Fulfilling the Promise

Transcription

1 Fulfilling the Promise of Cardiac MR Non-contrast, free-breathing technique generates comprehensive evaluation of the coronary arteries By Maggie Fung, MR Cardiovascular Clinical Development Manager; Wei Sun, PSD Engineer; Jun Xie, PSD Engineer; and Naoyuki Takei, Applied Science Lab Scientist; GE Healthcare *At the time of printing, 3D Heart (as part of the Optima Edition 16.0 upgrade) is not licensed in accordance with Canadian law and is not available in Canada. 51

2 c a r d i a c i m a g i n g { Even with the continued development of high-field cardiac MR techniques and sequences, imaging the coronary arteries using MR remains a challenge. This is primarily due to the performance resolution required to image the coronaries using MR. Plus, the high spatial resolution and speed of CT imaging has made it a preferred modality over MR for ruling out coronary artery stenosis. However, heightened concerns of patient exposure to radiation and the potential to over-estimate stenosis due to calcification when using CT has led to a renewed interest in coronary MR. In particular, a non-contrast, free-breathing coronary MRA technique is increasingly attractive, especially for the assessment of proximal arteries and anomalous coronaries in congenital heart disease. 3D Heart* is a new addition to the GE Cardiac MR application suite for whole-heart volumetric coronary angiography and free-breathing bright blood imaging of cardiac chambers in congenital heart disease. 3D Heart provides three main techniques optimized for high-field MR imaging: 1. 3D FIESTA (1.5T): SSFP-based sequence with optimized SPECIAL FatSat to suppress epicardial fat (for better visualization of right coronary artery) and T2 preparation pulse to suppress myocardium (for better visualization of the left coronary artery) (Figure 1). The cardiac-gated bright blood SSFP technique provides good visualization of the coronaries, cardiac chambers, and great vessels without the use of a contrast agent. A new steady state preparation reduces off-resonance artifacts and improves the robustness of 3D FIESTA image quality. 2. 3D FGRE (3T): FGRE-based sequence with optimized SPECIAL FatSat, optimized for 3T. 3. 3D IR Prep FGRE (3T): FGRE-based sequence with IR Prep to enhance T1 weighting on 3T (Figure 2). This application can also be used for 3D myocardial delayed enhancement to assess viability of the myocardium on both 1.5T and 3T. Figure 1. PSD: 3D FIESTA with T2 Prep, Navigator Echo, Spectral IR FatSat, and FIESTA acquisition for 1.5T. T2p Nav FS FIESTA T2p Nav FS FIESTA Figure 2. PSD: 3D IR Prep FGRE with Nav Tipup, Navigator Echo, and FGRE acquisition for 3T. IR Prep Nav restore Nav FGRE IR Prep Nav restore Nav FGRE 52 SignaPULSE Spring 2011

