Echo-train spin-echo, and T2-weighted sequences in general, are important for evaluating the abdomen and pelvis. For liver masses, it is predominantly T2-weighted images that are important for lesion characterization, while T1-weighted images are important for both lesion detection and characterization. T2-weighted images are also
important for the assessment of diffuse liver disease, including iron deposition, edema related to active liver disease, and fibrosis. Echo-train T2-weighted sequences are important for the assessment of fluid-filled structures, including bile ducts, gall bladder, pancreatic duct, stomach and bowel, as well as cysts or cystic masses, abscesses or free fluid in the abdomen or pelvis.
The relative resistance of echo-train images to motion degradation generally yields better resolution of structures internal to cystic masses, such as the septations within a pancreatic serous or mucinous tumor. MR cholangiopancreatography (MRCP) is based on modified echo-train sequences, in which the effective TE is longer, in the
order of 250-500 msec. Lengthening the TE results in heavily T2-weighted high contrast images that
make most soft tissues dark, while the fluid in bile ducts, gallbladder, and the pancreatic duct is very bright.MRCP can be performed in thin sections of 3-4 mm for higher spatial resolution, or by using a single thick slab of 3-4 cm if the need is to include the majority of the pancreatic and bile duct in a single image.As a result of the repeated refocusing echo pulses, echo-train imaging is relatively insensitive to respiratory motion and bowel peristalsis. Combined with its relative resistance to the paramagnetic distortion effects of intraluminal bowel gas, echo-train imaging is therefore particularly well-suited to imaging the bowel.
Fat is high in signal intensity on echo-train spin-echo sequences in comparison to conventional spin-echo sequences, in which fat is intermediate in signal intensity. The MR imaging determination of recurrent malignant disease versus fibrosis for pelvic malignancies illustrates this difference. Recurrent malignant disease in the pelvis (e.g., cervical, endometrial, bladder, or rectal cancer) generally appears high in signal intensity on conventional spin-echo sequences because of the higher signal intensity of the diseased tissue relative to the moderately low-signal intensity fat. In contrast, fat is high in signal intensity on echo-train spinecho images, and recurrent disease will commonly appear relatively low in signal intensity. Unfortunately, the fact that abnormal tissue is not high in signal intensity on echo-train T2-weighted images
relative to fat is not specific for neoplasm, as fibrosis can have a similar appearance. This is particularly problematic in post-therapy patients.
Fat may also be problematic in the liver because fatty liver is high in signal intensity on echotrain spin-echo sequences, thereby diminishing contrast with the majority of liver lesions, which are also generally high in signal intensity on T2- weighted images. It is often necessary to use fat suppression on T2-weighted echo-train spin-echo sequences for liver imaging. Fat suppression should generally be applied to at least one set of
images of the abdomen or pelvis, to ensure optimal contrast between high signal abnormalities, such as fluid collections or cystic masses, and adjacent intra-abdominal or pelvic fat.
Gaa J, Hutabu H, Jenkins RL, et al. Liver masses:
replacement of conventional T2-weighted spin
echo MR imaging with breath-hold MR imaging.