Brain imaging techniques are used in almost all fields of medicine. What follows are the most commonly practiced methods used in hospitals all over the world at this time.
The use of angiograms and x-rays in examining the brain
- An angiogram is a method of radiographing blood vessels.
- X-rays: high photons of energy are directed through the tissue being studied, some of these are either absorbed or deflected creating an image on photographic film representing (in varying degrees of density)the tissue being studied.
Radiographic studies of the cerebral arteries have been made possible for many years by angiograms and x-rays, although CT scans are now more widely used as a 3D image can be produced which conveys more detailed information.
- A catheter is placed into the Femoral artery (although others may be used) and passed up to an extracranial cervical artery (carotid or vertebral).
- Radio-opaque contrast material (usually iodine) is injected through the catheter into the artery lumen, consequently being carried round the brain with the blood.
- Multiple x-rays (radiographs) can then be taken which clearly shows the distribution of blood in the brain highlighting any occlusions that could be caused by such problems such as an embolus.
- The contrast material is denser than the bone in the skull and so is absorbed over bone and therefore is displayed more clearly.
- Angiograms often undergo techniques such as Subtraction to improve clarity of images: subtracting one radiograph from another so only the differences remain. Originally it was performed photographically although digital computer systems rapidly made this an easier technique.
Disadvantages of this procedure:
- Invasive and carries risks (1:100) of complications occasionally leading to death, especially in children, the elderly, and people with allergies (although this is highly reduced by good procedural techniques).
PET scans – “Positron Emission Tomography”
There are three basic steps to this visualising technique:
- Label a selected compound with a positron emitting radioactive isotope, e.g. Carbon-11, Nitrogen-13, or Oxygen-15. These are used since they have short half-lives and are found in almost all body tissues.
- Administer the compound to the subject via an injection or inhalation, into the bloodstream.
- The image distribution of positron activity is by tomography (slices). When a positron is emitted, it collides with an electron (it’s antiparticle) and they annihilate each other, giving 2 photons of EM(electromagnetic) radiation at 180° to one another, and this is detected by the computer and translated into the image.
- PET scans were developed in the 1960s in USA hospitals, universities, and research institutes.
- Early machines had low resolution and only made one “slice”.
- They were later developed using more detectors, enabling lots of slices and higher detail to be seen.
- They cost up to 5 million dollars.
- There are around 200 PET scanning machines in the world today.
Cerebral Blood Flow (CBF) technique:
Oxygen-15 is attached to a compound and then injected into the patient’s bloodstream. They are left for 5-10 minutes and then the distribution is measured. Assuming that increased blood flow in a specific area of the brain indicates greater activity, i.e. when a person speaks, reads, etc, the active area will “light up”.
MRI scans - Magnetic Resonance Imaging
- The patient lies within a large electromagnetic field.
- There are vast quantities of water and fat in our bodies, and it is the hydrogen atoms that they contain that are important for MRI scanning.
- The 1-proton nuclei of hydrogen atoms spin and wobble (this is procession) continuously in random directions.
- However, once in the magnetic field, most of the nuclei line up and spin together with their axes all aligned.
- EM waves, usually radio waves, are then passed through the person. As the waves pass through the body, they cause the atoms to change from low energy to high energy states against and along the magnetic field. They do this only when the right frequency is applied and this is called the resonant frequency.
- The radio waves are then switched off and some of the protons then fall back to their low energy states. As they do this, they loose their energy in the form of a wave, and this is picked up by a radio receiver. The signals are then converted by the Fourier Transform (a mathematical technigue that interprets the data) into frequencies with phase and amplitude.
- The amplitude = a position on the “grey scale”. – The stronger the signal, the brighter it appears and thus the more hydrogen atoms there are present in that area.
- All this is fed to a computer that constructs a map of the tissue.
- Slices are made using 3 planes (each perpendicular to each other – x, y, z). Each “plane” is a magnetic gradient in addition to the main field, and the position of the planes determines the position of the slice. The magnetic field, however, isn't uniform. In a brain scan its strength would be higher at the patient's head than at his feet so therefore the response to the radiowave pulse depends on the strength of the field. That is how the machine knows it is taking a picture of the head - because the resolution is much clearer there.
Applications of MRI:
- Black and white images because the machine is measuring one thing at a particular position.
- Very sensitive. Can detect a tumour more readily than a CT scan.
- Can detect white matter diseases such as MS, dementia, and memory disorders.
- Can detect infarctions (damage to blood vessels in CNS).
- Shows up aneurysms and legions.
- Can detect oedema (increase of water content of area) after a stroke.
- Uses bright/dark blood to visualise blood flow in tissues.
fMRI scans - Functional Magnetic Resonance Imaging
- This is basically the same as MRI except it uses oxygenated haemoglobin verses deoxygenated haemoglobin.
- It is used for brain mapping of functional areas.
- Activation of a brain area (e.g. looking at a picture will activate the visual cortex) causes an increase in blood flow to the region in order to keep up with the oxygen consumption in that area of tissue. Therefore there is an increase in oxygenated haemoglobin and a decrease in deoxygenated haemoglobin.
- Deoxygenated haemoglobin is less “active” so there is a decrease in signal. Oxygenated haemoglobin gives off a stronger signal.
Although this does not map actual neural response, it is still valid as a mapping technique.
Applications of FMRI:
- Maps flowing blood. Can detect blockage of brain arteries.
Advantages of MRI scans
- Can be done in any plane desired.
- More detailed than other techniques.
- Does not require radiation that could be dangerous.
Disadvantages of MRI scans
- The patient must stay still for 5-10 minutes at a time. Can last up to an hour.
- Patients may experience claustrophobia.
- Due to it essentially being a gigantic magnet, no metallic objects are allowed. Pacemakers, some fillings, shrapnel, etc make it impossible to use the MRI scanning technique.