Time-resolved Magnetic Resonance Research Articles

Time-resolved 3-dimensional magnetic resonance phase contrast imaging (4D Flow MRI) analysis of hemodynamics in valve-sparing aortic root repair with an anatomically shaped sinus prosthesis

ObjectiveThe anatomically shaped sinus prosthesis (Uni-Graft W SINUS; Braun, Melsungen, Germany) used in valve-sparing aortic root replacement promises physiological hemodynamics believed to grant physiologic valve function. Using time-resolved 3-dimensional magnetic resonance phase contrast imaging (4D Flow MRI), we analyzed sinus vortex formation and transvalvular pressure gradients in patients with sinus prosthesis compared with age-matched and young healthy volunteers. MethodsTwelve patients with sinus prosthesis (55 ± 15 years), 12 age-matched and 6 young healthy volunteers (55 ± 6 years, 25 ± 3 years, respectively) were examined at 3T with a 4D flow magnetic resonance imaging sequence. Sinus vortices visualized by streamlines and time-resolved particle paths were graded on a 4-point Likert scale. Time resolved pressure differences of the left ventricular outflow tract and the ascending aorta to a reference point in the aortic bulb as well as the transvalvular pressure gradient were evaluated. Results4D flow visualizations revealed a propensity of the sinus prosthesis toward intermediate (50%) and large (28%) vortices compared with age-matched volunteers with small (61%) and intermediate (36%) vortices. Vortices in sinus prostheses had a similar configuration compared with those in volunteers. The peak transvalvular pressure gradient did not vary significantly between patients and age-matched volunteers (4.0 ± 0.9 mm Hg, 3.8 ± 0.7 mm Hg, P = .373), its temporal evolution resembled that of volunteers with a prolonged positive phase in patients. ConclusionsHemodynamics closely relating to those of volunteers were confirmed in sinus prostheses, believed to grant physiological valve function. Minor differences are presumably attributed to graft compliance and temporal resolution of the acquisition. Nevertheless, long-term deterioration of valve function as it was described for straight grafts could potentially be decelerated using sinus prostheses.

Magnetic Resonance Angiography in the Diagnosis of Cerebral Arteriovenous Malformation and Dural Arteriovenous Fistulas: Comparison of Time-Resolved Magnetic Resonance Angiography and Three Dimensional Time-of-Flight Magnetic Resonance Angiography.

BackgroundTraditional digital subtraction angiography (DSA) is currently the gold standard diagnostic method for the diagnosis and evaluation of cerebral arteriovenous malformation (AVM) and dural arteriovenous fistulas (dAVF).ObjectivesThe aim of this study was to analyze different less invasive magnetic resonance angiography (MRA) images, time-resolved MRA (TR-MRA) and three-dimensional time-of-flight MRA (3D TOF MRA) to identify their diagnostic accuracy and to determine which approach is most similar to DSA.Patients and MethodsA total of 41 patients with AVM and dAVF at their initial evaluation or follow-up after treatment were recruited in this study. We applied time-resolved angiography using keyhole (4D-TRAK) MRA to perform TR-MRA and 3D TOF MRA examinations simultaneously followed by DSA, which was considered as a standard reference. Two experienced neuroradiologists reviewed the images to compare the diagnostic accuracy, arterial feeder and venous drainage between these two MRA images. Inter-observer agreement for different MRA images was assessed by Kappa coefficient and the differences of diagnostic accuracy between MRA images were evaluated by the Wilcoxon rank sum test.ResultsAlmost all vascular lesions (92.68%) were correctly diagnosed using 4D-TRAK MRA. However, 3D TOF MRA only diagnosed 26 patients (63.41%) accurately. There were statistically significant differences regarding lesion diagnostic accuracy (P = 0.008) and venous drainage identification (P < 0.0001) between 4D-TRAK MRA and 3D TOF MRA. The results indicate that 4D-TRAK MRA is superior to 3D TOF MRA in the assessment of lesions.ConclusionCompared with 3D TOF MRA, 4D-TRAK MRA proved to be a more reliable screening modality and follow-up method for the diagnosis of cerebral AVM and dAVF.

