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.     

GENERATIONS OF CT SCANNERS

GENERATIONS OF CT SCANNERS

The different generations of CT scanner was developed over the years to increase scan time and efficiency of scanning. The changes were majorly on the X-ray tube and detector arrangements.

Major adjustment in the technology of CT scanners were manifested in 

• Tube orientation and shape of beam (from pencil beam through narrow beam to fan beam)
• Number of detectors (from single detectors to multiple detectors).
• Detector arrangement.
• Slip ring technology.

This primarily led to decrease in scan time and increase in efficiency of the scanners.

FIRST GENERATION CT SCANNERS:

   

This picture shows the very 1^st generation of CT scanners. It was used to scan only the head.

The scans were done using a water filled box. A narrow pencil width X-ray beam and a single detector mechanism were used for data acquisition. Scan was performed in a rotate-translate motion.

Let me explain a rotate-translate motion.

For scan 1, the X-ray tube and the single detector scan from one end of the patient to the other end.  E.g. when scanning the head, the x-ray tube and the detector moves from the right side of the head to the left. This is translation (a horizontal movement across the patient)

For scan 2, the X-ray tube and the single detector rotate into a new position (circular movement around the patient) and the translational scans are taken in this new position.

For scan 3 to the last scan, the procedures are repeated.

Note: scans are only recorded during the translational movement and for one complete slice; the rotation has to complete 360⁰.

This scanners where limited because

1.       Only head scans could be performed.

2.       Generates a lot of heat requiring water and an elaborate cooling system.

3.       Scan time was very slow. About 1 minute per slice

This led to the development of the 2^nd generation scanners.

SECOND GENERATION CT SCANNER

The 2^nd generation CT scanners were developed to overcome some of the challenges of the 1^st generation scanners. The scanners have narrow fan beam (10⁰), multiple detectors, multiple angles of acquisitions but still perform rotate-translate scans. Scan time was greatly reduced to about 20seconds per slice.

THIRD GENERATION CT SCANNER

This generation of scanners was developed to primarily reduce scan time

It has the following features

•          Fan beam
•          Multiple detector array (500 -1000 detectors)
•         Only rotational movement. Rotate-rotate (no more rotate-translate)
•          Scan time of as fast as 0.5sec per rotation or per slice.

Most modern day scanners have this design. However, the 3^rd generation scanners has a limitation which is the presence of a rotate-rotate ring artifact.

 FOURTH GENERATION CT SCANNERS

This generation of CT scanners was designed to eliminate the ring artifact.  It has the following modifications

1.       A fan beam.

2.       Fixed ring of detectors. The detectors are arranged round the gantry.

3.       Only the tube rotates.

Electron Beam CT:

This was designed for ultrafast scans to freeze cardiac motion in Cardiac CT scans. An electron beam is emitted from a cathode and focused on a track of tungsten anode target from which X-rays are produced and collimated into a fan beam for use in CT scans.

Slip Ring Technology and Helical Scans:

All generations of CT scanners (except 4^th gen.) required winding and unwinding of connection cables causing inter-scan delays. Slip ring was designed to eliminate this. A slip ring is a drum with grooves along which electrical contactor brushes slide. Data are transmitted from detectors via various high capacity wireless technologies thus allowing continuous rotation. This enables the helical scan where data are collected continuously as the patient moves through the rotating gantry.

Most modern day 3rd generation CT scanners are incorporated with slip rings. This has eliminated the need for 4th generation scanners in eliminating ring artifacts.

next we will discus CT scanning protocols

COMPONENTS OF A COMPUTED TOMOGRAPHY SCANNER

COMPONENTS OF A COMPUTER TOMOGRAPHY SCANNER

HOW DOES A COMPUTED TOMOGRAPHY SCANNER WORK

A computed tomography scanner is a special type of X-ray machine. It was invented to solve 2 major challenges associated with a conventional X-ray machine

1.       Inability of an X-ray machine to make 3 dimensional images of the objects/ body parts X-rayed. The organs of the body are in 3 dimensions of breadth, length and height, but an x-ray image is in 2dimensions of breadth and height. This makes an X-ray image a limited or false representation of the organs under view.
2.        Superimposition of anatomical structures on each other on X-ray films. For example, when you chest, the ribs lie on top of the lungs. The sternum is superimposed on the thoracic spine, and the right side of the heart lies in between the sternum and thoracic spine and cannot be visualized.

