Nuchal Translucency Scan
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Magnetic resonance imaging is increasingly being used for diagnosis and follow up. The wide range of indications can be grouped under:
Gadolinium contrast media is used for similar cases to iodinated contrast in computed tomography (CT), but only totals approximately 30% of studies. This is mainly due to MSK, spine and cerebral MRI scans, which are most frequently carried out, not usually requiring a contrast injection.
Because of the potentially serious complications from safety issues with MRI (interaction of the magnetic field with implanted devices and ferromagnetic objects), each patient is required to answer an MRI safety screening form. This will be checked during a verbal interview with the patient before the scan to ensure they do not have any implants that are contraindicated.
The make and model of certain implants will need to be checked against relevant databases and MRI safety websites by the staff in the MRI department to ascertain their safety. This might take some time, particularly if medical notes or operation reports need to be obtained from another hospital to verify the exact nature of the implant or device. Therefore, if a patient is known to have an implant, it is best to obtain this information before the appointment so that the scan does not need to be delayed or cancelled.
The decision of whether or not to undertake an MRI scan should be made by the referring doctor depending on the clinical and medical indications of the patient. Discussion with the radiologist might be useful to determine if MRI is the most suitable imaging modality.
Metal-containing implants comprise a set of possible contraindications to MRI scanning, as they might heat or move during the procedure. The functioning of mechanical or electronic implants might be interrupted or permanently altered, so many of these devices or patients who have these devices in situ will not be allowed to enter the scan room or be scanned. Devices or metal foreign bodies that are most likely absolute contraindications include:
Please contact the MRI department if you are unsure about any contraindications or requirements necessary.
There are a multitude of implants within patients that need to be assessed as safe. Many are safe and will be reviewed on the safety screening form before the scan.
These implants include, but are not restricted to:
There are many other implants that might be contraindicated and should be discussed with the MRI department individually. Some implants are compatible, but might require assessment befpre and after the scan; for example, programmable shunt devices.
Other conditions can act as relative contraindications for MRIs:
There are no known side-effects of an MRI, providing the patient does not have a contraindicated implant in situ.
Scanning while the patient is in the first trimester of pregnancy is generally avoided, although the risks are theoretical and probably small, but not clinically established. Scanning at this time should be discussed with the radiologist. Gadolinium-based contrast media are usually contraindicated in pregnancy.
In patients with poor renal function, there is a risk of nephrogenic systemic fibrosis (NSF) associated with gadolinium chelate injections (see Gadolinium contrast medium (MRI contrast agents)). Patients with known, or at risk of, renal impairment need to have their renal function assessed before MRI in order to determine whether administration of gadolinium contrast is best avoided.
Gadolinium contrast is generally very safe, but does have a low rate of allergic reactions (about 1 in 1,000 minor reactions and 1 in 10,000 significant reactions). MRI departments using contrast should have appropriate staff and medication to deal with reactions to contrast.
There is no specific post-procedural care required.
The advantage of MRI should be determined on a case-by-case basis. If unsure whether MRI is the modality of choice, clinical discussion of the case with the radiologist is warranted.
Often it is best to start imaging investigations with the simpler modalities – especially plain films in the musculoskeletal system. Normal ultrasound and CT might obviate the need for more complex imaging.
Ultrasound, like MRI has no ionising radiation, and is often the first line of investigation. It has a well-established role, and is also an evolving imaging modality.
CT has the advantage of being a much faster imaging technique, and is thus especially useful for trauma/road trauma, where time is critical. CT has no immediate safety issues, but does involve ionising radiation. It also has the contraindication of contrast medium injection with poor renal function (but not leading to NSF).
CT is better for acute haemorrhage, calcification, fine bony detail and most fractures. Imaging of the lung parenchyma remains in the CT field, and it is also currently used for pulmonary emboli and the coronary arteries (over MRI at the present time). Claustrophobia is unusual in a CT scanner.
If imaging is not helpful in determining the nature of a lesion, surgical intervention or biopsy might be required.
There are many types of MRI scans (referred to as sequences) that can be carried out during an MRI procedure. These sequences are determined by the clinical indications given on the request and so specific clinical indications must be given to ensure the correct sequences are carried out.
Page last modified on 18/8/2017.
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