4D Contrast-enhanced MR Angiography with the Keyhole Technique in Children: Technique and Clinical Applications.

Unlike in adults, contrast agent-enhanced magnetic resonance (MR) angiography in the pediatric population raises unique challenges such as faster heart rates, more rapid arteriovenous transit, smaller structures, smaller volumes of contrast agent used, and more complex disease processes. A need exists for a rapid contrast-enhanced MR angiographic technique that can separate the arterial and venous phases of contrast enhancement in sedated pediatric patients breathing freely during the course of an examination. In time-resolved contrast-enhanced MR angiography with the keyhole method (four-dimensional [4D] contrast-enhanced MR angiography), various spatial and temporal frequency undersampling schemes are used to substantially reduce the time of acquisition without markedly compromising spatial resolution. The keyhole method can be briefly described as an undersampling approach in which only a small region of the k-space (keyhole) around the center is repeatedly sampled while the periphery is sampled only once during acquisition. This method provides a wide range of options that can be used to overcome conventional limitations of contrast-enhanced MR angiography in children and opens the door for several new pediatric applications, including evaluation of congenital heart disease in neonates and infants, thoracic and extremity vascular pathologic conditions, high-flow vascular malformations, systemic vein thrombosis, and pediatric portal hypertension. This review provides a technical overview of 4D contrast-enhanced MR angiography, outlines its advantages and pitfalls in the pediatric population, and also describes various applications in children, including modifications of the technique needed for each application.

Abstract No. 474 - Feasibility of time-resolved magnetic resonance angiography for detecting reperfusion of visceral artery aneurysm embolized with platinum coils

Abstract No. 474 - Feasibility of time-resolved magnetic resonance angiography for detecting reperfusion of visceral artery aneurysm embolized with platinum coils

Is the Lecompte technique the last word on transposition of the great arteries repair for all patients? A magnetic resonance imaging study including a spiral technique two decades postoperatively.

To compare the Lecompte technique and the spiral anastomosis (complete anatomic correction) two decades after arterial switch operation (ASO). Nine patients after primary ASO with Lecompte and 6 selected patients after spiral anastomosis were evaluated 20.8 ± 2.1 years after ASO versus matched controls. Blood flow dynamics and flow profiles (e.g. vorticity, helicity) in the great arteries were quantified from time-resolved 3D magnetic resonance imaging (MRI) phase contrast flow measurements (4D flow MR) in addition to a comprehensive anatomical and functional cardiovascular MRI analysis. Compared with spiral reconstruction, patients with Lecompte showed more vortex formation, supranatural helical blood flow (relative helicity in aorta: 0.036 vs 0.089; P < 0.01), a reduced indexed cross-sectional area of the left pulmonary artery (155 vs 85 mm²/m²; P < 0.001) and more semilunar valve dysfunctions (n = 5 vs 1). There was no difference in elastic aortic wall properties, ventricular function, myocardial perfusion and myocardial fibrosis between the two groups. Cross-sectional area of the aortic sinus was larger in patients than in controls (669 vs 411 mm²/m²; P < 0.01). In the spiral group, the pulmonary root was rotated after ASO more towards the normal left position (P < 0.01). In this study, selected patients with spiral anastomoses showed, two decades after ASO, better physiologically adapted blood flow dynamics, and attained a closer to normal anatomical position of their great arteries, as well as less valve dysfunction. Considering the limitations related to the small number of patients and the novel MRI imaging techniques, these data may provoke reconsidering the optimal surgical approaches to transposition of the great arteries repair.

Time-resolved magnetic resonance angiography (MRA) at 3.0 Tesla for evaluation of hemodynamic characteristics of vascular malformations: description of distinct subgroups.

