|
|
MRI: |
Ultrasound |
Expense |
Expensive |
Most expensive |
Cheapest |
Scan time |
10-45 mins |
Longest |
Shortest |
Availability (linked to waiting lists) |
Most big hospitals can do it
|
So expensive that it only available in specialised units |
Most widely available |
Ionising - therefore increasing probability of cancer developing |
|
No |
No |
Availability - due to trained operators as well as equipment |
|
Least available |
Most available |
Contrast agents |
Nephrotoxic contrast agents, and adverse reactions to the iodinated contrast agents
|
The usual contrast agent [gadolinium chelates] has a good safety profile |
None used |
Contrast and detail in images |
State-of-the-art spiral CTÂ gives very detailed images - but not as good as MRI |
MRI has better detection and descriptive powers than does CT. It provides excellent contrast that can reveal subtle variations in tissues of differing histology. |
Detailed images are not obtained in Ultrasound of the quality of the other two - but we do get pretty good ones nowadays - see 3-d imaging! |
Spatial Resolution (the ability to distinguish two structures an arbitrarily small distance from each other as separate) |
CT provides good spatial resolution |
MRI provides comparable resolution to CT with far better contrast resolution (the ability to distinguish the differences between two arbitrarily similar but not identical tissues).
The basis of this ability is the complex library of pulse sequences that the modern medical MRI scanner includes, each of which is optimized to provide image contrast based on the chemical sensitivity of MRI. |
Other considerations |
|
The scan can be done in any direction, not just axially like CT scans. |
|
Contraindications |
Pregnancy - can't have ionising radiation scan if pregnant
|
Can't be used if patient has implanted metal like: pacemakers, aneurysm clips, heart valves, vascular stents |
|
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