The Early Postoperative Sella

Abstract

Reading a postoperative sellar MRI is generally considered as one of the most difficult challenges during imaging of the sellar region, particularly in the immediate postoperative period. Early postoperative MRI is usually not performed in most pituitary centers, and the corresponding literature is scarce. Presence of blood products and packing material have been accused of masking the sellar content and possible residual tumor in the first weeks after surgery. In our experience, this is true only with pituitary tumors with liquid (hemorrhagic or necrotic) content (Fig. 26.1). Early MRI, at day 3 or 4 after transsphenoidal surgery, is routinely obtained in our center and compared with the operative report. We do not perform immediate postoperative MRI, as proposed by some, the T1 signal of blood being still at day 1 isointense to the brain. At this stage, we strongly recommend noncontrast T1W sequences, which are the most informative. Conversely, we consider T2WI more prone to differentiate normal residual pituitary tissue from tumoral remnant beyond 3 months postoperatively. It is our opinion that gadolinium perfusion, at this stage, complicates the interpretation of MRI and must be avoided. Early postoperative MRI has numerous advantages: it can detect complications before the patient leaves the hospital, such as hemorrhage outside the tumoral limits (Fig. 26.2), overpacking of the sella (Fig. 26.3) or CSF fistula (Fig. 26.4). Some depletion of vasopressin storage can be observed on day 4 postoperative MRI, best appreciated on axial T1W fat-saturates noncontrast images. In case of postoperative diabetes insipidus, complete extinction of the bright spot will occur some days later, i.e., after complete depletion of vasopressin storage. MRI permits visualization of the surgical bed and mostly a tumoral remnant, if any, more easily if a hemostatic packing has been put in place. In any case, early postoperative MRI facilitates the reading of future examinations. MRI characteristics of hemostatic materials (Gelfoam, Surgicel) packed in the few days after surgery are as follows. It is a mass with regular contours, T1 isointense with the gray matter, with a peripheral rim of T1 hyperintensity corresponding to methemoglobin deposit (Fig. 26.5). Air bubbles caught in the fabric appear as inframillimetric rounded T1-hypointense images. The part of surgical bed free of hemostatic material corresponds to either tumoral remnant or normal residual pituitary tissue, the latter being usually smaller than the former. In the case of cavernous sinus invasion, the normal anterior pituitary gland is found contralaterally, but is frequently not clearly identified on early postoperative MRI. Of course, MR signal of tumoral remnant is identical to that of the tumor before surgery. Apart from cavernous sinus, location of the tumoral remnant is driven by the laws of ballistics, i.e., superiorly in the suprasellar cistern, and inferiorly and posteriorly within the sphenoid sinus (Fig. 26.6). Autogenous subcutaneous fat graft taken from the abdominal wall or from the lateral thigh is used in case of CSF leak; it is easily recognized in the form of a T1-hyperintense brilliant homogeneous polylobulated mass surrounded by a dark T1-hypointense line oriented perpendicular to the frequency-encoded direction and corresponding to chemical shift artifact (Fig. 26.7). Fat signal is suppressed with the use of fat-saturated sequence. If necessary, surgeons can also use vaporized glue instead of Surgicel to assure hemostasis of the surgical area. In these cases, the sellar content appears homogeneously isointense to the brain, potentially making it difficult to differentiate serous collection from residual tumor. T2WI can help (Fig. 27.4). Muscle, fascia, cartilaginous matter, and bone collected from the nasal approach or silicone plate used to seal the sella are better identified on follow-up MRI.