Quantitative evaluation of hemodynamic characteristics of arteriovenous and venous malformations using time-resolved magnetic resonance angiography (MRA) at 3.0 Tesla. Time-resolved MRA with interleaved stochastic trajectories (TWIST) at 3.0 Tesla was studied in 83 consecutive patients with venous malformations (VM) and arteriovenous malformations (AVM). Enhancement characteristics were calculated as percentage increase of signal intensity above baseline over time. Maximum percentage signal intensity increase (signalmax), time intervals between onset of arterial enhancement and lesion enhancement (tonset), and time intervals between beginning of lesion enhancement and maximum percentage of lesion enhancement (tmax) were analyzed. All AVMs showed a high-flow hemodynamic pattern. Two significantly different (p < 0.001) types of venous malformations emerged: VMs with arteriovenous fistulas (AVF) (median signalmax 737%, IQR [interquartile range] = 511 - 1182%; median tonset 5s, IQR = 5 - 10s; median tmax 35s, IQR = 26 - 40s) and without AVFs (median signalmax 284%, IQR = 177-432%; median tonset 23s, IQR = 15 - 30s; median tmax 60s, IQR = 55 - 75s). Quantitative evaluation of time-resolved MRA at 3.0 Tesla provides hemodynamic characterization of vascular malformations. VMs can be subclassified into two hemodynamic subgroups due to presence or absence of AVFs. • Time-resolved MRA at 3.0 Tesla provides quantitative hemodynamic characterization of vascular malformations. • Malformations significantly differ in time courses of enhancement and signal intensity increase. • AVMs show a distinctive high-flow hemodynamic pattern. • Two significantly different types of VMs emerged: VMs with and without AVFs.

Experimental and Theoretical Study of the Effectiveness and Stability of Gold(I) Catalysts Used in the Synthesis of Cyclic Acetals

Different [AuL]+ fragments (L = tertiary phosphines, ylides, or NHC carbene) have been tested under mild conditions as suitable catalysts for the transformation of terminal or internal alkynes into the corresponding cyclic acetals upon reaction with ethylene glycol. We have obtained a moderate to negligible activity when using tertiary phosphines or nonstabilized ylides as ligands. However, a very high catalytic activity is reached when the IPr N-heterocyclic carbene ligand is used. We have analyzed the key stages in this type of gold-catalyzed reaction, namely, (i) electronic activation (alkynophilicity); (ii) protodeauration; and (iii) decomposition of the gold catalyst. The first two stages have been analyzed through DFT computation of the minimum-energy reaction pathways employing different catalysts. An explanation of the catalysts’ stability has been proposed through the analysis of in situ time-resolved nuclear magnetic resonance spectra of the catalysts.

Renal Arteriovenous Shunts: Clinical Features, Imaging Appearance, and Transcatheter Embolization Based on Angioarchitecture.

Renal arteriovenous (AV) shunt, a rare pathologic condition, is divided into two categories, traumatic and nontraumatic, and can cause massive hematuria, retroperitoneal hemorrhage, pain, and high-output heart failure. Although transcatheter embolization is a less-invasive and effective treatment option, it has a potential risk of complications, including renal infarction and pulmonary embolism, and a potential risk of recanalization. The successful embolization of renal AV shunt requires a complete occlusion of the shunted vessel while preventing the migration of embolic materials and preserving normal renal arterial branches, which depends on the selection of adequate techniques and embolic materials for individual cases, based on the etiology and imaging angioarchitecture of the renal AV shunts. A classification of AV malformations in the extremities and body trunk could precisely correspond with the angioarchitecture of the nontraumatic renal AV shunts. The selection of techniques and choice of adequate embolic materials such as coils, vascular plugs, and liquid materials are determined on the basis of cause (eg, traumatic vs nontraumatic), the classification, and some other aspects of the angioarchitecture of renal AV shunts, including the flow and size of the fistulas, multiplicity of the feeders, and endovascular accessibility to the target lesions. Computed tomographic angiography and selective digital subtraction angiography can provide precise information about the angioarchitecture of renal AV shunts before treatment. Color Doppler ultrasonography and time-resolved three-dimensional contrast-enhanced magnetic resonance angiography represent useful tools for screening and follow-up examinations of renal AV shunts after embolization. In this article, the classifications, imaging features, and an endovascular treatment strategy based on the angioarchitecture of renal AV shunts are described.

