Imaging Cranial Base Chordoma and Chondrosarcoma

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Imaging Technique

MR is essential for determining the signal characteristics and soft tissue extent of chordoma and chondrosarcoma. An MR with gadolinium-based contrast agent should be performed if the patient has no absolute contraindication to MR imaging. Serum creatinine should be checked prior to gadolinium administration to calculate the glomerular filtration rate (GFR). Gadolinium is not generally administered to patients with a GFR less than 30 (mL/min/1.73 m ² ), even if they are on dialysis. MR sequences should include T1-weighted precontrast as well as T2-weighted, diffusion-weighted, and T1-weighted postcontrast fat-suppressed images. In most cases, the information from these four sequences is sufficient to make the correct diagnosis. To obtain additional information about the internal characteristics and the extent of skull base tumors, many also perform additional sequences, including heavily T2-weighted steady-state imaging, gradient echo T2∗ imaging, and/or fluid-attenuated inversion recovery (FLAIR) imaging. Heavily T2-weighted steady-state sequences are called fast imaging employing steady-state acquisition (FIESTA) or constructive interference in steady state (CISS) depending on the manufacturer of the MR imaging machine. These steady-state sequences provide thin-section T2-weighted imaging of the basal cisterns and adjacent fluid-containing structures. They are relatively insensitive to cerebrospinal fluid motion and are helpful for evaluating small structures that traverse the basal cisterns such as blood vessels and cranial nerves. Diffusion-weighted imaging is also typically performed, not only to evaluate the diffusion characteristics of the tumor but also to assess for any complications such as posterior fossa infarction or other tissue injury. This sequence is especially important in the postoperative setting. Postgadolinium T1-weighted images are typically performed with fat suppression to help distinguish enhancing tumor from intrinsically bright fat, which is abundant in the marrow spaces of the skull base and the adjacent soft tissues. Fat-suppressed images do often suffer from artifacts related to the air, bone, and soft tissue interfaces that are common at the skull base, but these artifacts can be mitigated by careful attention to imaging technique.

CT may be the first imaging study performed in a patient who initially presents with nonspecific symptoms such as headache and who is found to have a skull base tumor. If CT is not the initial study obtained, then it should be performed to complement the MR, as CT provides essential information about the integrity of the bones of the skull base and can be very helpful in the differential diagnosis of an unknown lesion. On CT, bones should be evaluated with a sharp or bone kernel reconstruction using bone windows. Soft tissues should be evaluated with a soft tissue kernel reconstruction using soft tissue windows. If an MR examination using a gadolinium-based contrast agent has already been performed, then a CT examination without iodinated contrast is appropriate to evaluate the adjacent bony structures. If no MR has been performed or if no gadolinium-based contrast agent was administered, then CT images should be acquired with iodinated contrast to assess the enhancement characteristics of the skull base mass.