Brian P. Jacob, David C. Chen, Bruce Ramshaw, Shirin Towfigh (eds.) - The SAGES Manual of Groin Pain-Springer International Publishing (2016)

13. Imaging for Evaluation of Groin Pain
181
magnetic field. This opposition is relatively unstable and the flipped
hydrogen atoms will eventually switch back to their natively aligned
state, emitting their own radio- frequency pulse and communicating back
with the scanner as they do so. Since every tissue is different, the rate of
this process varies dramatically throughout the body and allows for tissue
discrimination. Foreknowledge of how fat, water, and other body
substances will behave under these conditions allows for the targeting
by specific “sequences” such as T1-weighted sequences for fat,
T2-weighted sequences for water, and short - tau inversion recovery
(STIR) for edema, among many others. Compare this to computed
tomography, where there is only one parameter (i.e., density) that significantly
impacts tissue discrimination.
MR excels at differentiating between many subtle soft tissue and
musculoskeletal pathologies responsible for groin pain, such as those
seen in iliopsoas tendinosis, bursitis, osteitis pubis, and athletic pubalgia
(Figs. 13.9 , 13.10 , and 13.11 ). Avascular necrosis of the femoral head in
particular is apparent on MR long before it is demonstrable by CT (Fig.
13.12 ). Yet MR is not always a definitive examination, particularly with
respect to certain musculoskeletal pathologies in which the relative lack
of hydrogen in bone can necessitate correlation with conventional radiography
or CT. While radiographically occult stress fractures are clearly
identified on MR, many other benign and malignant osseous lesions
may be indistinguishable from each other on the basis of MR alone.
Definitive evaluation of acetabular labral tears is via MR arthrography,
wherein contrast material is directly injected into the hip joint (Fig. 13.13 ).
Dynamic MR has become the primary evaluation of pelvic floor dysfunction
now that most centers have stopped performing colpography
and defecography, which involve the respective administrations of vaginal
or rectal contrast agents followed by fluoroscopic visualization during
Valsalva maneuver (Fig. 13.14 ). Dynamic MR can demonstrate
ligamentous laxity and organ prolapse (rectocele, cystocele, enterocele),
as well as less conspicuous pathologies such as vesicovaginal and rectovaginal
fistulae. Occult inguinal hernias also benefit from evaluation
with dynamic MR, as will be discussed later in this chapter. Other gynecologic
sources of groin pain such as endometriosis, uterine fibroids,
and ovarian masses/cysts are well evaluated on MR but can often be
more readily and economically demonstrated with ultrasound.
Intravenous MR contrast material is fundamentally different than the
iodinated material used for CT. The risk of contrast reaction is significantly
lower with gadolinium-based MR contrast agents, and there is no