Diagnostic significance of interslice gap and imaging volume in body MR imaging

Thin interslice gaps and large imaging volumes are detrimental to MR signal and contrast, especially when the body coil is used. To show the influence of these two factors on fat and water signal and contrast, we performed a series of in vitro experiments. A cylinder filled with water and another filled with oil were imaged transaxially (TR = 2000 msec; TE = 20 and 70 msec) with different interslice gaps (0-150% slice thickness). A series of images was obtained to cover a 40-cm imaging volume. Increasing interslice gap thickness increased water signal without affecting fat signal, resulting in a decrease in fat/water contrast on TE = 20 msec (less T1-weighting) and increasing contrast on TE = 70 msec (more T2-weighting). Contrast nearly doubled when the interslice gap was increased from 10% to 75%. As slices moved away from the central slice, fat and water signals decreased slowly to 12.5 cm off center. Signal loss was significant with offsets greater than 12.5 cm. These results emphasize that to obtain proper contrast on T2-weighted images, the optimal interslice gap should be used and the region of interest should be near the central slice. Otherwise, images should be interpreted with caution.