A clinical combined gadobutrol bolus and slow infusion protocol enabling angiography, inversion recovery whole heart, and late gadolinium enhancement imaging in a single study.

BackgroundThe use of gadolinium contrast agents in cardiovascular magnetic resonance is well-established and serves to improve both vascular imaging as well as enable late gadolinium enhancement (LGE) imaging for tissue characterization. Currently, gadofosveset trisodium, an intravascular contrast agent, combined with a three-dimensional inversion recovery balanced steady state free precession (3D IR bSSFP) sequence, is commonly used in pediatric cardiac imaging and yields excellent vascular imaging, but cannot be used for late gadolinium enhancement. Gadofosveset use remains limited in clinical practice, and manufacture was recently halted, thus an alternative is needed to allow 3D IR bSSFP and LGE in the same study.MethodsHere we propose a protocol to give a bolus of 0.1 mL/kg = 0.1 mmol/kg gadobutrol (GADAVIST/GADOVIST) for time-resolved magnetic resonance angiography (MRA). Subsequently, 0.1 mmol/kg is diluted up to 5 or 7.5 mL with saline and then loaded into intravenous tubing connected to the patient. A 0.5 mL short bolus is infused, then a slow infusion is given at 0.02 or 0.03 mL/s. Image navigated (iNAV) 3D IR bSSFP imaging is initiated 45–60 s after the initiation of the infusion, with a total image acquisition time of ~5 min. If necessary, LGE imaging using phase sensitive inversion recovery reconstruction (PSIR) is performed at 10 min after the infusion is initiated.ResultsWe have successfully performed the above protocol with good image quality on 10 patients with both time-resolved MRA and 3D IR bSSFP iNAV imaging. Our initial attempts to use pencil beam respiratory navigation failed due to signal labeling in the liver by the navigator. We have also performed 2D PSIR LGE successfully, with both LGE positive and LGE negative results.ConclusionA bolus of gadobutrol, followed later by a slow infusion, allows time-resolved MRA, 3D IR bSSFP using the iNAV navigation technique, and LGE imaging, all in a single study with a single contrast agent.

Improved receiver arrays and optimized parallel imaging accelerations applied to time-resolved 3D fluoroscopically tracked peripheral runoff CE-MRA

ObjectivesThree-station stepping-table time-resolved 3D contrast-enhanced magnetic resonance angiography has conflicting demands in the need to limit acquisition time in proximal stations to match the speed of the advancing contrast bolus and in the distal-most station to avoid venous contamination while still providing clinically useful spatial resolution. This work describes improved receiver coil arrays which address this issue by allowing increased acceleration factors, providing increased spatial resolution per unit time. Materials and methodsReceiver coil arrays were constructed for each station (pelvis, thigh, calf) and then integrated into a 48-element array for three-station peripheral CE-MRA. Coil element sizes and array configurations for these three stations were designed to improve SENSE-type parallel imaging taking advantage of an increase in coil count for all stations versus the previous 32 channel capability. At each station either acceleration apportionment or optimal CAIPIRINHA selection was used to choose the optimum acceleration parameters for each subject. Results were evaluated in both single- and multi-station studies. ResultsSingle-station studies showed that SENSE acceleration in the thigh station could be readily increased from R=8 to R=10, allowing reduction of the frame time from 2.5 to 2.1s to better image the typically rapidly advancing bolus at this station. Similarly, the improved coil array for the calf station permitted acceleration increase from R=8 to R=12, providing a 4.0 vs. 5.2s frame time. Results in three-station studies suggest an improved ability to track the contrast bolus in peripheral CE-MRA. ConclusionsModified receiver coil arrays and individualized parameter optimization have been used to provide improved acceleration at all stations in multi-station peripheral CE-MRA and provide high spatial resolution with frame times as short as 2.1s.

Identifying the Adamkiewicz artery using 3-T time-resolved magnetic resonance angiography: its role in addition to multidetector computed tomography angiography.

This study assessed Adamkiewicz artery (AKA) detectability using multidetector computed tomography angiography (MDCTA) and time-resolved magnetic resonance angiography (MRA) at 3 T. This Institutional Review Board-approved retrospective study included 117 patients with thoracoabdominal aortic disease scheduled for aortic repair. A total of 111 patients underwent MDCTA for AKA identification; 43 patients whose AKA identification was not definitive on MDCTA underwent additional MRA. The remaining six patients, who were not indicated for iodine-contrast MDCTA, underwent only MRA. Two reviewers independently evaluated both MDCTA and MRA data. The 4-point confidence index was used. Grades 3-4 were considered sufficient for AKA diagnosis. AKA detectability was at 80.2% (89/111) using MDCTA and 89.8% (44/49) with MRA. In the 43 patients who underwent both MDTCA and MRA, the AKA detectability and consensus grades were significantly elevated using MRA vs. MDCTA (detectability: 88.4 vs. 69.8%, respectively, p = 0.043). AKA detectability was also higher in aortic aneurysm than aortic dissection patients on MDCTA (90.9 vs. 69.6%, respectively, p < 0.01), but not on MRA (92.9 vs. 88.6%, respectively, p = 0.99). Time-resolved MRA at 3 T increases AKA detectability and is recommended for patients without definitive AKA identification on MDCTA.

A novel application of four-dimensional magnetic resonance angiography using an arterial spin labeling technique for noninvasive diagnosis of Moyamoya disease

BackgroundNoncontrast-enhanced time-resolved four-dimensional magnetic resonance angiography using an arterial spin labeling technique (ASL-4D MRA) is emerging as a next generation angiography for the management of neurovascular diseases. This study evaluated the feasibility of ASL-4D MRA for the diagnosis of Moyamoya disease (MMD) and MMD staging by using digital subtraction angiography (DSA) and time-of-flight MRA (TOF MRA) as current standards. MethodsEleven consecutive non-operated patients who underwent DSA for the diagnosis of MMD were recruited. Two independent observers evaluated the three tests. The data were analyzed for inter-observer and inter-modality agreements on MMD stage. Nine of 22 hemispheres underwent surgical revascularization and ASL-4D MRA was repeated postoperatively. ResultsTime-resolved inflow of blood through the cerebral vessels, including moyamoya vessels, was visualized in all the 22 non-operated hemispheres. MMD stages assessed by DSA and ASL-4D MRA were completely matched in 18 hemispheres, with a significant positive correlation between these modalities (r=0.93, P<0.001). Inter-observer agreement with ASL-4D MRA (κ=0.91±0.04, P<0.001) and inter-modality agreement between ASL-4D MRA and DSA (κ=0.93±0.04, P<0.001) were both excellent. MMD stages assessed by ASL-4D MRA have also a significant positive correlation with those assessed by TOF MRA (r=0.68, P=0.004). Repeated ASL-4D MRA clearly demonstrated the bypassed arteries and changes in the dynamic flow patterns of cerebral arteries in all the nine hemispheres after surgical revascularization. Of these, postoperative focal hyperperfusion was detected by single photon emission tomography in 7 hemispheres. In five of the seven hemispheres (71%) with postoperative hyperperfusion, ASL-4D MRA demonstrated focal hyperintense signals in the bypassed arteries, although TOF MRA did not. ConclusionsNoninvasive ASL-4D MRA is feasible for the diagnosis of MMD staging. This next generation angiography may be useful for monitoring disease evolution and treatment response in cerebral arteries after revascularization surgery in MMD.

