Cardiac MRI Phase contrast imaging David M Chaky

Cardiac MRI Phase contrast imaging David M. Chaky, MD

Cardiac MR primer The 2 primary sequences used in cardiac imaging are the spin-echo and gradient-echo sequences. § The spin-echo, or dark blood, sequence provides excellent spatial resolution and is used to study anatomy and morphology. § The gradient-echo, or bright blood, sequence samples multiple data points during the cardiac cycle. § Because there is intrinsic contrast between flowing blood and static tissue, no contrast agents are required. The resulting gradient-echo images can be displayed in a continuous loop of ventricular contraction and relaxation, so-called cine-MRI, to assess regional and global ventricular function. §

§ Common orientations used in MRI of the heart include the vertical long axis (so-called 2 -chamber view), horizontal axis (socalled 4 -chamber view), and short axis (see Figure 1). From these standard views, cardiac chamber sizes and volumes can be measured and calculated. Wall thickness can be measured to determine myocardial mass.

Cardiac MR primer MRI is invaluable in visualizing complex congenital heart disease. Venoarterial, atrioventricular, and ventriculoarterial connections are readily identified, as are chamber morphology, position, and relation to other visceral organs and great vessels. § MRI can demonstrate the location, extent, and attachment of intracardiac and paracardiac masses. § Generalities regarding tissue composition can be made based on the signal intensity and homogeneity, because there are different relaxation properties between normal and diseased tissue - MRI can help elucidate infiltrative processes such as hemochromatosis, amyloidosis, and fatty replacement of myocardium in right ventricular dysplasia. §

§ § § Ventricular function Global and regional ventricular function can be measured from cine-MRI. The ejection fraction is calculated from tomographically determined end-systolic and end-diastolic volumes. Ventricular remodeling and myocardial thinning after myocardial infarction can be appreciated easily

Valvular assessment In cine-MRI, turbulent blood flow creates an area of low signal (“signal void”) due to dephasing (asynchronous precession) of the hydrogen nuclei. § A qualitative assessment of the degree of valvular disease can be made based on the size of the jet, the duration of the jet in the cardiac cycle, and the presence of a proximal convergence zone (an area of flow acceleration) on the opposite side of the valve. § §

Cardiac MR primer § Pericardial evaluation § Pericardial effusions can be defined, and some generalities about fluid composition can be made, based on signal intensity. The thickness of the pericardium can be measured and should be <3 to 4 mm.

Cardiac MR primer Magnetic resonance angiography § Most MRAs can be performed without administration of a contrast, although contrast agents may improve image quality in certain circumstances. § Magnetic resonance angiography depicts normal laminar flow as bright signal, whereas turbulent, diminished, or absent flow is depicted as signal voids. §

Intimal flips, true and false lumen identification, thrombus formation, origin of the tear, involvement of the brachiocephalic vessels, and the presence of aortic regurgitation or hemopericardium all can be seen. § Aortic aneurysms are well delineated with MRA, and 3 -Dl reconstruction is helpful in planning surgical repair. §

Cardiac MR – other common adult indications § Myocardial perfusion imaging § Magnetic resonance spectroscopy § Coronary MRA

Case § History - § Sinus venosus septal defect with partial in almost pulmonary venous return of the right pulmonary veins. Evaluate pulmonary venous drainage.

TECHNIQUE- Axial gated FIESTA, two chamber, four chamber, and short axis FIESTA images were performed. § Three dimensional gadolinium enhanced magnetic resonance angiography of the chest was obtained with multiplanar reformatted images. § Cine phase contrast imaging of the aorta and main pulmonary artery was obtained with calculation of Qp/Qs. §

Moderately to severely dilated right atrium and ventricle. § LV FIESTA 2 chamber

Moderately to severely dilated right atrium and ventricle. § LV FIESTA axial nongated

Sinus venosus septal defect associated with partial anomalous pulmonary venous return of the right upper and middle lobe pulmonary veins to the superior vena cava. § MIP

§ PC LVOT

§ PC RVOT

Cardiac MR report § Sinus venosus septal defect associated with partial anomalous pulmonary venous return of the right upper and middle lobe pulmonary veins to the superior vena cava. Qp: Qs = 4: 1 § The upper pulmonary vein drains to the superior vena cava approximately 2. 3 cm above the expected location of the caval atrial junction. Moderately to severely dilated right atrium and ventricle. Preserved right and left ventricular systolic function. Enlarged main, right, left pulmonary arteries.

Ventricular function § Right ventricle- The right ventricle is moderately to severely enlarged with preserved systolic function. - RV end diastolic volume- 138 ml - RV end diastolic volume indexed to body surface area 199 ml/m 2 - RV ejection fraction- 61% Left ventricle- The left ventricle appears structurally normal, with good systolic function. - LV end diastolic volume- 33 ml - LV end diastolic volume indexed to body surface area 48 ml/m 2 - LV ejection fraction- 65%

Phase contrast MR Dark blood (spatial saturation) – air, bone, & other substances also have low signal § Bright blood (TOF or CE) – other tissues may have high signal § PC – changes from motion itself are used to depict blood flow § PC – better background suppresion & flow measurement §

