Magnetic resonance imaging of the thoracic aorta

Calibration of the Mechanical Boundary Conditions for a Patient-Specific Thoracic Aorta Model Including the Heart Motion Effect

OBJECTIVE

We propose a procedure for calibrating 4 parameters governing the mechanical boundary conditions (BCs) of a thoracic aorta (TA) model derived from one patient with ascending aortic aneurysm. The BCs reproduce the visco-elastic structural support provided by the soft tissue and the spine and allow for the inclusion of the heart motion effect.

METHODS

We first segment the TA from magnetic resonance imaging (MRI) angiography and derive the heart motion by tracking the aortic annulus from cine-MRI. A rigid-wall fluid-dynamic simulation is performed to derive the time-varying wall pressure field. We build the finite element model considering patient-specific material properties and imposing the derived pressure field and the motion at the annulus boundary. The calibration, which involves the zero-pressure state computation, is based on purely structural simulations. After obtaining the vessel boundaries from the cine-MRI sequences, an iterative procedure is performed to minimize the distance between them and the corresponding boundaries derived from the deformed structural model. A strongly-coupled fluid-structure interaction (FSI) analysis is finally performed with the tuned parameters and compared to the purely structural simulation.

RESULTS AND CONCLUSION

The calibration with structural simulations allows to reduce maximum and mean distances between image-derived and simulation-derived boundaries from 8.64 mm to 6.37 mm and from 2.24 mm to 1.83 mm, respectively. The maximum root mean square error between the deformed structural and FSI surface meshes is 0.19 mm. This procedure may prove crucial for increasing the model fidelity in replicating the real aortic root kinematics.

Magnetic resonance imaging of the thorax. Past, present, and future

Magnetic resonance imaging is a valuable modality of extreme flexibility for specific problem-solving capability in the thorax. This article reviews MR applications in the imaging of great vessels, which are currently the most important applications in the thorax; other established applications in the thorax; and pulmonary functional MR imaging.

Magnetic resonance imaging of the thorax. Past, present, and future

Magnetic resonance is a valuable modality of extreme flexibility for specific problem-solving capability in the thorax. This article reviews MR applications in the imaging of great vessels, which are currently the most important applications in the thorax; other established applications in the thorax; and pulmonary functional MR imaging.

Imaging of thoracic aortic disease

Computed tomography, magnetic resonance imaging, and transesophageal echocardiography represent the relatively noninvasive techniques available for imaging thoracic aortic disease, especially in the evaluation of aneurysms and dissections. The article discusses the technique and application of these modalities in the evaluation of thoracic aorta. Imaging appearances of the commonly encountered pathologies of the thoracic aorta are presented and discussed, and potential pitfalls of technique and diagnosis are addressed.

Gradient echo MRI for measurement of the pulmonary autograft diameter after transplantation to the aortic root: validation and comparison with ultrasound

The purpose of this study was to s the value of MRI for measurement of pulmonary autograft diameters after transplantation to the aortic root in adults. Thirty-eight adults underwent this operation. MRI and transesophageal echocardiography (TEE) were performed in 30 and 27 patients, respectively, after a mean follow-up period of 2.8 years. For internal validation of MRI, measurements at the diastolic short and long axes of the sinus level were used. Pulmonary autograft diameters were measured and compared with MRI and TEE at five different levels: the subannular region (1), annulus (2), sinus (3), sinotubular junction (4), and the distal part of the autograft (5). The correlation coefficient (r2) between long- and short-axis measurements for corresponding sinuses was .97. Diameters obtained with MRI were 1 to 3 mm larger than those obtained with TEE (P < .05), except for the annulus at systole (P > .3). Cine gradient echo MRI is an appropriate technique to evaluate pulmonary autograft diameters during follow-up. Concordance with TEE was good, apart from a systematic difference of approximately 2 mm.

Procedures for estimating growth rates in thoracic aortic aneurysms

Thoracic aortic aneurysms (TAAs) are potentially lethal medical conditions often requiring surgical intervention. Reliable information on TAA growth rates and associated risk factors is important for managing this challenging patient population. Unfortunately, a number of studies have employed questionable statistical methods, leading to biased and imprecise estimates. The present study describes these statistical problems in existing studies and delineates procedures for obtaining more reliable results. Using data from the Yale Center for Thoracic Aortic Disease, the study compares TAA growth rate estimates using conventional methods versus the recommended approach of instrumental variables (IV) estimation. The IV approach is designed to mitigate problems of measurement errors inherent in existing estimates of TAA growth. The results demonstrate that IV estimation yields more robust and precise estimates of TAA growth rates and risk factors for TAA growth. For example, the conventional approach yields TAA growth rates that fluctuate substantially-from 0.12 cm/yr to 0.90 cm/yr-depending on (1) the minimum serial follow-up period for patient inclusion in the study and (2) how subjects with negative measured growth rates are handled. In contrast, growth rate estimates using the IV approach are much more robust, ranging from 0.12 to 0.13 cm/yr. The 95% confidence intervals of estimated TAA growth are much more compact using the IV approach as well. We conclude that the IV estimation procedure yields more reliable estimates of TAA growth than does the conventional approach.

Surgical management of ascending and aortic arch disease: refined techniques with improved results

BACKGROUND

Treatment of aneurysms of the ascending aorta, arch aorta, or both is surgically challenging and has traditionally carried a high hospital mortality rate. The use of refined operative techniques, including improved grafts, enhanced myocardial protection, retrograde cerebral perfusion with circulatory arrest, transesophageal echocardiography, and control of hematologic factors, has resulted in reduced hospital mortality rates.

METHODS

We conducted a retrospective analysis of records of 117 consecutive patients who underwent 118 procedures between March 1987 and September 1997, for graft replacement of the ascending or transverse aortic arch with or without aortic valve reconstruction or replacement. There were 67 men (57.3%) and 50 women (42.7%). The mean age was 61.4 years (range, 16 to 81 years). Aortic abnormalities were medial degeneration in 59 patients (50.0%), dissection in 28 patients (23.7%), atherosclerosis in 16 patients (13.6%), Marfan's syndrome in 8 patients (6.8%), and other in 7 patients (5.9%).

RESULTS

The ascending aorta alone was replaced in 58 patients (49.2%), ascending and arch aorta in 56 patients (47.5%), and isolated arch aorta in 4 patients (3.4%). Twenty-six patients (22.0%) required aortic valve reconstruction, 17 patients (14.4%) had separate aortic valve replacement, and 37 patients (31.4%) received a valve conduit. Overall hospital mortality rate was 3.4% (4 of 117 patients). Postoperative complications included myocardial infarction in 3 patients (2.5%), stroke in 7 patients (5.9%), pulmonary insufficiency in 22 patients (18.6%), renal insufficiency in 4 patients (3.4%), and reoperation for bleeding in 8 patients (6.8%). There were no deep sternal wound infections. Follow-up was completed for 112 (99.1%) of 113 survivors and ranged from 1 month to 10.6 years (mean, 39.5 months). Actuarial survival for patients discharged from the hospital was 87.9%+/-3.7% (standard error of the mean) at 3 years and 79.7%+/-5.8% at 6 years.

CONCLUSIONS

Graft replacement of the ascending and transverse aortic arch, although technically demanding, can be performed with low hospital mortality and morbidity rates.