Reperfusion Rates of Pulmonary Arteriovenous Malformations after Coil Embolization: Evaluation with Time-Resolved MR Angiography or Pulmonary Angiography

PurposeTo assess reperfusion rates after coil embolization for pulmonary arteriovenous malformations (PAVMs) using time-resolved magnetic resonance (MR) angiography or pulmonary angiography. Materials and MethodsPatients with PAVMs who underwent embolization and met the following inclusion criteria were included: (a) embolization was performed using bare or fibered platinum microcoils or both, (b) the complete cessation of flow was confirmed by digital subtraction angiography, and (c) follow-up examinations were conducted with time-resolved MR angiography or pulmonary angiography. Coil embolization was performed in 16 patients with 24 untreated or reperfused PAVMs. Sac embolization was performed for 12 untreated PAVMs. Feeding artery embolization was performed as primary embolization in each of the 12 reperfused PAVMs. Five PAVMs were treated 2 to 4 times because of reperfusion. The study included 32 coil embolizations. Follow-up images were reviewed, and reperfusion rates were assessed. The relationships between reperfusion and the location of PAVM, size of PAVM (feeding artery and venous sac), coils (number and total length), timing of embolization (primary or repeat embolization), and types of coils used (with or without fibered coils) were examined. ResultsReperfusion rates at 3, 6, 12, and 24 months were 8%, 27%, 36%, and 49%, respectively, for the 12 untreated PAVMs (primary embolization) and 50%, 50%, 92%, and 100%, respectively, for the 12 reperfused PAVMs (repeat embolization) (P = .0062). No significant differences were observed in the other parameters measured. ConclusionsWhen evaluated with time-resolved MR angiography or pulmonary angiography, reperfusion rates after coil embolization for PAVM were considerably high, particularly with repeat embolization.

The Role of Branch Vessels in Aortic Type B Dissection: An in vitro Study

In acute type B aortic dissection (ABAD) a patent false lumen portends a poor outcome. Patent branch vessels originating from the false lumen in a type B aortic dissection are assumed to contribute to persistent blood flow and patent false lumen. Therefore, the morphologic changes of the false lumen generated by different outflow rates in an in vitro model were investigated. An artificial dissection was created in two ex vivo porcine aortas. A thin cannula was placed in the false lumen, simulating a branch vessel originating from it. The aorta was positioned in a validated in vitro circulatory system with physiological pulsatile flow (1,500-2,700 mL/minute) and pressure characteristics (130/70 mm Hg). The cannula was attached to a small silicone tube with an adjustable valve mechanism. Three different valve settings were used for creating outflow from the false lumen (fully closed, opened at 50%, and fully opened at 100%). Measurements of lumen areas and flow rates were assessed with time-resolved magnetic resonance imaging. In order to study reproducibility, the experiment was performed twice in two different porcine aortas with a similar morphology. Increasing antegrade outflow through the branch vessel of the false lumen resulted in a significant (p < .01) increase of the mean false lumen area at the proximal and distal location in both models. The distal false lumen expanded up to 107% in the case of high outflow via the false lumen through the branch vessel. Increasing antegrade outflow through a branch vessel originating from the false lumen when no distal re-entry tear is present results in an expansion of the cross sectional false lumen area.

Free breathing contrast-enhanced time-resolved magnetic resonance angiography in congenital heart disease

Background Contrast enhanced magnetic resonance angiography (MRA) is generally performed during a long breath-hold (BH), limiting its use in infants and children. Time resolved MR angiography (TRA) often use data sharing techniques, making it is necessary to acquire data during a BH. This study proposes a high-resolution free breathing (FB) TRA sequence for use in adults and children with congenital heart disease (CHD).

Spin pumping in ferromagnet-topological insulator-ferromagnet heterostructures.

Topological insulators (TIs) are enticing prospects for the future of spintronics due to their large spin-orbit coupling and dissipationless, counter-propagating conduction channels in the surface state. However, a means to interact with and exploit the topological surface state remains elusive. Here, we report a study of spin pumping at the TI-ferromagnet interface, investigating spin transfer dynamics in a spin-valve like structure using element specific time-resolved x-ray magnetic circular dichroism, and ferromagnetic resonance. Gilbert damping increases approximately linearly with increasing TI thickness, indicating efficient behaviour as a spin sink. However, layer-resolved measurements suggest that a dynamic coupling is limited. These results shed new light on the spin dynamics of this novel material class, and suggest great potential for TIs in spintronic devices, through their novel magnetodynamics that persist even up to room temperature.