Phase contrast MR - 2 D § § § 2 D PC – useful for getting images in suspended respiration or differing phases of cardiac cycle Background tissue is gray Flow in the positive direction along the flow-encoded axis is light (white) Flow in opposite direction is dark (black) Can even be color coded similar to Doppler US

Phase contrast MR - 2 D Measurement of pulsatile flow most accurate w/ cardiac gating (aka cine phase contrast) § 2 D useful for blood velocity and volume §

Phase contrast MR - 3 D 3 D PC are useful for depicting flow w/ high SNR, high spatial resolution, and high background suppression § Major disadvantage is long acquisition time §

Phase contrast MR Flow encoded phase shifts result from the application of bipolar magnetic field gradients, which are composed of two lobes w/ opposite signals. § When the 1 st lobe is applied, the spins of the stationary and moving tissues begin to accumulate phase. § Immediately after, the 2 nd lobe is applied, and the stationary protons lose their phase and accumulate a net phase of 0. § Blood that moves in between the 2 gradients experiences unequal positive and negative gradients and therefore accumulates a net phase shift §

§ Phase-difference methods are based on the principle that hydrogen nuclei moving through a magnetic field gradient accumulate a phase shift proportional to their velocity. § Magnitude and phase images. Blood in the ascending aorta appears bright on the phase image, indicating flow in a cranial direction through this transverse image plane. Caudally directed flow in the descending aorta appears dark.

Phase contrast MR The accurate measurement of the ratio of pulmonic to systemic flow (Qp/Qs) is important in the evaluation of the patient with left-to-right intracardiac shunting, since it is often used to determine subsequent therapy. § The asymptomatic patient with a Qp/Qs of <1. 5 usually is managed conservatively, whereas the patient with a Qp/Qs of >1. 5 whose pulmonary vascular resistance is not markedly elevated usually is referred for surgical correction. §

Phase contrast MR The available methods of determining the magnitude of left-to-right intracardiac shunting include oximetry, indicator dilution, radionuclide scintigraphy, and Doppler echocardiography. The oximetric and indicator dilution techniques, although relatively easy to perform, are invasive. Radionuclide scintigraphy and Doppler echocardiography are noninvasive, but the former requires the injection of a radionuclide, and the latter may be technically difficult or impossible to perform in some patients. § PC MR is a rapid, accurate, noninvasive, and widely applicable method of estimating the magnitude of intracardiac left-to-right shunting is desirable. §

Circulation. 1995; 91: 29552960. ) § Pulmonary-to-systemic flow ratios (Qp/Qs) by catheterization (horizontal axis) and by magnetic resonance imaging (MRI) (vertical axis) for 21 subjects. Each symbol represents the data from 1 patient. The regression line is shown. All points fall within the two shaded areas, indicating that compared with oximetry and indocyanine green, MRI correctly identified all subjects with a Qp/Qs < or 1. 5.

Phase contrast MR With MRI, the cardiovascular system can be imaged with high spatial resolution. § Velocity-encoded, phase-difference MR images have been used to measure flow in the carotid artery, proximal aorta and pulmonary artery, and abdominal aorta. § Flow is calculated by multiplying blood velocity by the cross-sectional area of the vascular structure of interest. With this technique, flow can be measured quickly; scans can be performed in less than 2 minutes. §

§ Magnitude (top) and corresponding velocity maps (bottom) acquired for measuring flow in the proximal aorta. Frames 4 and 6 from the 9 frames acquired during the cardiac cycle are displayed. The letter A indicates the lumen of the proximal aorta. On the velocity map, the gray scale intensity for each pixel encodes for velocity. For each frame of the cardiac cycle, velocity within the vessel is calculated as the average velocity for all the pixels within the lumen.

§ Magnitude (top) and corresponding velocity maps (bottom) of the pulmonary artery in the same two frames. The letter P indicates the lumen of the main pulmonary artery. In frame 4, the area of the vessel and velocity of the blood flowing through it are larger than in frame 6, thus indicating substantially higher flows for earlier frames of the cardiac cycle. This patient had substantial left-to-right intracardiac shunting (pulmonaryto-systemic flow ratio of 2. 1 by magnetic resonance imaging).

Phase contrast MR Mean blood flow = spatial mean velocity x cross sectional area of vessel § Pressure gradients are calculated by the modified Bernoulli equation: Delta P = 4 v^2 (p is in mm of mercury and v is in m/sec) §

Phase contrast MR Flow measurements can be plotted on a graph (throughout the cardiac cycle) and the area under the curve can be used to derive flow volume § Aliasing is possible is the max velocity value is lower than the actual velocity § Small vessels are poorly evaluated §

§ Each point on the curve represents the mean blood flow within the artery at a phase of the cardiac cycle. § Each value is derived from a separate VEC MR image. § The blood flow is a product of the spatial mean velocity and the cross-sectional areas of the vessel.

Effect of different VENC values on 2 d PC images sensitive to flow in the superior/inferior axis

References § Cardiovascular MRI & MRA by Charles B. Higgins, Albert De Roos § MRI Principles by Donald Mitchell § Cardiovascular Magnetic Resonance by Warren Manning § Assessment of Left-to-Right Intracardiac Shunting by Velocity. Encoded, Phase-Difference Magnetic Resonance Imaging Circulation. 1995; 91: 2955 -2960.