therapy surveillance. In addition to possible comorbidities (e.g. bicuspid aortic valve) it is accompanied by various middle and long-term complications depending on the primary choice of the therapeutic procedure. Magnetic resonance imaging (MRI) plays an important role for the mostly young patients in the control of the aortic isthmus stenosis and therapy because it is non-invasive and there is no X-ray exposure. Radiologists should be well-informed on the principles of the therapeutic procedure in order to be competent in the interpretation of MRI findings. Due to the continuous development of MRI technology, techniques for functional evaluation (e.g. dynamic MRA, 4D PC flow measurement) are increasingly becoming available in addition to high-resolution MR angiography (MRA), which could predict the risk of possible complications, such as aneurysms. However, in this aspect further studies are necessary. Interventional therapy with stents and stent grafts is often employed for the therapy of possible complications following an operation (aneurysms, restenosis) but because of massive metal artefacts the use of MRI is often sometimes severely limited.