Acute Stroke Imaging: Feasibility and Value of MR Angiography with High Spatial and Temporal Resolution for Vessel Assessment and Perfusion Analysis in Patients with Wake-up Stroke

Magnetic resonance (MR) imaging (MRI) provides information that can be used to estimate the symptom onset in patients with wake-up stroke (WUS). Time-resolved MR angiography (MRA) is the fastest available MR sequence technique for vessel assessment, and the different phases acquired can provide information about cerebral perfusion. The aim of this study was to evaluate the diagnostic performance of time-resolved MRA both for the assessment of vessel morphology and for the feasibility of perfusion. Nineteen patients with WUS were included. Image quality and vessel pathologies were evaluated and correlated to time-of-flight-MRA (n = 14), computed tomography-angiography (n = 4), sonography (n = 12), and conventional angiography (n = 6). The temporal delay of signal enhancement in all pixels of the time-resolved MRA measurement after contrast injection was evaluated and compared to dynamic susceptibility contrast-enhanced (DSC) perfusion imaging (n = 13). Time-resolved MRA resulted in the diagnosis of large vessel disease in 14 of 19 patients, involving the internal carotids (n = 4), the vertebral arteries (n = 3), and the circle of Willis (n = 10). All severe vascular pathologies which influence patients' acute stroke therapy were obtained by time-resolved MRA. Overestimation of stenoses in two of 14 patients resulted in sensitivity and specificity of 100% and 71%, respectively. Time-to-peak (TTP) estimations were hampered by movement artifacts in four patients (31%). Compared to DSC, the area of TTP delay was comparable in size and localization without relevant overestimation or underestimation. Time-resolved MRA is a valuable technique in patients with WUS with high sensitivity and high negative predictive value. Cerebral perfusion estimation can be performed in selected cases for therapy decision but can be hampered by patient movement.

Free breathing contrast-enhanced time-resolved magnetic resonance angiography in pediatric and adult congenital heart disease.

BackgroundContrast enhanced magnetic resonance angiography (MRA) is generally performed during a long breath-hold (BH), limiting its utility in infants and small children. This study proposes a free-breathing (FB) time resolved MRA (TRA) technique for use in pediatric and adult congenital heart disease (CHD).MethodsA TRA sequence was developed by combining spiral trajectories with sensitivity encoding (SENSE, x4 kx-ky and x2 kz) and partial Fourier (75% in kz). As no temporal data sharing is used, an independent 3D data set was acquired every ~1.3s, with acceptable spatial resolution (~2.3x2.3x2.3mm). The technique was tested during FB over 50 consecutive volumes. Conventional BH-MRA and FB-TRA data was acquired in 45 adults and children with CHD. We calculated quantitative image quality for both sequences. Diagnostic accuracy was assessed in all patients from both sequences. Additionally, vessel measurements were made at the sinotubular junction (N = 43), proximal descending aorta (N = 43), descending aorta at the level of the diaphragm (N = 43), main pulmonary artery (N = 35), left pulmonary artery (N = 35) and the right pulmonary artery (N = 35). Intra and inter observer variability was assessed in a subset of 10 patients.ResultsBH-MRA had significantly higher homogeneity in non-contrast enhancing tissue (coefficient of variance, P <0.0001), signal-to-noise ratio (P <0.0001), contrast-to-noise ratio (P <0.0001) and relative contrast (P = 0.02) compared to the FB-TRA images. However, homogeneity in the vessels was similar in both techniques (P = 0.52) and edge sharpness was significantly (P <0.0001) higher in FB-TRA compared to BH-MRA. BH-MRA provided overall diagnostic accuracy of 82%, and FB-TRA of 87%, with no statistical difference between the two sequences (P = 0.77). Vessel diameter measurements showed excellent agreement between the two techniques (r = 0.98, P <0.05), with no bias (0.0mm, P = 0.71), and clinically acceptable limits of agreement (-2.7 to +2.8mm). Inter and intra observer reproducibility showed good agreement of vessel diameters (r>0.988, P<0.0001), with negligible biases (between -0.2 and +0.1mm) and small limits of agreement (between -2.4 and +2.5mm).ConclusionsWe have described a FB-TRA technique that is shown to enable accurate diagnosis and vessel measures compared to conventional BH-MRA. This simplifies the MRA technique and will enable angiography to be performed in children and adults whom find breath-holding difficult.Electronic supplementary materialThe online version of this article (doi:10.1186/s12968-015-0138-9) contains supplementary material, which is available to authorized users.

