CT BRAIN: a review of practice

In the last edition we made an introduction to CT where we discussed how a CT image is formed, the component parts of a CT scanner, and the generations of a CT scanner. In the module, we shall discus the CT scan of the brain. For want of space, we shall skip sectional anatomy of the brain. Please refer to your anatomy textbook  for a review of sectional anatomy of the head. We are going to talk about  

1. Clinical indication
2. Contra-indication
3. Patient preparation
4. Scanning protocols/ parameters
5. Scanning procedures
6. Image review
7. Post processing
8. After care for patients

INDICATION

Brain aneurysm                       Stroke                                                 

Hemorrhage                             Brain infections/ meningitis

Hydrocephalus                                    Arteriovenous malformation

Head injury                              Birth defect

Seizures                                   Space occupying lesions

Metastasis                                severe headache

Abnormal bone formation       etc

CONTRA-INDICATIONS

For every medical examination requiring ionizing radiation, the Radiographer must ensure that the examination is of good medical benefits before carrying out the procedures. It is standard practice to only accept patients referred for radio-diagnosis by the physician.

Risk of harm from ionizing radiation and adverse reaction from administering iodine contrast are the two major concerns in Computed tomography. Other minor contra-indications include claustrophobia while harm from ionizing radiation to pregnancy hinders administering CT to pregnant women.

However, the general rule is to weigh the benefit with the risks involved and where the benefit outweighs the risk, it is advisable to go ahead with the examination. Precautions are taken to keep radiation to fetus to the barest minimum and where risk of contrast reaction is suspected, adequate emergency medications are kept within reach.

 SCANING PROTOCOLS

Most scanners have preset protocols for scans of specific regions. However, a radiographer must be aware of these protocols and when necessary, make adjustments for optimal results.

Scout

Scan type         : Scout

Start location    : foramen magnum

End location    : vertex of skull

kVp                 : 100

mA                  : 10

scout plane      : Lateral 90⁰

                          AP 180⁰

Scout windows            : width 500

                          Level 50

Scouts              : AP and Lateral

Scan type                     : Axial

Scan plane                   : Transverse

Start location                : Foramen magnum

End Location               : Vertex of skull

IV contrast                   : 50 – 60 ml (if requested)

Oral contrast                : None

DFOV                          : ~23cm

SFOV                           : Head

Algorithm                    : Standard

Gantry Rotation time   : 1 – 2sec

Slice thickness             : 5.0mm

Slice interval                : 5.0mm

kVp                             :120- 140

mA                              : 120- 330

Please note: most scanners come with options for recon 1, 2, 3, etc. Recon 1 is the reconstruction platform for primary data acquisition. This is to say that any adjustments to this recon will have direct impact on the patient and image data acquisition. For example, adjusting the slice thickness in recon one will increase the radiation dose to the patient as more radiations will be given to the patient in an attempt to get a higher number of images.

The general technique is to keep parameters in recon 1 as constant as possible. Adjustments to recon 2, 3…10 are made to meet specific needs. For instance, Road traffic accident or head injury patients will require a bone or bone+ algorithm. This is achieved by changing the algorithm to “Bone or Bone+”. To acquire volume images for 3D or multi-planar volume reformation (MPVR), the slice thickness and slice intervals are reduced to 2.5mm or lesser.

The radiographer is responsible for determining which protocols needs to be adjusted to meet what needs. The ultimate goal is to achieve optimal images of best contrast and resolution for best diagnosis.

PATIENT POSITIONING

The gantry is fixed with a table/ couch for positioning the patient. It also has lights in 3 coordinates. The standard positioning for brain CT scans is supine with head first. Most scanners have dedicated head rest for CT brain scans. The head is adjusted to bring the chin down. The midline of the head coincides with the midline of the table and the gantry light. While positioning the patient, the sagittal coordinate of the gantry light coincides with the mid-sagittal plane of the patient’s head, the coronal coordinate at 2.5cm above the external auditory meatus (EAM) and the axial coordinate on the nasion (or glabella).

Positioning is however different for different scanners. While the positioning described above applies to a GE scanner, Toshiba scanner centers the axial light coordinate 2cm above the vertex of the skull. Understanding departmental routine in patient positioning is very important.

SCAN PROCEDURE

Once the appropriate positioning is done, the radiographer enters the patients date (name, age, weight, examination required etc.), selects the protocol for brain scans from the computer screen (usually preset). The 1^st scan is the scout (also called topogram). Scouts are usually taken in 2 planes ( AP and lateral).

The essence of having a scout is to plane the scan using localizers that actually represents slices.

Scanners differ in design, terminologies etc. but basic scanning procedures/ steps can be generally describes as follows

1.      Acquire the scout/ topogram

2.      Plan the scan using the scout. The localizers are placed to cover from the base of skull (foramen magnum) to the vertex of the skull. The localizer/ slices are adjusted to run parallel to the orbitomeatal or glabelomeatal line.

3.      Cross check the protocols selected and make adjustments to recons where necessary. It is standard practice to keep slice thickness and interval for brain scans at 5mm. adjustments in algorithms may also be necessary. However, there is rarely any need to adjust kV and mA settings. Pathology may necessitate adjustment in algorithms (e.g. Bone algorithm for head injury patients) and patient cooperation may require adjustments in scan time.

