Download MDD_MR1_generic_response

Medical Devices Directorate and MR sequences – GENERIC RESPONSE
File: MDD_MR1_generic_response
Version – January 13th 2005
1. Introduction
a. Purpose.: This document gives generic (i.e. general, non-specific) responsive information
to be read along with the replies to the specific questions (see MDD_MR2_questionnaire). It
is intended for the device user, not the provider. In addition there is an instruction manual
for each device.
b. Types of device
i. Sequence devices – developed by MR pulse programmer, implemented on a
scanner.
ii. Data analysis devices – developed by computer programmer, implemented on
computers separate from the MRI scanner.
2. General responses to the Essential Requirements – MR sequences:.
a. GR1. MR sequences run on a clinical MRI scanner. At Queen Square these are from
General Electric Medical Systems, or from Siemens. These scanners are medical devices
and are CE marked. They are also approved for use in the USA by the Food and Drugs
Administration (FDA). They come with a comprehensive set of instruction manuals. Any
use of a scanner in ‘clinical mode’ is covered by this CE mark. The manufacturer provides a
set of pulse sequences which enable the scanner to collect images (and spectra) with various
image contrasts, and sensitive to various MR parameters. At Queen Square research is
carried by MR physicists into novel ways of collecting MR data, using new (‘home-made’)
pulse sequences. Siemens scanners are only used in clinical mode. GE scanners are
generally used in ‘research mode’ when developing and using home-made sequences, and
therefore do not, in general, have a blanket coverage arising from the manufacturer’s CE
mark. Although the static field cannot be altered in research mode, the RF power, and
possibly gradient switching rate, can be and therefore need to be considered in detail. There
is use of electromagnetic radiation, for the RF pulses (see GR5 below). The gradient
switching produces a low frequency radiation (see GR4 below). There is no ionising
radiation. Subjects are screened for pacemakers and metal objects before being allowed into
the scanner environment. The moving couch is driven by motors; this operation is not
altered in any of the home-made devices. The gradient coils can become hot; however these
are water cooled and thermally shielded, and home-made sequences will not alter this
operation. See ER1, ER2, ER4, ER6, ER7.6, ER9.2, ER11.1.1,ER12.1,ER12.3,ER12.5,
ER12.7.1, ER12.7.3, ER12.7.4, ER12.7.5, ER12.8.2, ER13.1,ER13.6k, ER13.6l,
b. GR2. There are general safety guidelines from various bodies. In the UK, the relevant
bodies are the National Radiation Protection Board (NRPB) and Health and Safety
Executive (HSE). NRPB guidelines are similar to those produced by the International
Commission on Non-Ionising Radiation Protection (ICNIRP http://www.icnirp.de). ICNIRP
produced a comprehensive 20-page paper on safety in 2004. In the USA, the Food and
Drugs Authority (FDA) is relevant. These guidelines affect the static field, the rate at which
the gradients can be switched (dB/dt), and the Radiofrequency (RF) power deposition
(Specific Absorption rate SAR). Safety effects are considered in more detail in GR3-7
below. An additional issue is that the HSE seems to be recommending against anything
above 2.0T. This seems to be a problem affecting MR in the whole UK, and has not been
resolved. This recommendation originates with ICNIRP and applies particularly to staff and
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volunteers. Patients may undergo imaging at higher fields as long as there is medical cover
available. In general the risk associated with MRI is very low, and there is a general move
in healthcare towards MRI and away from other (more risky) imaging techniques such Xray Computed Tomography and nuclear medicine, both of which involve ionising radiation.
