Feasibility and reproducibility of biventricular volumetric assessment of cardiac function during exercise using real-time radial k-t SENSE magnetic resonance imaging

PURPOSE

To assess the feasibility and reproducibility of real-time radial k-t sensitivity encoding (SENSE) magnetic resonance imaging (MRI) for biventricular volumetric assessment during exercise.

MATERIALS AND METHODS

In all, 12 healthy young adults underwent MRI at rest and during supine exercise at three different workload intensities. Biventricular volumes and function were assessed with 1) a radial k-t SENSE real-time sequence and 2) a scanner vendor supplied (standard) real-time sequence. Global image quality, motion fidelity, and agreement in right ventricular (RV) and left ventricular (LV) stroke volume (SV) as a surrogate measure for accuracy were assessed. Exercise MR was repeated within 1 month for assessment of reproducibility.

RESULTS

Imaging scores were superior for radial real-time k-t SENSE images (P < 0.001). Agreement in RV and LV SV during exercise was better with radial k-t real-time (SD of difference +/-3.43 vs. +/-8.97 mL; P < 0.001). Agreement in cardiac output (CO) in the same subject at two different imaging sessions was better for radial k-t SENSE. This was significant for the CO calculated for the RV (SD of difference +/-0.6 vs. +/-0.95 L/min; P = 0.01) and LV (+/-0.45 vs. +/-0.92 L/min; P < 0.001).

CONCLUSION

Radial k-t SENSE real-time imaging represents a feasible and reproducible imaging technique for biventricular assessment during exercise.

Real-time assessment of right and left ventricular volumes and function in patients with congenital heart disease by using high spatiotemporal resolution radial k-t SENSE

PURPOSE

The purpose of this study was to compare ventricular volumes in patients with congenital heart disease measured by using (a) a cardiac gated sequence, (b) a standard real-time sequence, and (c) a radial real-time k-space and time (k-t) sensitivity encoding (SENSE) sequence.

MATERIALS AND METHODS

The local research ethics committee approved this study, and written consent was obtained from all participants. Of 40 patients with congenital heart disease, ventricular volumes were measured by using the three sequences. Global image quality and motion fidelity were scored and compared with a Wilcoxon signed rank test. Image contrast, edge sharpness, and summed perimeters (the total length of the endocardial tracings for a given ventricle at systole and diastole) were quantified and compared by using paired t tests. Ventricular volumes were compared with paired t tests, Bland-Altman analysis, and correlation coefficients.

RESULTS

Global image quality, motion fidelity, image contrast, edge sharpness, and summed perimeters were all greater for radial real-time k-t SENSE imaging compared with standard real-time imaging (P < .05). However, the gated acquisitions were significantly superior to radial real-time k-t SENSE (P < .05). For cardiac gated versus radial k-t real-time acquisitions, there was no difference between right ventricular (RV) volumes and ejection fraction (EF) (P > .15). There was a small difference in left ventricular (LV) end-diastolic volume (EDV) and thus, LV stroke volume and EF (P < .05). For cardiac gated versus standard real-time acquisitions, both RV and LV EDV and thus, stroke volume and EF were significantly lower (P < .05).

CONCLUSION

Ventricular volumes and function can be accurately quantified by using radial k-t SENSE real-time imaging.

Percutaneous pulmonary valve implantation: impact of evolving technology and learning curve on clinical outcome

BACKGROUND

Percutaneous pulmonary valve implantation was introduced in the year 2000 as a nonsurgical treatment for patients with right ventricular outflow tract dysfunction.

METHODS AND RESULTS

Between September 2000 and February 2007, 155 patients with stenosis and/or regurgitation underwent percutaneous pulmonary valve implantation. This led to significant reduction in right ventricular systolic pressure (from 63+/-18 to 45+/-13 mm Hg, P<0.001) and right ventricular outflow tract gradient (from 37+/-20 to 17+/-10 mm Hg, P<0.001). Follow-up ranged from 0 to 83.7 months (median 28.4 months). Freedom from reoperation was 93% (+/-2%), 86% (+/-3%), 84% (+/-4%), and 70% (+/-13%) at 10, 30, 50, and 70 months, respectively. Freedom from transcatheter reintervention was 95% (+/-2%), 87% (+/-3%), 73% (+/-6%), and 73% (+/-6%) at 10, 30, 50, and 70 months, respectively. Survival at 83 months was 96.9%. On time-dependent analysis, the first series of 50 patients (log-rank test P<0.001) and patients with a residual gradient >25 mm Hg (log-rank test P=0.01) were associated with a higher risk of reoperations.

