A 2D-TOF MR venography pitfall

Clinical History

Lytic skull metastasis, near the confluence of sinuses. MR venography using 2D-TOF and 3D contrast-enhanced MP-RAGE to analyse the permeability of dural sinuses.

Imaging Findings

A patient with a medical history of a metastatic follicular thyroid carcinoma, treated by thyroidectomy and iodine-131-radioablation therapy, was referred to the neurosurgery department because a swelling of the nucha appeared. CT scans demonstrated a lytic occipital mass. MR venography was justified to appreciate before surgery the permeability of the torcular Herophili, transverse sinuses and superior sagittal sinus.

MR imaging studies were performed on a 1.5-T imager using two-dimensional fast low-angle shot sequence (2D-FLASH, TR/TE 30/9, flip angle 50, matrix 224*256, slice thickness 3mm, acquisition time 8’14’’), T1-weighted (SE, TR/TE 500/12, matrix 192*256, slice thickness 6mm) and T2-weighted (TSE, TR/TE 3000/16;98, matrix 230*512, slice thickness 6mm) spin echo sequences, and a three-dimensional contrast-enhanced magnetisation-prepared rapid gradient-echo sequence (3D-gadolinium MP-RAGE, TR/TE 9.7/4, flip angle 12, matrix 150*256, slice thickness 2mm, acquisition time 4’11’’). MR venography without gadolinium injection was carried out using 2D time-of-flight (TOF) technique and the maximum intensity projection (MIP) algorithm.

2D-TOF MR venography showed no flow signals in the confluence of sinuses and in the superior sagittal sinus, whereas 3D contrast-enhanced MP-RAGE demonstrated that this hypervascular skull mass was pressing on the dural sinuses without any sign of dural invasion or thrombosis. The neurosurgeon was informed of the permeability of the intracranial venous system. The tumour was explored through a midline suboccipital craniectomy. It was a very haemorrhagic extradural mass but excision of the tumour was performed to its full macroscopic extent.

Discussion

Skull metastases from thyroid carcinoma are often single lytic (1) hypervascular lesions (2), as in this case report.

2D-TOF MR venography, which is an in-flow based method, failed to depict flow in the torcular Herophili and in superior sagittal sinus in this patient. Using TR = 30ms and a 3mm slice thickness, the minimum velocity encoding (3) by this FLASH sequence is: 0.3/0.03 = 10cm/s. However, 3D contrast-enhanced MP-RAGE clearly delineated the dural sinuses without a filling defect. Therefore the flow velocity in this superior sagittal sinus must be lower than 10cm/s.

In cases of doubt over tumour invasion of the dural sinuses or in cases with very slow flow, it is recommended not to use the TOF technique, but to perform either 3D contrast-enhanced MP-RAGE (4) or phase-contrast MR angiography (2D or 3D) with a low-velocity encoding (5).

Differential Diagnosis List

Final Diagnosis

Limitation of 2D-TOF MR venography

URL:	https://www.eurorad.org/case/1624
DOI:	10.1594/EURORAD/CASE.1624
ISSN:	1563-4086