See ER1, ER2, ER4, ER6, ER7.6, ER9.2,
c. GR3 static field: At Queen Square, including associated Chalfont, there are clinical
machines operating with ethical approval at 1.5T, 3.0T and 4.7T.
d. GR4 The gradient switching time and slew rate determine the dB/dt (this increases with
distance from the magnet isocentre). [PAUL – give typical times and rates]. In pulse
sequences this is not usually increased above the manufacturer’s value. The consequences
of a high dB/dt are stimulation of some muscles, usually in the chest wall, and are
considered to be uncomfortable but not dangerous. (See FDA guidance).
e. GR5 Radiofrequency (RF) Power deposition, as defined by the Specific Absorption Rate
(SAR), is potentially the most hazardous factor. Small amounts can cause a feeling of being
warm; large amounts could in principle cause local heating in tissues that do not have a
blood supply, such as the vitreous humour or lens in the eye, and as a worst case a cataract
could result. The GE scanner has a variety of SAR checks, in software and hardware, some
of which can be turned off in research mode. The software checks are complex; they attempt
to take into account the mass of the subject, they make some assumptions about the
distribution of the deposited power, and also set rules about the time course of the power
deposition (i.e. controlling both instantaneous and time-averaged power deposition). This is
an area about which we are currently investigating, and where each scanner manufacturer
will have a different Research Mode environment. We are currently considering starting a
project to measure deposited power directly, in a phantom. (ER12.8.2)
f. GR6 Acoustic noise can be annoying, and for many sequences ear plugs are offered to the
subject. The sound level is dependent primarily on the gradient switching rate and the
number of gradient pulses per second, and provided this is not increased (see above), the
sound level is not expected to increase above that found in noisy sequences used in clinical
mode. [ER12.7.3]
g. GR7 Other factors, such as ferromagnetic projectiles, internal metal structures, or
quenches, are no more risky than when conventional sequence are used in clinical mode.
h. GR8. A sequence device must operate with the lowest reasonable static field, SAR and
dB/dt. In general, the characteristics of the device would be degraded by going to lower
static field, SAR and dB/dt. The signal-to-noise ratio and hence precision would suffer.
SAR could not be reduced without removing or reducing RF pulses that are required for the
sequence to work. Reduced dB/dt would increase scanning time. [ER9.2b, ER11.1.1]
i. GR9. Injected contrast agents. For some pulse sequence devices, a contrast agent (CA),
based on Gadolinium, may be injected at the same time. The CA does not form part of the
device; permission is obtained separately from the local hospital ethics committee. See
ER7.3
j. GR10. EEG connections. EEG connections may be made at the same time as functional
(fMRI) imaging. More information from Louis Lemieux. Other hardware devices may also
be added to a MRI machine, although these are not considered here. See ER9.1, ER9.2,
ER12.6, ER13.6c
3. General responses to the Essential Requirements – data analysis programs
a. GR20 Image analysis programs take MR data from the scanner (in fact these can be in the
form of images or spectra). The data include header information (such as echo time, patient
ID).
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i. On the MS and epilepsy GE scanners, post 2004-upgrade scanner data are provided
by GE in DICOM format, and then converted to UNC format by an in-house
program called gedicom2unc. Old (pre-2004 upgrade) data will be available from
GE in DICOM format, although the exact DICOM implementation may be subtly
different. The accurate carrying through of header information is currently being
checked. Pre-upgrade data was converted using a program called signa2unc.
b. GR21. The general programming environment used in the NMR Research Unit at the
Institute of Neurology, UCL, is UNIX/LINUX based. Most programs are written in c,
although other standard programming languages are alsp used. Binaries for public use are
kept in a central directory and all new programs must have their source code in a central
source directory. This allows control of binary ownership and version number through
linking of these directories. This does not apply to historical software although we are in the
process of cataloguing this. When a new version of a binary is released the old versions are
kept for compatibility. The central directories and system are under the control of the
computer manager, who can therefore keep track of what changes are made.
c. GR22. Data analysis can also include programs to analyse spectra, convert file formats or
to register images.
References
a. FDA document Criteria for Significant Risk Investigations of Magnetic Resonance
Diagnostic Devices
http://www.fda.gov/cdrh/ode/guidance/793.pdf
Paul Tofts
Queries to be resolved before final version:
1. is dB/dt ever increased in research mode?
2. what are symptoms of dB/dt too high
3. GR20,21 – more needed from e.g. Mara, Dan
4. can the noise be increased by any sequence?
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