General background information
SED is the Specific Energy Dose. This is the accumulated energy as a result of whole body SAR for the whole examination. It is expressed in terms of units of energy per unit weight i.e. Joules per Kilogram.
Even when SAR is monitored, for very long examinations, the SED becomes important in relation to the risk of tissue damage, particularly in those patients who may have restricted thermal regulation.
Siemens specific background information
On Siemens MR systems (The following excerpt is heavily based on the guidance siemens provide in the operator manual under the ‘scanning’ section.)
The SED limit for the Siemens scanner is 14,400 J/Kg. This is equivalent to scanning in normal operating mode (2W/kg) for 2 hours. Siemens will display a warning threshold at 6,000 J/Kg. This will occur if scanning in normal mode (2W/Kg, continously for 50mins).
Below this levels, persons with normal thermoregulation are considered safe. Above the threshold, depending on the applied SAR and the individual patient’s condition, the patient may suffer physiological stress or may even experience tissue damage.
Practical advice for Siemens users (non-NHS GGC sites or non-NHS GGC SLA partner use this at their own risk)
We outline two scenarios with advice to minimise risk to patients.
Scenario 1: Dealing with a live SED warning
If clinical support is available, confirm whether further scans are indeed necessary
If clinical support is not immediately available, proceed with the MR but prioritise the order of the remaining scans and apply low SAR pulses
Make sure in-bore fan is on
On remaining sequences change the sequence pulse type to low SAR pulses
Check-in with patient between scans if this is possible (if not possible, continue following the guidance above)
Scenario 2: Plan ahead to avoid SED limits
If you know there are protocols or circumstances which are likely to or consistently lead to SED warnings then:
Make sure in-bore fan is on
Ensure the patient is not too warm from the start of the scan i.e. is not covered in blankets
aim to prioritise the scan based on clinical need (if this guidance is available)
If the full time for the scanning protocol is going to be long (>50mins) then use low SAR pulses on sequences from 40mins