CONCLUSIONS

Percutaneous pulmonary valve implantation resulted in the ability to avoid surgical right ventricular outflow tract revision in the majority of cases. This procedure might reduce the number of operations needed over the total lifetime of patients with right ventricle-to-pulmonary artery conduits.

Whole-heart cine MRI using real-time respiratory self-gating

Two-dimensional (2D) breath-hold cine MRI is used to assess cardiac anatomy and function. However, this technique requires cooperation from the patient, and in some cases the scan planning is complicated. Isotropic nonangulated three-dimensional (3D) cardiac MR can overcome some of these problems because it requires minimal planning and can be reformatted in any plane. However, current methods, even those that use undersampling techniques, involve breath-holding for periods that are too long for many patients. Free-breathing respiratory gating sequences represent a possible solution for realizing 3D cine imaging. A real-time respiratory self-gating technique for whole-heart cine MRI is presented. The technique enables assessment of cardiac anatomy and function with minimum planning or patient cooperation. Nonangulated isotropic 3D data were acquired from five healthy volunteers and then reformatted into 2D clinical views. The respiratory self-gating technique is shown to improve image quality in free-breathing scanning. In addition, ventricular volumetric data obtained using the 3D approach were comparable to those acquired with the conventional multislice 2D approach.

Interactive MR imaging and tracking of catheters with multiple tuned fiducial markers

PURPOSE

The lack of magnetic resonance (MR) safe catheters and guide wires remains an important obstacle to widespread clinical use of MR-guided endovascular procedures. The authors looked at the feasibility of using multiple tuned fiducial markers (TFM) and novel imaging sequences to track catheters reliably under MR and to evaluate the safety of such markers in terms of heating.

MATERIALS AND METHODS

The visualization and tracking of a catheter with six quadrature tuned fiducial coils was carried out in a special designed in-vitro setup within a 1.5-T MR imager simulating an MR-guided endovascular intervention. The fiducial markers were also tested for heating.

RESULTS

The excellent signal contrast between the fiducial and the background when using novel interleaved real time and interactive sequences allowed for rapid and reliable identification of the fiducial markers and therefore the catheter. No significant heating of the marker was noted.

CONCLUSIONS

The authors have shown that catheters with multiple tuned fiducial markers are superior to passive catheter designs in terms of visualization and do not carry the risk of heating that is commonly associated with active catheters.

Cardiac magnetic resonance imaging after stage I Norwood operation for hypoplastic left heart syndrome

BACKGROUND

After the Norwood operation, a patient's suitability for proceeding to a bidirectional cavopulmonary connection (BCPC) is assessed by a combination of echocardiography and diagnostic cardiac catheterization. In this study, we describe the results of 37 patients who underwent cardiovascular magnetic resonance (MR) assessment before BCPC.

METHODS AND RESULTS

Cardiovascular MR and echocardiography were performed in 37 infants with hypoplastic left heart syndrome before BCPC, and the findings were compared with surgical findings. MR assessment of ventricular function and valvar regurgitation were compared with echocardiography. MR exhibited high sensitivity and specificity for identification of neoaortic (sensitivity 86%, specificity 97%) and left pulmonary artery (sensitivity 100%, specificity 94%) obstruction. Echocardiography exhibited poor sensitivity for identification of vascular stenosis. The mean right ventricular ejection fraction calculated from the MR data was 50+/-10%. There was general agreement between MR and echocardiographic measures of ventricular function, although patients with good function on echocardiography demonstrated a wide range of ejection fractions. There was good agreement between MR and echocardiography for identification of valvar regurgitation.

CONCLUSIONS

Cardiovascular MR can be used to define ventricular and valvar function and vascular anatomy in infants with hypoplastic left heart syndrome after the Norwood operation. We have shown how this information can be used to plan the BCPC and identify any revisions or additional valvar surgery.