In order to overcome these challenges /limitations, a Computed Tomography scanner works based on 3 basic principles:

1.        That the body is made up of thousands of tissues which different densities (and atomic numbers) and these tissues have different inherent abilities to absorb X-ray photons passing through them owing to their different atomic masses/ numbers. This means that these tissues absorb X-ray photons at different rates to the limit of their natural densities.
2.       . The amount of X-ray photons absorbed by these tissues can be measured and mathematically processed to calculate their attenuation coefficients. These attenuation coefficients form the bases of CT image generation.       
3. The ability to reconstruct 3-Dimensional (cross-sectional) images by systematic computed manipulation.

The process of image generation in computed tomography is divided into 3 which includes

1.       The joint process of passing a uniform X-ray beam through the patient (incident beam) and obtaining a non-uniform X-ray beam that comes out from the patient (emergent beam). Scan phase.
2.        The process of calculating the differences in the amount of X-ray photons that passed through the patient and the amount that came out of the patient. This involves calculating the differential attenuation coefficients the tissues under view. Reconstruction phase.
3.     . The process of  assigning CT numbers to the different attenuation coefficients that represent the different tissues in the body under view. It is the CT numbers that are turned into shades of white and grey that are visible on the screen as CT images. Display phase.

In other to further understand how a CT scanner works, we will discuss these 3 phases separately and appreciate how they are all linked to producing the 3Dimensional CT images.

SCAN PHASE

The scan phase can be acquired in either a step and shoot mode (axial scans) or a continuous motion mode (helical scans). Understanding the difference between a helical scan and an axial scan mode is very important. The gantry is in continuous circular motion during the scan; the table moves patient continuously through the rotating gantry during a helical scan. But during an axial scan, the table moves, then stops and takes a scan, then moves again and stops again and in that order till the scan is complete.

 During a scan (both helical and axial), X-ray is generated by the tube and focused by the collimator system to pass through the body in sections. As the X-ray beam passes through the body, there is differential attenuation by the tissues in the path of the beam. This differential attenuation is characterized by the differences in atomic number and densities of these tissues. The emergent beam carrying data of different linear attenuation of the tissues is recorded, amplified and digitized by a joint action of the detector array and DAS

Let me explain this mechanism in a much simple manner. The human body is made of hundreds of tissues and these tissues have different atomic numbers and masses. Because of this, the tissues absorb X-rays in different proportions. Let’s imagine a particular section of the body is made of up 1000 tissues labeled T_1, T₂, T_3, to T₁₀₀₀. The X-ray beam coming from the tube is labeled X and the X-ray beam coming out from the patient that has now passed through the tissues will be a product of the atomic numbers of the tissues so we will have tissue T₁ giving X₁ and tissue T₁₀₀₀ giving X₁₀₀₀. So what happened is that there are now about 1000 different X-ray energies. When this different energies fall on the detectors, they will make the detector give out different signals each depending on the energy of the energies of the X-rays. So the differential attenuations are used to calculate differential attenuation values by the Data acquisition system. This will be discussed in details later

RECONSTRUCTION PHASE:

The differential (linear) attenuation values recorded by the detectors and converted to electrical signals by the DAS are reconstructed by a complex mathematical algorithm in the CPU. Filtered back projection is the reconstruction programme used in modern day CT scanner. During this process, the image is divided into a matrix of voxels (pixels) and the attenuation coefficients calculated.  The attenuation depends on the density and atomic number of the tissues and energy of the X-ray photons. The objective of CT image reconstruction is to determine how much attenuation of the narrow beam occurred in each voxel of the reconstruction matrix. The calculated attenuation value (coefficients) is what is displayed on the screen

DISPLAY PHASE:

The attenuation values are replaced with CT numbers. A CT number is calculated and assigned to each of the individual tissue voxel of the matrix from the linear attenuation coefficients. The CT numbers are determined by the density of the tissues.

Water is the reference material for CT numbers and has a CT number of Zero. Tissues with attenuation coefficient (density) greater than water have positive CT number; those that are less dense will have negative CT numbers.

CT number is measured in Housfield Unit (HU)

CT number =    U_tissue-U_water   X1000

    U_water

The digital image consisting of matrix of pixels (voxels) with each pixel having a CT number is converted into visible image represented by different shades of gray or brightness level.

Having understood the general overview of how a CT image is formed, we will take a step backwards to discuss the different components of a CT machine and how they function tomorrow.