BLOCK 12 WEEK 6 Flashcards
NON INVASIVE BRAIN IMAGING
ULTRASOUND
Pros:
- live images
- no radiation
- Cross sectional images allow for the visualisation of structures
- Relatively portable imaging modality
Con:
- Low spacial resolution
ULTRASOUND
- you know what your looking at by seeing where the probe is placed but its not very clear hence the low ‘spacial resolution’
X- RAYS used for:
- Skull fractures
- Sutures
- Haemorrhage
X -rays:
- Normally a erect PA image is taken and a abdominal supine AP.
- You need at least 2 projections to determine anatomic placement.
X RAYS
CT SCANS
- CT scans are used to image structures associated with the brain but not the brain itself e.g. odema, bone fractures and HAEMORRHAGE
CT SCANS
- CT scans are measured on a HOUNSFIELD SCALE normal X-Rays are not
PROS:
- Cross sectional images which allow for the easy visualisation of structures
- Easy to tell the differences between tissues
CONS:
- Relatively high radiation dose
- Relatively high cost
MRI
MRI are to do with water
- Bone and air have no water so appear black
- Tissues with water appear white to grey
- Tissues with a high amount of signal will appear white
- Tissues with a low level of signal will appear grey
What can we see:
An MRI scanner can be used to examine almost any part of the body including:
Brain and spinal chord / bones and joints / breasts / heart and blood vessels / internal organs such as the liver, womb or prostate gland.
MRIs
MRI Imaging can be:
-T1
-T2
-Fat Saturated Images
-Vascular Contrast Images
How does MRI scan work?
-The human body is about 70% water
-Water molecules are made up of hydrogen and oxygen atoms
At the centre of each hydrogen atom is an even smaller particle called a proton. Protons are tiny magnets and are very sensitive to magnetic fields.
When you lie under a powerful scanner magnet, the protons in your body line up in the same direction, in the same way that a magnet can pull the needle of a compass.
Short bursts of radio waves are then sent to certain areas of the body, knocking the protons out of the alignment.
When the radio waves are turned off, the protons realign. This sends out radio signals, which are picked up by receivers.
These signals give information about the exact location of the protons in the body.
They also help to distinguish between various types of tissues in the body, because the protons in different types of tissues realign at different speeds and produce distinct signals.
In the same way that millions of pixels con a computer can produce complex pictures, the signals from the millions of protons in the body are combines to create a detailed image.
Magnets on – atoms align
Radiofrequency on – atoms disperse
Radiofrequency off – the atoms re-align releasing energy ( different for each tissue type).
Pros:
- good spatial resolution
T1 weighted image
- The timing of radiofrequency pulse sequences used to make T1 images results in images which highlight FAT tissue within the body.
-T1 – ONE tissue is bright: fat
T2 weighted image
- The timing of radiofrequency pulse sequences used to make T2 images results in images which highlight FAT and WATER within the body.
-T2 – TWO tissues are bright: fat and water
T1 and T2 images
Fluid attenuated inversion recovery (FLAIR) MRI
- FLAIR is also similar to T2, however, the CSF signal is nullified. This is particularly useful for evaluating structures in the central nervous system (CNS), including the periventricular areas, sulci, and gyri.
- For example, FLAIR can be used to identify plaques in multiple sclerosis, subtle oedema after a stroke, and pathology in other conditions whereby CSF may interfere with interpretation
MRI (ANGIOGRAPHY)
Clinical use:
- Artherosclerosis (plaques)
- Stenosis (narrowing)
- Aneurysms
- Atriovenous malformations
- Interventional radiology - guide stent placement
DIFFUSION WEIGHTED IMAGING (DWI)
DWI is an imaging modality that combines T2 images with the diffusion of water.
-With DWI scans, ischaemia can be visualised within minutes of it occurring (Figure 5).
- This is because DWI has a high sensitivity for water diffusion, thereby detecting the physiological changes that happen immediately after a stroke.
PET SCAN
- Positron emission tomography (PET) scans produce detailed 3-dimensional images of the inside of the body.
- PET scans are often combined with CT scans to produce even more detailed images. This is known as a PET-CT scan. With MRI its a PET-MRI.
-
Why PET scans are used?
A PET scan can show how well certain parts of your body are working, rather than simply showing what they look like.
PET scans are particularly helpful for investigating confirmed cases of cancer to determine how far the cancer has spread and how well it’s responding to treatment.
PET scans are sometimes used to help plan operations, such as a coronary artery bypass graft or brain surgery for epilepsy.
HOW PET SCANS WORK?
- PET scanners work by detecting the radiation given off by a substance injected into your arm called a radiotracer as it collects in different parts of your body.
- In most PET scans a radiotracer called fluorodeoxyglucose (FDG) is used, which is similar to naturally occurring glucose (a type of sugar) so your body treats it in a similar way.
- By analysing the areas where the radiotracer does and does not build up, it’s possible to work out how certain body functions are working.
- For example, using FDG in the body’s tissues can help identify cancerous cells because they use glucose at a much faster rate than normal cells.
PROCESS OF A PET SCAN
- The radiotracer is injected into a vein in your arm or hand about an hour before your scan, as it takes time for it to reach the right cells in your body.
- During the scan, you lie on a flat bed that’s moved into a large, cylindrical scanner.
-The scan usually takes 30 to 60 minutes.
- ut the amount of radiation you’re exposed to in a standard PET scan is safe.
-The radiotracer becomes quickly less radioactive over time and will usually be passed out of your body naturally within a few hours.
- Drinking plenty of fluid after the scan can help flush it from your body.