Introduction to Computed Tomography (CT), Magnetic Resonance imaging (MRI) Flashcards

Question

CBCT

Answer 1

Advantages: Detailed cross-sectional images Improved low-contrast detectability Use of IV contrast to further enhance difference between structures High spatial resolution for imaging bone/fractures/trauma Very quick scans (whole body scan in about 20 sec) Image reconstruction/3D in various anatomical planes Disadvantages: High radiation dose examination Large and expensive equipment Images can be susceptible to artefacts

Answer 2

Developed into an important imaging modality from 1978 Utilises the fact that magnetic nuclei in a static magnetic field exhibit a characteristic resonance frequency that is proportional to the field strength Primarily MR images map the distribution of hydrogen nuclei within the body The radiographic community have recognised MRI’s unique ability to image soft tissue & differentiate between benign and malignant tissue

Answer 3

Stands for: Magnetic Involves magnetic fields Resonance Involves resonance – frequency has to be matched to a natural process Imaging Produces images

Answer 4

magnetic forces randomly distributed. place patient in mr machine create a magnetic field - strong magnet see 2 physical principles alignment of magnetic field in same direction not randomly distributed additional movement - net magnetic vector (nmv)

Answer 5

MR Any MRI exam MRA MR Angiography – blood vessels MRV MR Venography – veins only MRA MR Arthrography – joints MRS MR Spectroscopy – chemical composition of the brain fMRI Functional MRI – how the brain works iMRI Interventional MRI – real-time image guided surgery

Answer 6

open magnetic - no claustrophobia with patients.

Answer 7

series of coil wire generate magnetic fields

Answer 8

In MR, z direction is defined by direction of B0 (longitudinal) No difference between x and y (both are perpendicular to B0) - called transverse plane

Answer 9

Can’t measure it while it is lined up with B0 (B0)z ~ 1T M0 ~ 0.000001 T insignificant field If we can move M0 we can measure it (B0)xy = 0 T M0 ~ 0.000001 T a measurable field

Answer 10

Can’t measure it while it is lined up with B0 (B0)z ~ 1T M0 ~ 0.000001 T insignificant field If we can move M0 we can measure it (B0)xy = 0 T M0 ~ 0.000001 T a measurable field

Answer 11

Frequency of precession is called Larmor frequency and depends on external magnetic field strength Larmor Equation 0 =  B0 γ = 42 MHz per Tesla

Answer 12

The observed effect of rf irradiation is to rotate M0 away from z axis and down towards x-y plane

Answer 13

Pulse of rf radiation leaves M0 exactly in transverse plane called a 90° pulse If strength of rf is doubled (or rf is switched on for twice as long) M0 will turn through 180° pulse is then called a 180° pulse Smaller flip angles are easily produced The magnetism from the patient can be read at a flip angle of as little as 10°

Answer 14

After 90° pulse, magnetization vector lies in the x-y plane, and is still rotating at ω 0 Receiver coil looking only at x-y plane detects changing magnetic field as magnetization rotates A current is generated in the coil, producing the MR signal

Answer 15

MR signal after 90° pulse is switched off known as a Free Induction Decay (FID) Oscillates at Larmor frequency Decays to zero exponentially

Answer 16

Advantages: Does not use ionising radiation Images acquired directly in multiple planes (sagittal, coronal, transverse and oblique) Superior soft tissue contrast compared to CT/US/RNI so excellent to image the brain, spine, joints etc. Allows images to be weighted to visualise fat, water etc. Accurate anatomical, metabolic and functional information – advanced techniques such as MR spectroscopy allow for precise tissue characterisation rather than merely 'macroscopic' imaging Some angiography can be done without the use of intravenous contrast media (unlike CT) Disadvantages: Longer scans Less comfortable and may be a suboptimal patient experience – small bore, noise, claustrophobia Large equipment, very expensive and less readily available Not safe for patient with certain implants, pacemakers, foreign bodies Higher safety considerations for staff – MRI safe/compatible equipment MR images are more susceptible to artefacts due to the sensitivity of the magnet – patient motion, external disturbances

Answer 17

It can provide additional information in order to more precisely define tumour localization for treatment planning using radiation therapy It has better soft-tissue contrast than CT Provides better visual discrimination between tissues that should be treated and those that should not However MRI cannot identify the mass attenuation coefficient or attenuation characteristics for high-energy photons, X-rays, and gamma-rays, as this is critical for precise dose calculation Increased used of MRI technology in radiation therapy

Answer 18

The Magnetic Resonance Linear Accelerator (MR Linac) combines an MRI scanner and linear accelerator to precisely locate tumours, tailor the shape of X-ray beams in real time and accurately deliver doses of radiation to moving tumours

Answer 19

Fusion of two or more imaging modalities to form a new technique Fusing imaging technologies synergistically combining structural and molecular imaging PET/CT SPECT (Single Positron Emission CT)/CT PET/MRI SPECT/MRI