Gradient Coils And Rf Coils

Question 1

Gradient coils

Answer 1

• 3 coils placed in 3 orthogonal planes – X Y Z
• Create a gradient (variation) in the main magnetic field –
• This gradient enables MR signal to be localised by selecting the slice and by frequency and phase encoding
• The phase encoding gradient is applied along one of the sides of the image plane. The frequency encoding gradient is applied along the remaining edge of the image plane.
• According to Faraday’s law of electromagnetic induction changing magnetic fields generates a current in a conductor
• It states that induced current in a conducting circle is proportional to the rate of change of magnetic flux withing thr circle
• Any wires inside the patient might be influenced by the rapid chnaging magnetic fields of the gradient coils
• Current may also stimulate excitable tissues such as nerves with some sensitive patients
• The switching of the gradient coils on and off is responsible for the loud noise in MRI

Question 2

RF Coils

Answer 2

• Send or receive the signal
• 2 types – volume coils and surface coils

Question 3

Volume coils

Answer 3

• Transmit and receive the rf pulse
• Gatherthe signal from a volume of anatomy enclosed within the coil
• Example body coil– coil inside the bore of the magnet- transmits frequency for most examinations
• When the anatomy is properly fitted in the volume coil, noise is reduced and improves SNR
• Should be insulated to prevent burns

Question 4

Surface coils

Answer 4

• Places directly on the area being examined
• Receive only coils with the rf signal being transmitted by thr body coils
• Due to the close proximity to the anatomy they can receive the MR signal from nearby tissues
• Better SNR- the volume imged ia determined by their size -reduces rhe amount of surrounding noise-increases resolution and detail
• Shoukd not occupy a plane perpendicular to the main magnetic field- should not cut the field
• Always run in a straight line to avoid loops – avoid loops of wire and loops by the patient himself such as holding hands
• Preferably placed in the isocentre of the magnet- where the magnetic field is the strongest and most uniform
• Sharp bends shoukd be avoided

Question 5

MR linac

Answer 5

• Tracks tumour movement and makes up for these using mri
• Monitors movement in real time
• Good for lung tumours
• Modifies the radiation deluvery to account for any changes