Gas–Gas Dispersion Coefficient Measurements Using Low-Field MRI

The dispersion coefficient of displacing fluids within porous media is an important parameter to measure accurately for a range of applications. For example, enhanced gas recovery is an emerging technology where carbon dioxide is injected into natural gas reservoirs as a means of maintaining the pressure of the reservoir and to hence enhance the natural gas recovery, with the additional benefit of carbon dioxide sequestration. Research is currently being done to measure the dispersion coefficient of carbon dioxide into methane because carbon dioxide is a contaminant that reduces the value of the natural gas. Such dispersion experiments are difficult to perform in the laboratory using conventional rock core flooding equipment as erroneous contributions to the dispersion process from entry and exit effects (into and out of the rock core respectively) are impossible to eliminate completely. Previously, we have estimated the resultant error to be of the order of 25 % (Hughes et al., Int J Greenh Gas Control 9:457–468, 2012). Here we effectively deploy spatially resolved breakthrough curves which are characteristic of the dispersion process for carbon dioxide replacing methane. These are obtained from time-resolved 1D magnetic resonance imaging (MRI) profiles. By analysis of the additional dispersion between these time-resolved profiles, we are able to eliminate the entry/exit effects and obtain more accurate values for the dispersion coefficient. We demonstrate this using a model porous medium (mono-dispersed glass beads) in a cell capable of holding gas pressures up to 80 bar in a low-field MRI rock core analyser. Via simultaneous conventional IR analysis of the carbon dioxide effluent to determine the dispersion coefficient, we are also able to directly quantify the magnitude of the error due to these entry/exit effects.

Preoperative Peroneal Perforator Mapping with Dynamic Time‐Resolved Magnetic Resonance Angiography

Objectives: Dynamic time-resolved magnetic resonance angiography (DTR MRA) is a novel imaging algorithm that allows for better visualization of small-caliber vessels when compared with standard MRA. We sought to estimate the technique sensitivity when compared with intraoperative findings. Methods: Retrospective review of 25 patients undergoing reconstruction with free fibula flap at an academic tertiary setting from 2009 to 2013. Preoperative DTR MRA was performed in all cases. Two radiologists performed blind reviews of the images and their findings were compared with surgical descriptions of the perforator anatomy. Results: Surgical exploration identified 32 perforators while DTR MRA identified 45 vessels. The technique confirmed the presence of perforators in 22 out of 23 patients with at least one perforator, and their absence in 1 out of 2 patients who had a negative exploration. There was a significant correlation in the number of perforators identified clinically and radiologically in each case ( P = .005). Twenty-seven perforators were localized to their correct anatomic location by the technique, yielding a sensitivity of 84.4% and a specificity of 16.7%. Ten out of the 18 vessels incorrectly identified by this technique were located within 30 mm of a confirmed perforator. Conclusions: DTR MRA is a highly sensitive technique for determining the presence and location of cutaneous perforators in patients undergoing reconstruction with free fibula flap. The majority of the false-positive findings corresponded to vessels located within 30 mm of a confirmed perforator; we hypothesize that they could represent distal branching patterns presenting radiologically as separate entities. These results compare favorably to perforator mapping with standard MRA techniques.