Radiology, Vol 157, 95-101, Copyright © 1985 by Radiological Society of North America
MR technology: effect of even-echo rephasing on calculated T2 values and T2 images
W Kucharczyk, M Brant-Zawadzki, L Lemme-Plaghos, A Uske, B Kjos, DA Feinberg and D Norman
In multiple spin-echo image sequences of blood flow, the "even-echo"
phenomenon produces an absolute increase in signal magnitude from first- to
second-echo images of normal vessels harboring slow flow. Distinguishing
this from the apparent relatively high signal intensity seen on second-echo
images in pathologic foci of stationary tissue is important to the
diagnostician. Selected case material containing two tissue types was
reviewed retrospectively: tissues known to harbor slow flow, such as normal
veins and venous sinuses and vascular malformations, and tissues that have
long transverse (T2) relaxation times and appear as intense structures on
second-echo images, such as neoplasms, infarcts, and regions of
demyelination. Calculations of T2 parameters were made by computer for
defined regions of interest. T2 images were also generated. Visual
inspection of the acquired images did not reliably distinguish increased
intensity due to even-echo rephasing from the relative changes between
adjacent tissues seen on second-echo images. More definitive
differentiation of the even-echo phenomenon was provided by calculated
values of T2 and computer- synthesized T2 images representing acquired
intensity data of two-echo sequences. The synthesized images were
especially useful when stationary tissue with lengthened T2 values was
adjacent to or in proximity to vessels or vascular lesions. A five
spin-echo image sequence was valuable for separating slow flow from
stationary tissue by a technique of synthesizing T2-difference images using
three consecutive echoes.
