MRI Flashcards
contrast agent
how does it work
dose required
chelates of godolinium
Typically only 15-20mls needed vs 150mls for iodinated agents: much lower risk of CIN.
Gadolinium alters the properties of nearby hydrogen nuclei causing increased signal intensity
rate of adverse reaction gadolinium
0.04%
NSF
what is it
when does it occur
potentially fatal
development fibrotic tissue, collagen deposition in skin and muscles due to scarring and contractures
can occur up to 3 months post contrast
MRI work
Protons of hydrogen atoms usually spin in a random fashion
In MRI scanner, they align with the magnetic field in the longitudinal plane and produce a secondary spin (precession)
When a radiofrequency pulse is applied the nuclei receive energy to move out of alignment and into the transverse plane
When this pulse is removed the atoms release their energy in 2 ways: T1 and T2
The release of energy is picked up as an electrical voltage by a receiver coil and converted to a image
t2 vs t1
Time taken for realignment – relaxation time
Relaxation along static axis / longitudinal plane – T1
Energy released back into surroundings when realign back to longitudinal plane
Relaxation occurs more rapidly in fat
Large molecules give energy back to environment quicker
Relaxation along secondary axis / x-y axis– T2
Energy loss between adjacent nuclei, lose their precessional coherence and dephase
T2 decay occurs more slowly in water, resulting in higher signal
Relaxation times varies with protons in different tissue types
T2 water bright, fat dark
what is DWI
The random movement of water molecules is restricted in the body by boundaries formed by cell membranes
Tissues typically demonstrating restricted diffusion include cancer, oedema, fibrosis and abscess
Densely cellular prostate cancer displays restricted diffusion compared to normal adjacent peripheral zone tissue
Prostate cancer results in increased cellularity, and a reduction of the extracellular space, and results in restricted diffusion on DWI
ADC and B values
The strength and duration of the diffusion gradient detected are referred to as “b-values” with higher values resulting in the production of a higher signal intensity
Calculation of aforementioned b-values facilitates the construction of an apparent diffusion coefficient (ADC) map that demonstrates tumors as having low signal intensity secondary to the increased cell density, reduced interstitial fluid and reduced free water when compared to normal prostatic tissue such as that in a normal gland or in BPH (Figure 4B).
what is dce??
DCE imaging utilizes intravenous gadolinium contrast to visualise blood flow in and out of prostate
Serial imaging of wash-in and wash-out periods enables tumours to be identified and correlates with tumour aggression
Malignant prostate lesions have increased tumour vascularity, and show early, rapid and intense enhancement, followed by quick washout of contrast administered for imaging purposes
PROMIS trial year number of men, PSA threshold, type of MRI tests used result NPV of MRI biopsies avoided higher detection rate of mri with trus vs trus alone spec MRI
576 men psa less than 15 1.5T MRI 2017 mri, trus, saturation mri plus trus 18% more cases mri more sensitive than trus at higher risk disease - gg 3 or above avoid 1 in 4 biopsies MRI spec 41% NPV 89%
PRECISIOn study men inclusion tests used result
men 500
inclusion - clinical suspicion prostate cancer, no prev biopsy
tests used - trus vs mri plus targeted trus
result
more clinically sig prostate cancer found with trus and mri group vs trus alone
T1 used to exclude prostatic haemorrhage
Intraprostatic hemorrhage is the most common biopsy-related morphological change. It can be identified as high signal intensity on T1–WI and can compromise the interpretation on T2–WI because of its similar appearance to that of PCa (low T2 signal intensity) in up to 80 percent of cases. However, the degree of hemorrhage has been shown to be significantly less in areas of PCa than in areas of normal or benign prostatic tissue. This feature has been referred to as the T1 hemorrhage exclusion sign.
disadvantages of DCE
Contrast using gadolinium, increases time on scanner further, iv access, medical supervision with rare contrast reactions
ferrous body size danger in MRI
not dangerous if below 0.1mm
CI to MRI
aneurysm clips artificial heart valves cardiac pacemaker metal fragments in eyes metallic implants, shrapnel injury patient claustrophic surgical clips foreign bodies
PIRAD 3 upgrade to PIRAD 4
look at DCE
