Cardiac Application Tips: Interpreting Phase-Contrast Flow Quantification for Cardiac Images

Interpreting Phase-Contrast Flow Quantification for Cardiac Images
(Sample case)

Noninvasive detection of blood velocity within the heart is the most commonly reported with M-mode and 2D color Doppler echocardiography. Time-resolved phase contrast magnetic resonance imaging (PC-MRI) is an alternative to Doppler, which offers many advantages for quantitative flow evaluation. In particular, MRI can be used to examine blood and tissue using arbitrary image-plane orientations without restrictions due to access windows. Additionally, PC-MRI is insensitive to velocity errors caused by source angle sensitivity. Most importantly, PC-MRI can provide three-component velocity information, compared to the single component from Doppler ultrasound.

Phase contrast cine MRI combines the flow-dependent contrast of phase contrast MRI with the ability of cardiac cine imaging to produce images throughout the cardiac cycle. With the sequences available, PC measurement can be performed in a breath hold or during normal respiration. Prospective, as well as, retrospective methods for synchronization of the data to the cardiac cycle are available. Retrospectively interpolated methods can image the entire cardiac cycle efficiently. This technique produces images in which contrast is related to flow velocity as well as magnitude images, such as those of conventional cine MRI. The data can be interpreted qualitatively to demonstrate the presence, magnitude, and direction of flow, and quantitatively to provide estimates of flow velocity, volume flow rate, and displaced volumes. Phase contrast cine MRI can be used in the diagnosis of aortic dissections, in the study of flow distributions in large vessels (e.g. pulmonary arteries), as well as in smaller vessels such as the carotid and basilar arteries, and in the evaluation of complex anatomical variants.

Phase-contrast (PC) pulse sequences are often performed as an integrated part of cardiovascular MR for the purpose of flow quantification. Two-dimensional PC is most commonly used for measuring the velocity and volume of pulsatile blood flow during different phases of the cardiac cycle. Optimal cardiac gating is therefore essential for successful application of a PC sequence.

The patient is put in a supine position. Before the coils are put on the patient and the study initiated, make sure the cardiac gating is working reliably. Usually anatomic sequences such as double inversion recovery single-shot turbo spin echo (e.g., db-HASTE ), true FISP cine loops or even contrast-enhanced MRA sequences are performed first. These sequences provide anatomic information for prescribing the optimal plane of PC sequence for flow quantification. In addition, the cine images can provide information as to whether valvular or vascular stenosis or regurgitation is present. If significant stenosis is present, consider increasing velocity-encoding variable (Venc) to avoid aliasing artifacts. Usually in adult cardiac studies, a Venc of 250cm/sec is routinely used for aorta (a Venc of 500cm/sec is used in the presence of aortic stenosis).

Quantification of LV and RV cardiac output :
• For this purpose, the plane of PC sequence should be placed at the root of the aorta or main pulmonary artery (MPA) respectively, just above the aortic or pulmonic valves and perpendicular to the vascular lumen.

Quantification of valvular dysfunction :
• For this purpose, the plane of PC sequence is placed just distal to the valve of interest and perpendicular to the direction of blood outflow.

Flow quantification of a (peripheral) blood vessel :
• For this purpose, the plane of PC sequence is placed perpendicular to the blood vessel lumen. If flow quantification of a focal stenosis is desired, the plane should be placed just distal to the site of interest. (In some institutions the plane of the PC sequence is prescribed in the plane of flow, but we routinely perform through-plane PC application as described above).

The acquired PC data set can then be analyzed using semi-automated commercially available software such as Argus. Flow velocity, peak velocity, luminal area, volume of blood flow (within a cardiac cycle or a certain period of time such as 1 minute) can be measured and calculated using such software. The amount of total flow, forward flow and reverse flow through a particular valve can also be depicted, thus regurgitation factor can be calculated. The volume of blood flow through the aorta and main pulmonary artery can be calculated and compared (i.e., LV and RV cardiac output), thus the presence and degree of cardiac shunting can be calculated. In addition, the peak velocity can be further used to calculate pressure gradient across a stenotic vessel or valve using the following formula: