CT Scan Technique of Head

CT Scan Technique of Head

Patient positioning:

The patient lies supine on the scanner table, head resting in the head support and positioning is aided by alignment lights. The orbitomeatal baseline is positioned parallel to the transverse alignment and the median sagittal plane is perpendicular to the table and coincident with the sagittal alignment light. To ensure that the skull is symmetrically positioned, the external auditory meatuses must be equidistant from the skull support and the interorbital (interpupillary) line is parallel to the scan plane. The head is secured in positioned with the aid of Velcro straps. The patient is moved into the scanner and the table is raised to bring the scan reference point to the level of the EAM.

Technique:

A lateral scan projection radiograph is obtained, from C2-C3 level upto the level of vertex. From this image, 5 cm contiguous sections are prescribed from the foramen magnum, the superior border of the petrous bone, parallel the to the orbitomeatal baseline (8). Further 10 mm contigous sections are prescribed from the superior border of the petrous bone to the vertex. Medium smooth reconstruction algorithm for bone and sharp kernel for bone (optional - in case of trauma or suspected or known calvarial lesion) is chosen (9).




During scanning avoid the orbits being included in the FOV to minimize radiation to the orbits. Although radiation induced cataract is believed to be associated with exposures of over 5000 mGy (1-3), the threshold of low linear energy transfer (LET) radiation giving rise to an increasing frequency of ophthalmologically detectable opacities in atomic bomb survivors has been estimated at approximately 500 to 2000 mGy (3,4). Repeated scans may exceed this threshold.



In 1982, Lund and Halaburt demonstrated that variation in the plane of scanning with different gantry angulations could affect the radiation dose to the lens (5). Yeoman et al reported a mean lens dose of 43.44 mGy using an orbitomeatal (OM) baseline, and 5.58 mGy when a supraorbitomeatal baseline was used (SOM) baseline (6). This corresponds to an 87% dose reduction to the lens, without significantly increasing posterior fossa artifacts.



In 2000, Lai et al showed that the most popular baseline plane for CT scan of the brain was parallel to the OM line. This finding is similar to the results of an international survey (6), and probably reflects the historical practice of positioning the chin of the patient downwards to image the posterior fossa in the original water bath scanners. Tozeik et al have demonstrated, however, that the OM baseline is the worst in terms of minimising posterior fossa artifact (12). Gantry angulation of 5 to 10 degrees below the Reid baseline was the optimum, and an intermediate result was obtained when the SOM baseline was used. Thus, CT scanning of the brain should be performed either with the beam parallel to, or below the Ried baseline for less posterior fossa artifact, or with the beam parallel to SOM baseline for a substantial reduction in the radiation dose to the lens.



In 2009, Kim et al concluded that the the line connecting the tue of tuberculum sellae and the occipital protuberance i.e. TS-OP lines are nearly parallel to the Anterior Commissure-Posterior Commissure i.e. AC-PC line. They measured the angles between the AC-PC line and the OML, the line connecting the tuberculum sellae and the internal occipital protuberance (TS-IOP line), and the line connecting the tuberculum sellae and the external occipital protuberance (TS-EOP line) on midsagittal brain MR images.of 223 patients.

In clinical brain MR imaging, the AC-PC line has been widely used as the standard imaging reference line since 1988, when Talairach et al (11-13) reported the AC-PC line as the reference for co-planar stereotaxis in the human brain. The original Talairach and Toumoux AC-PC line passes through the superior edge of the anterior commissure and the inferior edge of the posterior commissure. It follows a path essentially parallel to the hypothalamus sulcus, thereby dividing the thalamic and the hypothalamic regions (11). The Talairach AC-PC line provides more efficient brain coverage and may lead to more reproducible and readily interpretable clinical brain images (13).



Because the OML could not be viewed in the midsagittal brain MR images, the OML observed in the parasagittal image was projected onto the midsagittal image, and the angle to the AC-PC line was subsequently measured.


With use of CT topography on the basis of x-ray radiography, it is not possible to determine the anterior commissure and posterior commissure, which are anatomic landmarks for the AC-PC line. Thus, the AC-PC line cannot  used as a reference line for brain CT imaging. As a result, the OML which connects the outer canthus of the orbit and the center of the external auditory meatus, has been widely used as the traditional imaging reference line in brain CT imaging.


OML is defined by direct visual inspection of the patient and is aligned with a laser on the scanner table because it is based on soft tissue landmarks. From a historical perspective, this was established as a standard plane for CT scanning of the brain because, in the original water bath scanners, the chin of the patient had to be placed downward to image the posterior fossa (14). Hoewever, OML line cannot be drawn precisely by using the lateral scout image, in addition to the observation that the posterior fossa may induce beam hardening artifacts. Furthermore, because the OML is based on the extracranial skull structure, some problems are expected when this reference is used as a scanning landmark for deep brain structures (15).

To solve these problems, Weiss et al (16) suggested a new method of use of an imaging reference line elevated 12 degrees from the hard palate. However, the practical difficulty of using this line for brain CT scanning hs prohibited its wide usage.

However, use of the hard palate as a landmark can introduce problems, as the hard palate is located at an anatomically distal position to the AC-PC line and because the hard palate lies outside of the intracranial cavity, where the brain is located. In addition, it may not be feasible to use a line slanted 12 degrees upward from the hard palate as the reference line for brain CT scanning.

Because of the aforementioned disadvantages of using the OML as a reference line for brain CT scanning, several studies have proposed new baselines for brain CT scanning. Shimada (18) recommended the fronto-occipital pole (FO) line, joining the frontal pole to the occipital pole for observation or measurement of the brain. In 1977, Tokunaga et al (19) suggested the Glabella-inion (GI) line as the baseline for brain CT imaging. The GI line was shown to be parallel to the FO line, which was substituted for a line joining the deepest gyral impressions in the frontal and occipital parts of the cranial cavity.

In 1991, Rozeik et al (20) suggested using a line that was 5 degrees lower than the Reid baseline as the imaging reference line for brain CT imaging. The Reid baseline is often called the inferior orbitomeatal line, and it connects the inferior edge of the orbit to the external auditory meatus (15). The Reid line, or the inferior OML represents another reference line used when brain CT imaging is performed. This line connects the inferior edge of the orbit and the external auditory meatus. Use of a line that is 5 degrees lower than the Reid baseline as imaging reference line minimizes interpetrous artefacts.

S F S Halpin (21) used frontal and lateral localizers, and define three group of slices, angled approximately parallel to the floor of the anterior cranial fossa- this usually equates to an angle rotated 15 degrees from the radiographic baseline. Since the chance of a CT scan of the brain causing cataracts is zero, scans need not be angled to avoid the orbits. Furthermore, a conventional slice angle leads to better detection of pathology in the orbits, whilst assessment of Chiari malformation is difficult when the gantry is angled steeply from the anterior to the posterior fossa.

The first group of scans is throught the posterior fossa, and starts just above C1, terminating at the lower third ventricle. Colllimation is1.25 mm, and images are reconstructed in contiguous 5 mm slices. Tube voltage (kV) and tube potential (mA) are set relatively high because the bone is relatively thick in this region, but also because we wish to retain the ability to retrospectively reconstruct the data into high quality 1.25 mm slices, should that prove necessary. The second group uses 3.75 mm slices, with images reconstructed at 7.5 mm, using lower kV and mA, and continues to the upper part of the lateral ventricles, where a third group consists of an identical slice profile, but lower mA. This scan protocol usually produces 22 images, which are printed on one sheet of film. Merging thin slices into larger one dramatically reduces beam hardening artefact and produces images of far higher quality than those that would be produced by scans using 5 mm collimation. Images through the posterior fossa are routinely reconstructed at 2.5 mm: these are not normally filmed but are available for soft copy review.

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7.      Clark’s

8.      CT and MRI protocol

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