Title: Blood flow effect in MRI SSFP imaging
Authors: Saeid Ahmadinia1, Abbas Nasiraei Moghaddam1,2
Affiliation: 1 Biomedical Engineering, Tehran Polytechnic, Tehran, 2Radiological Science, David Geffen
School of Medicine, UCLA,CA
Degree: 1(SA) Master student, 2(ANM) PhD
Introduction: Steady State Free Precession MRI (SSFP MRI) is a technique of interest for
cardiovascular imaging since its fast and also provides the highest contrast between blood and
myocardium. Time resolved SSFP images in a dynamic view present moving shadows that are
correlated to the blood flow as if it specifies the direction and the speed of blood. This intuitive
interpretation can be useful in many cases, however, needs to be validated and quantified to
avoid misinterpretation on the flow motion. This article studies this effect of blood flow in SSFP.
Methods: In this study an SSFP simulator has been used to discover the effects of moving spins
inside the flow on attaining the steady state condition. The position of spins, their motion track
and also the number of pulses applied to each spin were considered in this simulation. The main
idea is to monitor unsteady spins inside the imaging slice to verify their effects on development
of moving shadows.
Results: In the first simulation a uniform flow of 40 cm/s enters perpendicular to the imaging
plane as shown by the green Rectangle (fig.1). Simultaneously a parabolic flow –with a
maximum velocity of 40 cm/s- drives the spins to the right side. (fig.1)
Fig.1. Spins are imported into the slice through the green rectangular inlet by a uniform 40 cm/s flow profile and
driven to the right side by a parabolic flow profile with a maximum velocity of 40 cm/s. b) area including fresh spins
with high magnetized component a) dephasing area.
Referring to fig.1 the fresh un-saturated spins from out of slice increase the intensity of the
image in the center of parabolic flow profile (fig.1-b).This bright region is surrounded by an area
that looks darker than the background as a result of spin dephasing (fig.1-a). A similar situation
is seen in some actual MR images (fig.2).
Fig.2. A real image of left ventricle and aorta in systole phase. The green arrow refers to the dephasing area and the
red one refers to the area with higher signal intensity because of fresh unsteady spins.
In the next simulation the in-plane maximum velocity decreased to 6 cm/s; As a result of spins
dephasing a dark area (indicated by the red ellipse) is formed. The dynamic view of the images
in the cine mode presents a shadow moving towards the left side while the actual motion is
towards right (fig.3). It is, therefore, a potential source of misinterpretation on the flow motion
and needs to be quantified cautiously.
Fig.3. Spins are imported into the slice through the green rectangular inlet by a uniform 40 cm/s flow profile and
driven to the right side by a parabolic flow profile with a maximum velocity of 40 cm/s. Referring to the red ellipse
there is a moving darkness directed to the left side.
Discussion and conclusions: The motion of spins disturbs the SSFP imaging technique by
making up changes in the intensity of images. Interpretation of blood flow motion based on these
moving shadows should be treated cautiously.
Key Words: SSPF MRI imaging, Flow profile, Pixel
Electronic Mail: Saeid.ahmd@aut.ac.ir
Address: Iran, Tehran, Amirkabir University of Technology, Biomedical faculty, Imaging
Laboratory.
Cell Phone: 09133917106