Imaging Of The Brain

Question 1

Why is non-invasive brain imaging used?

Answer 1

To preserve the anatomy, structure and function of the brain.

Question 2

What are the uses of ultrasound in brain imaging?

Answer 2

Ideal for babies who have fontanelles that are susceptible to sound waves. It is ideal for investigating suspected hypoxia, meningitis and haemorrhage.

Question 3

What are the uses for X-rays in brain imaging?

Answer 3

Ideal for viewing hard tissues such as bones, and for brain imaging this is the skull for gross anatomy changes like skull fractures, haemorrhaging and surgery. It can be combined with MRI for a soft tissue contrast.

Question 4

What are the uses of CT scans?

Answer 4

Uses ionising radiation of X-rays for a 360 degree view of hard tissues such as bone and structures associated with the brain such as oedema, bone fractures and haemorrhage. It has a cheap and quick preparation time.

Question 5

What are the uses of MRI scans?

Answer 5

Ideal for providing high resolution images of the soft tissues for injury, tumours and oedema. It is a quick procedure however it is expensive for hospitals to offer.

It is sensitive to perfusion, vessel permeability and extra-cellular volume. It is ideal for angiography for assessing atherosclerosis, stenosis and aneurysm.

Question 6

How do MRIs work?

Answer 6

Water molecules in the body carry protons, which are highly magnetic. When a magnetic field is generated by a machine, these protons will align along its direction. Depending on the strength of the magnetic field, they will rotate around it, called procession. Applying an electromagnetic frequency will prevent procession and measuring the change as hydrogen ions realign provides the contrast for images.

Question 7

How are water and fat structures imaged by MRIs?

Answer 7

After applying the electromagnetic frequency, water structures realign more quickly than fat structures

Question 8

What is a T1-weighted MRI?

Answer 8

Enhances the viewing of fat structures so it is brighter and suppresses water signals so water and inflammation appear darker on images. It has a shorter relaxation time after applying electromagnetic frequency

Question 9

What is a T2-weighted MRI?

Answer 9

Enhances the viewing of water signals so water and inflammation appear brighter and fat appears darker. T2-weighted MRIs have a longer relaxation time after applying electromagnetic frequency before returning to normal.

Question 10

How do the structures of the brain appear in a T1-weighted MRI?

Answer 10

CSF= Black
White matter=white
Grey matter=Grey

Question 11

How do the structures of the brain appear in a T2-weighted MRI?

Answer 11

CSF= White
White matter= grey
Grey matter= White

Question 12

What are the contrasts used for MRI?

Answer 12

Gadolinium-metals which improve the sensitivity of the MRI to blood vessels and soft tissues. Can help point to highly vascularised areas, indicative of tumours and the drug delivery.

Question 13

What is magnetic resonance spectroscopy?

Answer 13

Used to assess the chemical composition of metabolites of normal brain tissue with abnormal brain tissue containing tumours.
These metabolites are: choline, creatinine, N-acetylaspartate and lactate.

Question 14

What does choline indicate?

Answer 14

High levels of cell division.

Question 15

What does high levels of N-acetylaspartate indicate?

Answer 15

Normal neural function in the brain, typical for normal brain tissue.

Question 16

What does high levels of lactate indicate?

Answer 16

High levels of glycolysis, due to tumours.

Question 17

What are PET scans?

Answer 17

PET scans are typically used to image brain tumours by localising metabolic activity of glucose uptake. It is very expensive and has a high sensitivity

Question 18

How do PET scans work?

Answer 18

Radioactive tracers of isotopes are injected into the body and when it interacts with the electrons target cells, it decays/annihilates to emit positrons that produce gamma rays. These gamma rays are picked up by a scanner. These are picked up and converted into a 3D image.

Question 19

What is SPECT?

Answer 19

Single Photon Emission Computed Tomography is a cheaper alternative to PET scans which use gamma emitting radioactive isotope tracers, with a lower image resolution.

Question 20

What is an EEG?

Answer 20

Used in brain imaging to detect electrical activity of neuronal signalling for brain function, depending on the number of electrodes used. It is ideally used for epilepsy.

Question 21

What is electrocorticography? (ECOG)

Answer 21

Invasive technique requiring surgical intervention to place electrodes directly on the brain’s surface to measure action potentials of neurons. It has a better spatial resolution than EEG, and is used for epilepsy which cannot be be controlled by medication, called intractable epilepsy.

Question 22

What is magnetoencephalography?

Answer 22

Used to detect the magnetic fields generated by the brain during neural activity but because the bones and skin are magnetically silent, it is used alongside structural MRI to localise brain activity. It is used to localise epilepsy and plan brain surgeries for epilepsy and tumour removal. MEGs are expensive and require a magnetically shielded room with scanners at the coldest possible temperature but provide a good spatial resolution.

Question 23

What is a functional MRI?

Answer 23

Magnetic resonance imaging which acts on protons in water molecules that produce a signal. It detects blood flow in the brain to assess brain function because it is typically used with blood level oxygen dependent contrast, to identify metabolically active areas in the brain and the changes in haemoglobin status of RBCs. Used in brain mapping studies in clinical research.

Question 24

What is functional NIRs?

Answer 24

Functional near infrared spectroscopy is when infrared light is emitted to pass through the skin, skull and brain tissue to measure blood flow and areas of high activity in the brain. Can be used while moving, ideal for children and babies.

Question 25

What do MRI techniques rely on?

Answer 25

Magnetic differences of protons. Hyperpolarisation is used to increase the frequency of proton alignment with the magnetic field in MRIs.

Question 26

What is single shot MRI?

Answer 26

A magnetic field signal is generated once and uses hyperpolarisation to boost the strength and detection of protons by scanners. It is used to measure perfusion flow and distribution of molecules depending on metabolic activity for tumour identification.

Question 27

What are the advantages of CT scans?

Answer 27

Sensitive to haemorrhage for identifying strok
Widely available and inexpensive

Question 28

What are the disadvantages of CT scans?

Answer 28

Exposure to radiation which increases cancer risk
Less sensitive to acute infarction so difficult to distinguish from chronic infarction
Poor imaging of posterior fossa

Question 29

What are the advantages of MRI scans?

Answer 29

High soft tissue resolution
Sensitive to ischaemia
Differentiates acute and chronic infarction

Question 30

What are the disadvantages of MRI scans?

Answer 30

Limited availability
More expensive
Takes longer to produce image
Issues with metallic objects like pacemakers.