Measurement of total pulmonary arterial compliance using invasive pressure monitoring and MR flow quantification during MR-guided cardiac catheterization

Pulmonary hypertensive disease is assessed by quantification of pulmonary vascular resistance. Pulmonary total arterial compliance is also an indicator of pulmonary hypertensive disease. However, because of difficulties in measuring compliance, it is rarely used. We describe a method of measuring pulmonary arterial compliance utilizing magnetic resonance (MR) flow data and invasive pressure measurements. Seventeen patients with suspected pulmonary hypertension or congenital heart disease requiring preoperative assessment underwent MR-guided cardiac catheterization. Invasive manometry was used to measure pulmonary arterial pressure, and phase-contrast MR was used to measure flow at baseline and at 20 ppm nitric oxide (NO). Total arterial compliance was calculated using the pulse pressure method (parameter optimization of the 2-element windkessel model) and the ratio of stroke volume to pulse pressure. There was good agreement between the two estimates of compliance ( r = 0.98, P < 0.001). However, there was a systematic bias between the ratio of stroke volume to pulse pressure and the pulse pressure method (bias = 61%, upper level of agreement = 84%, lower level of agreement = 38%). In response to 20 ppm NO, there was a statistically significant fall in resistance, systolic pressure, and pulse pressure. In seven patients, total arterial compliance increased >10% in response to 20 ppm NO. As a population, the increase did not reach statistical significance. There was an inverse relation between compliance and resistance ( r = 0.89, P < 0.001) and between compliance and mean pulmonary arterial pressure ( r = 0.72, P < 0.001). We have demonstrated the feasibility of quantifying total arterial compliance using an MR method.

Percutaneous pulmonary valve implantation in humans: results in 59 consecutive patients

BACKGROUND

Right ventricular outflow tract (RVOT) reconstruction with valved conduits in infancy and childhood leads to reintervention for pulmonary regurgitation and stenosis in later life.

METHODS AND RESULTS

Patients with pulmonary regurgitation with or without stenosis after repair of congenital heart disease had percutaneous pulmonary valve implantation (PPVI). Mortality, hemodynamic improvement, freedom from explantation, and subjective and objective changes in exercise tolerance were end points. PPVI was performed successfully in 58 patients, 32 male, with a median age of 16 years and median weight of 56 kg. The majority had a variant of tetralogy of Fallot (n=36), or transposition of the great arteries, ventricular septal defect with pulmonary stenosis (n=8). The right ventricular (RV) pressure (64.4+/-17.2 to 50.4+/-14 mm Hg, P<0.001), RVOT gradient (33+/-24.6 to 19.5+/-15.3, P<0.001), and pulmonary regurgitation (PR) (grade 2 of greater before, none greater than grade 2 after, P<0.001) decreased significantly after PPVI. MRI showed significant reduction in PR fraction (21+/-13% versus 3+/-4%, P<0.001) and in RV end-diastolic volume (EDV) (94+/-28 versus 82+/-24 mL.beat(-1).m(-2), P<0.001) and a significant increase in left ventricular EDV (64+/-12 versus 71+/-13 mL.beat(-1).m(-2), P=0.005) and effective RV stroke volume (37+/-7 versus 42+/-9 mL.beat(-1).m(-2), P=0.006) in 28 patients (age 19+/-8 years). A further 16 subjects, on metabolic exercise testing, showed significant improvement in VO2max (26+/-7 versus 29+/-6 mL.kg(-1).min(-1), P<0.001). There was no mortality.

CONCLUSIONS

PPVI is feasible at low risk, with quantifiable improvement in MRI-defined ventricular parameters and pulmonary regurgitation, and results in subjective and objective improvement in exercise capacity.

Spontaneous improvement of severe right ventricular dysfunction in the setting of hypoplasia of the left heart

Right ventricular dysfunction is known to occur after the first stage of the Norwood sequence for treatment of patients with hypoplasia of the left heart. In a subset of patients, such ventricular dysfunction occurs without a specific anatomical cause. We describe two such cases with severe right ventricular dysfunction. In both cases, magnetic resonance imaging was used accurately to measure ventricular function and assess the arterial trunks. In both cases, cardiac transplantation was considered, but right ventricular function improved without invasive management. Transient right ventricular dysfunction in these cases may be due to the reduced ability of the right ventricle to adapt to the systemic vasculature. The improvement in ventricular function in these two cases may be due to delayed adaptation.

Visualization and tracking of an inflatable balloon catheter using SSFP in a flow phantom and in the heart and great vessels of patients

Passive catheter tracking involves direct interaction between the device and its surroundings, creating a local signal loss or enhancement of the image. Using only standard balloon catheters filled with CO(2) and imaged with a steady-state free precession sequence, it was possible to visualize and passively track catheters in a flow phantom and in the heart and great vessels of 20 patients without any additional image processing. The phantom work demonstrated that it was advantageous to sacrifice spatial resolution in order to increase temporal resolution. Frame rates greater than 10/sec were necessary for ease of catheter manipulation. Although only the tip of the catheter was visualized, this technique proved to be effective in patients undergoing cardiac catheterization.