4.      Once scan planning is concluded and protocols are seen to be ok, it’s time to confirm the scan

CONFIRM => MOVE TO SCAN=> (TILT TABLE) => START SCAN

5.      Repeat scan for contrast administration (if required)

CONTRAST ADMINISTRATION

Some CT scans of the brain may require iodine contrast to enhance suspected underlying pathology. Brain scans can be classified as Contrast enhanced (requiring IV iodine contrast) and Non-contrast enhanced CT scans (not requiring IV iodine contrast)

Clinical indications for Non-contrast CT brain scan include intracranial hemorrhage, head trauma, early infarction, dementia, hydrocephalus etc. Indications for contrast enhanced brain CT scans are space occupying lesions, aneurysm, seizures, metastases, tumours etc.

The contrast is given through an intravenous cannula or needle on a superficial vein especially the median cubital vein and the cephalic vein. 50 -60ml of contrast is administered either manually or with automatic injector (with a flow rate of about 1.5- 2.5ml per sec and delay of 3mins).

Please note that it is very important that the contrast phase scan is done with the patient in same position so that slice A is in same location in plain scan and post contrast scan.

IMAGE REVIEW

A radiographer must have broad knowledge of both normal and pathological anatomy and must be able to apply this knowledge in making critical decisions for obtaining optimal image for diagnosis. For example, on reviewing images after a Brain CT scan, the radiographer may decide to administer IV contrast even when it was not requested. In most institutions, this decision is usually made by a radiologist. But there are cases where the radiographer needs to make such decisions independently. Also it makes you a better professional to not just carry out instructions on IV contrast but to also understand why these instructions are necessary.

The radiographer is also in a position to identify conditions that need urgent/ emergency attention by the radiologist and/ or physicians (e.g. intracranial hemorrhage, ruptured aneurysm, fractures etc.). While reviewing scan images, the radiographer must look out for

          i.            intracranial bleed,

`         

        ii.          

ii.   dilatation of the ventricles,

      iii.            mass effect /midline shift

      iv.           

iv. enlargement/ mass effect in the pituitary fossa,

        v.            abnormal calcification within the brain parenchyma,

      vi.            inhomogeneity of the brain parenchyma/ edema/ infarcts,

    vii.            fractures of the skull,

etc.

The purpose of image review in CT scan by the radiographer is not to make diagnosis but to aid the radiographer to optimize parameters/protocols for best image quality (contrast and resolution) that will aid the radiologist in making diagnosis.

In a patient with intracranial bleed of less than 3 days, the hematoma is hyperdense to the brain tissues; after 4days, the hyperdense hematoma becomes progressively surrounded by a hypodense periphery. Within 11 days to 6months, the hematoma is now isodense surrounded by hypodense tissues. Hematomas of more than 6 month are hypodense to brain tissues.

Hydrocephalus patients present with enlarged/ dilated ventricles. A radiographer will be quick to spot this abnormality. Intracranial tumours, edema, empyema, hemorrhage etc. cause a mass effect shifting the lateral ventricles away from the affected cerebral hemisphere. This midline shift is an important indicator of a possible tumour even if it is not enhanced in the pre-contrast scan. Distortions in the pattern of the gyri and sulci are also indicators of underlying pathology.

POST PROCESSING

Post processing is procedure that takes place after the scan has been completed and involves manipulation of volume images to produces images other than the axial images acquired during scanning. It is divided into

i.                    Multiplanar Reformation (MPR): this gives images in coronal, sagittal, oblique or transverse planes. Images in other planes other than axial plan help the radiologist in making a more comprehensive diagnosis. Some scanners can automatically generate MPR images while other scanner will require a command by the radiographer. It is very important that a radiographer who is new to a CT scan machine gets hands on practical on image reformations.

           

           

 

           

ii.         3 dimensional reformation: This involves using special software installed in the scanner or in the workstation to make 3-D representation of the 2-D images on display. It could be

·         surface rendering where the surface of the object scanned is represented

·          Volume rendering: this technique makes a voxel image visible to the eye. The images are no longer in slices but appear as real objects and their positions and relationships to other organs and tissues are clearly shown.

·         Projection displays: This is maximum intensity projection (MIP) and minimum-intensity projection (MinIP). Maximum intensity projection displays only the voxels with the highest values while Minimum intensity projection displays only voxels with the lowest values. MIP is used to display contrast filled and bone images. MinIP displays low contrast images.

Please note that multiplanar is best done using volume images. You cannot do reformation using images acquired with slice thickness and interval of 5mm. the images will be blurring and grossly unsuitable for making diagnosis

PATIENT AFTERCARE

Patients are placed under close observation briefly after an IV contrast administration for adverse reaction. About 90% of the patients do not react.

Breastfeeding mothers always show concern on having their baby ingest contrast the breast milk. Research shows that about 1% of injected iodine contrast get into the breast milk. Only about 1% of the iodine contrast ingested by the baby gets absorbed by the GIT. This amount is considered safe. But mothers that show much concern as regards the harm that the contrast can cause are advised to withhold breast feeding for at least 24hours after the contrast administration. Patients with allergy to sea foods or iodine are put under longer close monitoring.