3 Cardiac motion suppression One of the major challenges in coronary artery imaging is cardiac motion. In order to visualize the lumen of the coronaries, the imaging window must occur during the quiet cardiac period when the cardiac motion is minimal. The quiet diastolic period normally ranges from 100 to 200 ms, with the systolic period as short as 70 ms, varying based on the patient s heart rate. Therefore, it is critical to select the trigger delay and views segmentation (# RR) carefully to achieve optimal image quality. Although a patient-specific trigger delay will always provide the best result, the 3D Heart application automatically recommends a diastolic trigger delay (calculated based on the heart rate) to the user to improve the robustness of this application. Several acceleration strategies such as ASSET and partial NEX can be employed to reduce the acquisition time within the imaging window. It s also important to note that for patients with high heart rate or arrhythmia, it has been shown that the systolic quiet period is more stable and suitable for acquisition. And, since the systolic period is usually shorter than the diastolic period, the ASSET factor or segmentation (#RR) should be increased accordingly to maintain a short imaging window. Table 1 and 2 provide a guide on the trigger delay and segmentation selection strategies for different patient types. HR Trigger delay 0.5 nex: 1 nex: <75 bpm Diastolic 3 4 >75 bpm Systolic 3 RR + ASSET 4 RR + ASSET Arrhythmic Systolic 3 RR + ASSET 4 RR + ASSET Table 1. Trigger delay and segmentation selection strategies for 3D FIESTA. Real-time prospective respiratory motion correction Another challenge of coronary imaging is breathing motion. Since high resolution imaging is needed and whole heart acquisition is often desired, the acquisition usually takes five to 10 minutes and requires free breathing. 3D Heart utilizes a pencil-beam or spin-echo-based navigator echo to monitor the diaphragm motion to accurately gate the acquisition at end-expiration. It also provides a real-time prospective motion correction using the slab-tracking feature, which automatically shifts slab positions based on the detected diaphragm location to improve motion suppression and increase scan efficiency. Multi-slab easy whole heart acquisition Previously, coronary artery imaging has been difficult due to the oblique orientation of the coronary vessels. However, a recent advancement enables whole-heart acquisitions to eliminate the need for expert-targeted prescription. 3D Heart further provides a multi-slab acquisition option (similar MOTSA) that allows three to four slabs to be acquired at two to three minutes each. Multi-slab acquisition provides several advantages over single-slab acquisition: Improves CNR between vessel and surrounding tissue because of inflow effect; and Minimizes the effect of respiratory drift and heart rate variability on image quality since each slab is acquired within a shorter period of time. HR Trigger delay 0.5 nex: 1 nex: <75 bpm Diastolic 3 4 >75 bpm Systolic 4 to 6 RR 6 to 8 RR Arrhythmic Systolic 4 to 6 RR 6 to 8 RR Table 2. Trigger delay and segmentation selection strategies for 3D IR Prep FGRE. Figure 3. Multi-slab (left) versus single slab (right). Multi-slab acquisition strategy improves vessel contrast due to inflow effect. 53

4 c a r d i a c i m a g i n g Maggie Fung, MR Cardiovascular Clinical Development Manager Wei Sun, PSD Engineer (Images courtesy of National Naval Medical Center, Bethesda, MD) (Images courtesy of Hartford Hospital, Hartford, CT) Figure 4. Stenosis in proximal LAD. Figure 6. Twenty-one-year-old with corrected transposition of the great arteries. (Image courtesy of Stanford University, Palo Alto, CA) (Image courtesy of Advanced Cardiovascular Imaging, New York, NY) Figure 5. Anomalous coronaries. Figure 7. Six-year-old with LV Aneurysm post AV canal repair (heart rate = 85 to 90 bpm). (Images courtesy of Anzhen Hospital, Beijing, China) Figure 8. 3D Heart (IR Prep FGRE) with 8-channel cardiac coil at 3T demonstrating enhancement. 54 SignaPULSE Spring 2011

5 Clinical applications of 3D Heart 3D Heart can be used to assess proximal coronary arteries in a non-invasive, radiation-free manner. It is especially indispensible in the assessment of congenital heart diseases, including the evaluation of the origin of anomalous coronaries, the assessment of cardiac structure in pre- and post-surgery in tetralogy of fallot, and transposition of the great vessel. By selecting the trigger delay time and # RR carefully based on the patient s heart rate and quiet systolic phase, coronaries can be visualized even at high heart rates in small patients, i.e., systolic trigger delay at 4 RR, ASSET x2 (Figure 7). In the 3D IR Prep FGRE imaging mode, the 3D Heart application can also be used for 3D late enhancement (3D MDE) to assess myocardial viability in various cardiac diseases such as myocardial infarction or non-ischemic cardiomyopathies. With free-breathing, navigator-gated acquisition, and resolution of 1.5 x 1.5 x 3 mm 3, high resolution MDE can be obtained. Jun Xie, PSD Engineer Naoyuki Takei, Applied Science Lab Scientist (Images courtesy of Asahikawa City Hospital, Japan) Figure 9. High-resolution 3D MDE acquired axially with 3D Heart (reformatted into short axis slices). Conclusion With various technological advancements, free-breathing MR imaging of the coronaries and other great vessels is now possible. 3D Heart is a versatile application that provides robust, free-breathing, volumetric acquisition of the entire cardiac anatomy. With this volumetric cardiac-gated acquisition, highresolution imaging of the cardiac morphology and adjacent great vessels can be obtained without contrast. This is particularly suited for the assessment of anomalous coronary arteries and other congenital heart diseases. In addition, the 3D Heart application also provides a robust method for imaging 3D late enhancement for the assessment of myocardial viability. These new capabilities provide more comprehensive information of various cardiac diseases in MR imaging. 55