Novel method of quantifying pulmonary vascular resistance by use of simultaneous invasive pressure monitoring and phase-contrast magnetic resonance flow

BACKGROUND

Pulmonary vascular resistance (PVR) quantification is important in the treatment of children with pulmonary hypertension. The Fick principle, used to quantify pulmonary artery flow, may be a flawed technique. We describe a novel method of PVR quantification by the use of magnetic resonance (MR) flow data and invasive pressure measurements.

METHODS AND RESULTS

In 24 patients with either suspected pulmonary hypertension or congenital heart disease requiring preoperative assessment, PVR was calculated by the use of simultaneously acquired MR flow and invasive pressure measurements (condition 1). In 19 of the 24 patients, PVR was also calculated at 20 ppm nitric oxide +30% (condition 2) and at 20 ppm nitric oxide +100% oxygen (condition 3), with the use of the MR method. This method proved safe and feasible in all patients. In 15 of 19 patients, PVR calculated by Fick flow was compared with the MR method. At condition 1, Bland-Altman analysis revealed a bias of 2.3% (MR > Fick) and limits of agreement of 50.2% to -45.5%. At condition 2, there was poorer agreement (bias was 28%, and the limits of agreement were 151.3% to 95.2%). At condition 3, there was very poor agreement (bias was 54.2%, and the limits of agreement were 174.4% to -66.0%).

CONCLUSIONS

We have demonstrated the feasibility of using simultaneous invasive pressure measurements and MR flow data to measure PVR in humans.

Cardiac catheterisation guided by MRI in children and adults with congenital heart disease

BACKGROUND

Fluoroscopically guided cardiac catheterisation is an essential tool for diagnosis and treatment of congenital heart disease. Drawbacks include poor soft tissue visualisation and exposure to radiation. We describe the first 16 cases of a novel method of cardiac catheterisation guided by MRI with radiographic support.

METHODS

In our cardiac catheterisation laboratory, we combine magnetic resonance and radiographic imaging facilities. We used MRI to measure flow and morphology, and real-time MRI sequences to visualise balloon angiographic catheters. 12 patients underwent diagnostic cardiac catheterisation, two had interventional cardiac catheterisations, and for two patients, MRI was used to plan radiofrequency ablation for treatment of tachyarrhythmias.

FINDINGS

In 14 patients, some or all of the cardiac catheterisation was guided by MRI. In two patients undergoing radiofrequency ablation, catheters were manipulated with use of fluoroscopic guidance and outcome was assessed with MRI. All patients received lower amounts of radiation than controls. There was some discrepancy between pulmonary vascular resistance calculated by flow derived from MRI and the traditional Fick method. We were able to superimpose fluoroscopic images of electro physiology electrode catheters on the three dimensional MRI of the cardiac anatomy.

INTERPRETATION

We have shown that cardiac catheterisation guided by MRI is safe and practical in a clinical setting, allows better soft tissue visualisation, provides more pertinent physiological information, and results in lower radiation exposure than do fluoroscopically guided procedures. MRI guidance could become the method of choice for diagnostic cardiac catheterisation in patients with congenital heart disease, and an important tool in interventional cardiac catheterisation and radiofrequency ablation.

Three-dimensional magnetic resonance imaging of congenital cardiac anomalies

We describe a new method of three-dimensional magnetic resonance imaging of the heart that has been used to produce high quality diagnostic images in 274 patients with congenital cardiac disease, ranging in age from 1 day to 66 years. Using a steady state free precession gradient echo technique and parallel imaging, rapid acquisition of the entire cardiac volume is possible during 8 to 15 sequential breath-holds, each lasting between 8 and 15 s. We obtained high-resolution images, with a resolution of 1 mm3, at between 3 and 10 phases of the cardiac cycle. While images of diagnostic quality were obtained in all cases, in 52 patients there was some degradation due to various factors. Children under 8 years were ventilated, and ventilation was suspended for the breath-holds. For patients breathing spontaneously a novel respiratory navigator technique was developed, using a navigator echo placed over the right hemidiaphragm. This was used successfully in 20 patients, and reduced the misalignment of images obtained during different breath-holds. Images were analysed using multi-planar reformatting and volume rendering. Image processing took approximately five minutes for each study. End-diastolic images were processed for all patients. Systolic images were also processed in selected cases. Further improvements in parallel imaging should reduce imaging times further, so that it is possible to obtain the full volume image in a single breath-hold. This will enable imaging of complex anatomy to be obtained using a standard imaging protocol that does not require the operator to understand the cardiac malformation, making the magnetic resonance imaging of congenital cardiac disease faster and more effective.