equipment Flashcards
MRI hardware includes a:
Magnet
Radiofrequency source
Image processor and
Computer system
The magnet aligns the nuclei parallel or anti-parallel.
Magnetic homogeneity is necessary so therefore a shim system is needed.
The RF system excites the nuclei and this system requires a transmitter and receiver.
Magnetic field gradients determine the spacial locations of the RF signals.
The MRI signal is changed from the FID into an understandable format by a series of mathematical equations known as the Fourier transform.
This process is overseen by a computer that also allow for operator interface.
Diamagnetic
Substances e.g. copper, silver and sulfur.
These substances have no net magnetic moment without an external magnetic field present.
Their electron currents caused by their motions add to zero.
When an external magnetic field is applied they show a small magnetic moment that opposes the magnetic field.
They are therefore not attracted to but slightly repelled by the magnetic field.
Paramagnetic
Substances e.g. oxygen and gadolinium (MRI Contrast)
They have a small magnetic moment as a result of their unpaired electrons within the atom.
Without a magnetic field the magnetic moments occur random and cancel each other out.
With an external magnetic field these substances align with the direction of the field so their magnetic moments are added together
Ferromagnetic
Substances such as iron show a strong attraction and alignment to an external magnetic field.
They can become dangerous projectiles in the MRI environment.
They also retain their magnetism even when removed out of the magnetic field and subsequently become permanent magnets.
Permanent Magnets
Substances such as Nickel, cobalt and iron are used in the production of permanent magnets since these ferromagnetic substances retain their magnetism after being exposed to a magnetic field.
Main advantage is that :
they require no power supply and are therefore
relatively low in operating costs.
Electromagnets
The law of electromagnetism states that moving electrical charge induces magnetic fields around themselves.
Resistive magnets
Field can be switched off
Flux lines run horizontal
High operating costs as power supply required
Large fringe fields
Poor homogeneity at higher field strength
Superconductive magnets
Flux lines are horizontal Lower power requirements Expensive to buy Large fringe field High field strength (higher SNR ,shorter scan times)
Large coil:
Large area of uniform signal reception
Increased likelihood of aliasing with the smaller FOV
Positioning of patient is not too critical
Lower SNR (Signal to noise ratio)and SR (spatial resolution)
Used in examinations where a larger anatomy coverage is needed e.g. abdomen. (Small bowel)
Small coil:
Small area of reception Less likely to produce aliasing artefact Patient positioning is critical High SNR and SR Used in examinations of small body parts e.g. wrist, knee
