32. CT and MRI Flashcards
What are the principles behind computed tomography (CT) scanning?
The name comes from the Greek ‘tomo’,
meaning slice and ‘graphein’,
to write.
CTs take a series of X-ray images
around a central axis, either in a
discontinuous ‘shoot and step’ process,
or
in a continuous ‘spiral’ manner.
The latter are much quicker and so
may reduce motion artefact, and enable
better 3D reconstruction of images.
What are the principles behind magnetic resonance imaging (MRI)?
> MRI is an alternative way of
producing images of the body.
> MRI visualises soft tissues much better
than does CT, and therefore is
more useful in the study of the brain,
spinal cord and musculoskeletal system.
> Atoms with unpaired electrons
or protons are in a state of spin
that can be affected by the application of
an external magnetic field.
> Hydrogen ions found in water and fat molecules
(which make up 60–70% of the body)
are affected in this way and so,
when the patient enters the
powerful magnetic field of the scanner
(1–2 tesla), their protons align in the
direction of the field.
> The protons then begin to resonate
at their ‘precision frequency’.
> The powerful magnet is called the ‘primary magnet’ and its magnetic field is generated by an electrical current passing through coils of wire, which are cooled with liquid helium.
> Once the atoms have lined up,
a radiofrequency coil is turned on,
generating a second current at
right angles to the first.
> The energy generated by this coil is absorbed by the hydrogen ions and
disrupts their alignment.
> When the radiofrequency coil is turned off, the protons release energy
(in the form of low-frequency radiation)
and return to their original position.
> It is this low-frequency radiation
that is detected by the scanner, and
reconstructed into images.
> Different tissues will give out
different amounts of energy and return to
their equilibrium position at different rates,
allowing for differentiation between them.
This exchange of energy between
spin states is called
‘resonance’.
> Another component of the MRI scanner
is the ‘gradient magnet’.
These are smaller magnets that
are applied to allow fine-tuning
and focusing of the image on the
area being studied.
The banging noise in the MRI is the
sound of these magnets being
turned on and off.
> MRIs are either T1 or T2 weighted and this refers to the amount of time elapsed between the radiofrequency magnet being switched off and the image being taken, i.e. the ‘relaxation time’.
T1 images are taken earlier than T2.
In T1 images fat is bright and water is black,
in T2 images fat is black and water is bright.
> The entire scanner is housed in a room
lined with copper or aluminium,
and this room is referred to as the Faraday cage.
> What are the indications for general anaesthesia in the scanner?
> Unstable patient (e.g. for airway protection or from ITU)
> Young child if they cannot cooperate and lie still
> Patient with learning difficulties, as above
> Very anxious or claustrophobic patient
> Patients with movement disorders or who
are unable to lie still for
sufficiently long.
What are the problems associated with anaesthesia in the scanning department?
> Generic problems:
• Patients are removed to an
often remote and isolated area:
This area may be unfamiliar to the
responsible doctor.
It is important, therefore, to consider who will be available to help should there be an emergency during the trip and if possible, to familiarise oneself with the department and the equipment available there before the transfer.
• Cold and noisy environment:
Ambient temperature in an MRI
scanner is cool in order to prevent
the magnet from overheating.
The magnets produce a lot of noise
and earplugs must therefore be used
and patients covered in order
to minimise risk of hypothermia.
• Claustrophobic environment:
Space within scanners is extremely
limited, more so in the MRI scanner,
and some patients can find this
very distressing.
- Limited space for anaesthetic equipment
- Limited access to the patient:
Once the patient is in the scanner it
can be practically impossible to get to them.
Before the scan begins it
is important to satisfy yourself
that all the leads reach far enough,
that the patient is stable and
that you can see the monitor. The scan may
take some time, especially if it is an MRI.
> Specific problems related to the MRI scanner:
The magnet in the MRI adds a
whole new layer of problems.
• Ferrous implants:
Within the magnetic field,
ferrous implants
(e.g. pacemakers, defibrillators, cochlear implants, some aneurysm clips and foreign bodies)
are prone to displacement or
torque forces,
which can lead to serious patient injury. Patients with any such implants must not enter the MRI scanner. Non-ferrous implants are prone to heating and patients must be warned of this.
Both types of implants can cause image artefacts.
• Ferrous equipment:
Ferrous-containing equipment such as laryngoscopes, stethoscopes, pagers, and gas cylinders are prone to significant movement
within the 50 G line and
should therefore not be
taken beyond this point
unless securely fastened.
Magnetic strips on identity badges
and credit cards will also be
wiped if taken within the
magnetic field.
Ideally, only ‘MR safe’ and ‘MR conditional’ equipment
should be used within the scanner.
• Monitoring:
Special ‘MR safe’ ECG electrodes,
BP cuffs and pulse
oximeters are required.
ECG leads are short and plaited to minimise
the risk of magnetically induced currents within them, which can burn the patient
(burns are the most common MRI-associated injury).
If a standard anaesthetic machine is used,
this is housed outside the
Faraday cage with an extra long
Bain circuit connecting it to the
patient.
Long gas analysis sampling lines cause a delay in monitoring.
Monitoring equipment can introduce stray radiofrequency currents,
which can degrade the image quality.
• Delivery of anaesthesia:
‘MR conditional’ infusion pumps
should ideally be used.
However, standard pumps
can also be used outside
the 100 G line (see below).
Volatile agents can be
administered using an ‘MR conditional’
anaesthetic machine.
If this is not available,
a standard anaesthetic machine
can be used outside the cage with an
extra-long Bain circuit.
How are items to be used within an MRI scanner classified?
The old term ‘MR compatible’
is no longer suitable and the ASTM
International and FDA have introduced the following classification system:
1
> MR Safe:
Items are completely free of
all metallic components.
They are non-metallic,
non-conductive and
non-radiofrequency reactive.
They pose no hazard in any MR environment.
> MR Conditional: I
tems are safe under certain tested magnetic
conditions,
which should be enumerated on the product
(i.e. the magnetic field strength in
which the product can be safely used is stated).
> MR Unsafe:
Items pose a hazard in any MR environment.
What is the standard international unit of magnetic strength?
> SI unit for magnetic flux is the weber (Wb)
> SI unit for magnetic flux density
is the tesla (T),
which is used for large densities.
For smaller densities,
a smaller unit,
the gauss (G) is used.
An average MR scanner produces
between 1 and 1.5 T
(although newer machines can now generate
up to 3–5 T),
while earth’s magnetic field is
about 1 G.
1 T = 1 Wb/m2
1 T = 10 000 gauss
