Biomedical Engineering Reference
In-Depth Information
7.5.2.2
In-Vivo MRI Flow Measurement
Phase-contrast-MRI was applied onto the short axis orientation through the atria of
the heart (Wong et al. 2009a). Scan acquisition of 25 phases or time frames (based
on their indices from n t = 1 to 25) was completed based on retrospective gating. The
human subject volunteer was a normal male of age 22 years at the time of scanning.
The imaging used a two-chamber view of the right and left atria. The main objective
was to capture information on the large-scale vortices in the right atrium. These
vortices serve as principle features of the flow structures to analyze. The acquisition
parameters include: echo time TE = 1.6 ms, repetition time TR = 47.1 ms, field of
view FOV = (298 × 340) mm 2 at a (134 × 256) pixel matrix, and a velocity encoding
(VENC) of 150 cms −1 . The in-plane resolution of 1.54 mm/pixel was determined by
the pixel spacing and the through-plane resolution based on a slice interval of 6 mm.
Figure 7.31 describes phase contrast in MRI based on spin phase shifts that
are proportional to the blood velocity (Markl et al. 2003a; Yu et al. 2003). Phase
contrast MRI signals can be represented using intensity images. The intensity of
each pixel corresponds to the blood velocity at the measured location. To quantify
a velocity in one spatial dimension, at least two phase images must be taken for
subtraction of flow-induced phase shift. These are taken from background phas-
es caused by susceptibility-induced inhomogeneities and coil sensitivity changes
(Baltes et al. 2005). The flow sensitivity of a phase contrast signal acquisition is
given the term velocity encoding and is usually abbreviated as VENC (Svensson
2003). The VENC limits are defined by a set criterion which affects flow measure-
ments (Lagerstrand et al. 2006). Blood velocity will be aliased to an artificially low
value if it exceeds the maximum VENC level by the flow sensitisation gradients.
The phase contrast MRI technology is clinically attractive because it is able to
provide quantitative information on blood flow without the need for contrast agent
to be introduced into the human body. Moreover, ECG-gated phase contrast MRI
can provide cine-images for temporal velocity mapping (Thompson and McVeigh
Fig. 7.31  Phase contrast MRI velocimetry. Phase contrast MRI works on the concept that hydro-
gen nuclei from blood that has been exposed to magnetic fields accumulate a phase shift in spin
that is proportional to the blood velocity in the x, y, and z directions. Velocities v x , v y and v z bare
functions of the subtractions of spin phases φ x , φ y and φ z bof measured volumes with that of the
reference phase φ Ref
Search WWH ::




Custom Search