Biomedical Engineering Reference
In-Depth Information
3.2.4.3 Flow-Encoding Gradients
In PC angiography, the bipolar flow-encoding gradients may be applied in a single
direction (e.g. superior/inferior) or in all directions—S/I, A/P, and R/L. In some
anatomic regions, the carotid bifurcation such as application of a single flow-
encoding axis may be sufficient. The resulting images represent flow direction
by the sign of the pixel value in an S/I flow image, for example superior-to-
inferior flow is represented by positive pixel values (brighter), while inferior-
to-superior flow is represented by negative pixel values (darker). In such a
case, single flow-encoding directions will not be adequate i.e., intracranial MRA
shows blood flow components in all directions. However, the total flow image
can be obtained by measurement of individual flow components and combin-
ing them mathematically into a composite image called “velocity image.” This
image is made of flows in multiple directions and has magnitude (in cm/sec)
but no specific direction. Velocity is defined as a vector with a magnitude (in
cm/sec) and direction such as S/I, A/P, and R/L. The individual flow measure-
ments can also generate a phase image with velocity and directional flow in-
formation. In the phase contrast angiograms, display pixel values are propor-
tional to the product of image magnitude and velocity encoding. This relation-
ship of velocity with image magnitude provides quantitative measurement of
velocity.
3.2.4.4 Spatial Misregistration Effects
The reason for spatial misregistration artifacts can be understood with the
pulse sequence. In this sequence, phase encoding fixes the position of an
isochromat in the phase-encoding direction, which occurs shortly after the
90
◦
nutation pulse. This fixing of the isochromat position in the read-out di-
rection is followed by read-out which occurs only at echo time (e.g. approx-
imately at TE, 2TE, etc.) after phase encoding. If spin isochromats move be-
tween these two events in an oblique in-plane direction, their signal is mis-
registered. Spatial misregistration occurs because the position of the flowing
isochromats is identified in the phase-encoding direction prior to the read-out
direction. The result is a shift in intravascular signal intensity in the direc-
tion of flow along the read-out gradient. Measurements of the displacement
of the signal delineating an apparent vessel and the angle between vessel and
