Contrast Enhanced Brain Imaging

Introduction

Since the development of brain CT in the early 1970s, intravenous iodinated contrast enhanced brain imaging has been recognized as providing additional information. It improves the definition of normal brain anatomy by showing clearly the position of the vessels and dura and enhances the definition of areas of blood-brain barrier (BBB) breakdown found in tumours and other pathologies. MDCT makes it possible to acquire of volume of data with sub-millimetre slices and to reformat images in any standard or curved plane with isotropic voxels. Volume rendered reformations are possible from any volume acquisition and can provide a unique display of pathology or anatomy so useful in communicating information to clinicians. Current workstations provide such rapid reconstructions that they are now used routinely in many situations. This MDCT technique challenges the standard features of MR and offers more flexibility in image manipulation than MR.

The technique described below is the optimal sequence for the demonstration of any lesion within the cranium or craniocervical junction when specific vascular information is not required. It may be used with limited coverage for the investigation of possible posterior fossa pathology, e.g. facial pain, deafness, ataxia, cough impulse headache. When bone detail is also required the same data can be reprocessed with a bone filter and viewed on bone windows.

This protocol may also be used without intravenous (IV) contrast in place of the routine brain study described earlier, if this is thought clinically more appropriate. This technique is the advanced workhorse of CT brain imaging. If there is doubt about the clinical diagnosis or a clinical observation, then this gives the highest likelihood of identifying any structural abnormalities in any plane.

Parenchymal lesions

The degree of contrast enhancement can be optimized and increased by giving a large dose of iodine (e.g.100 ml of 300 or 350 mg/ml or 60 ml of 400 mg/ml) and waiting 5 minutes before scanning starts. As contrast crosses a deficient BBB the more iodine given and the longer the delay (up to a point!) the more will accumulate in the abnormal tissues. In this way enhancement, equivalent in most respects to MR, can be obtained. CT can therefore be used with confidence in any patient in whom MR is contraindicated or impossible, routine follow-up of malignant brain tumours, or in any situation where optimal CT brain scanning is required.

Extra-axial lesions

This protocol is equally effective in the demonstration of extra-axial masses, i.e. meningiomas, neuromas. In these situations, as in metastatic neoplasia, contrast enhancement is due to the natural gaps in the walls of the capillaries as there is no BBB to break down. The clear simultaneous demonstration of any associated bone abnormality, e.g. hyperostosis or destruction, is advantageous. Since many of these lesions involve the skull base and the parasellar region, examples will be found in subsequent chapters.

Ventricular lesions

These are uncommon but the high resolution and multiplanar reformations will enable correct identification that the lesion is intraventricular. The craniocervical junction and the aqueduct between the third and fourth ventricles can be easily assessed when MR is not possible. It is also useful in the assessment of congenital or acquired hydrocephalus.

Indications

Follow on from an abnormal routine examination.
Used as the first examination for:

– Possible posterior fossa disease.
– Malignant brain tumour follow-up.
– Exclusion/proof of metastatic disease.
– Focal epilepsy as the chance of finding a causative lesion is relatively high.
– Progressive neurological deficit +/- papilloedema.
– Assessing a cause of/treatment for hydrocephalus.

Technique

Applying different algorithms or reconstructive zooms to focal areas of interest (e.g. orbits, internal auditory meati, pituitary) allows detailed examination from the one data set. The same protocol can be performed without contrast for simple structural imaging, e.g. the craniocervical junction. Table 2.1 presents optimal patient preparation, and Table 2.2 the protocol parameters. Figure 2.1 shows the surview.

Reconstruction and reformation

Usually simple axial, coronal, and sagittal reformations are sufficient but curved and oblique ones will allow a clearer depiction in an asymmetrical head. All reformations can be achieved at right angles to each other and the skull base by rotating the axial plane along the line of the skull base initially. Focused review is easily achieved to give more detail of a small region, e.g. orbit, parasellar area, posterior fossa. The raw data can also be reprocessed to focus on a smaller area and this slightly improves the resolution over simple photographic resizing.

Table 2.1 Patient preparation

•	The patient should be given 100 ml of contrast IV by hand or pump injection at 2 ml/sec
•	A full 5 min delay from the end of injection before scanning gives time for optimum enhancement of cerebral tumours/abscesses as contrast leaks out across the damaged BBB

Table 2.2 Protocol parameters for a ‘catch-all’ study

Patient position	Supine
Surview	Lateral
First slice	Through spinous process of C1
Last slice	Vertex
Field of view	Whole head (˜230 mm)
Slice width	0.9 mm
Slice increment	0.45 mm
Pitch	0.683
Collimation	64 × 0.625 mm
Rotation time	0.75 sec
kV/mAs	120 kV/300 mAs
Resolution	Standard
Filter	Soft tissue with bone/brain correction if available
Reconstructive zoom	To include whole head
Windowing	WC 40
	WW 100
Contrast	100 ml by hand or pump at 2 ml/sec
Wait full 5 min before scanning