The UK National Health Service (NHS) Purchasing and Supply Agency (PASA) Centre for Evidence-based Purchasing (CEP): has produced a series of
comparative reports on the specifications and technical performance of both 1.5 and 3 Tesla (T) closed system magnetic
resonance imaging (MRI)
equipment, to help inform buyers’ purchasing decisions.
Seven 1.5 T models, produced by GE, Phillips, Siemens and Toshiba, and four 3 T models, produced by GE, Phillips and Siemens, were evaluated (figure 1; table 1). The latest reports, which were first published in May 2007, form the basis of this article [1, 2].
Developments in MRI Effective diagnostic imaging has a key role in ensuring that patients have speedy access to appropriate treatment. The UK Department of Health has prioritised imaging as part of its progress towards meeting its 18-week referral-to-treatment target, due to be implemented by the end of 2008.
Most hospitals buy 1.5 T systems. These have an important role in cancer diagnostics, including the identification of metastases from primary tumours of the respiratory system. Until recently, the more expensive 3 T systems have been used primarily for research, but they are becoming increasingly
MRI SCANNERS:
A BUYER’S GUIDE
D. Price, I. Delakis, C. Renaud and R. Dickinson
CorrespondenceCentre for Evidence-based Purchasing 152C Skipton House 80 London Road London SE1 6LH UK Website: www.pasa.nhs.uk/cep
popular for clinical work. They offer the possibility of higher image quality and shorter scanning times. Hitherto, MRI has not been the modality of choice for imaging the respiratory system. The air content in the lungs usually makes it difficult to produce sufficient image contrast at an appropriate spatial resolution. Blurring can be present in the images because of patient motion and it can also occur because the magnetic resonance signal from the lungs decays very rapidly. This article discusses the
technological features available on the latest MRI systems that improve image quality and imaging speed and have enabled studies of lung structure and function using MRI [3]. These have included
examinations of blood flow in the pulmonary vessels and blood perfusion in lung tissue [4, 5]. Normally these studies are performed with the use of injected contrast agents. Respiratory gating may be needed to coincide the imaging with the patient’s breathing. Such gating can reduce motion blurring although ideally, if the scan time can be reduced enough, images can be acquired in a single breath-hold.
Further developments are opening up the potential for MRI to be used
much more widely in the diagnosis and management of respiratory disease. These include inhaled gas contrast agents (such as
hyperpolarised helium), which enable MRI to track how air is being used in the lungs [5, 6].
How does MRI work? MRI provides excellent structural and functional images of the soft tissues, organs and vasculature in any desired spatial plane. This is achieved when the patient is placed inside a powerful magnetic field. Most modern scanners are based on superconducting magnets, which are cooled with liquid helium. The billions of protons that constitute the nuclei of hydrogen atoms in the body have magnetic properties. These continually spin in different directions, but line up with the pull of the main magnetic field. Once the protons are spinning in unison, they can all be tipped out of their alignment with the main magnetic field by applying a pulse of energy in the radiofrequency (RF) range. This is achieved with an RF transmit coil that is normally built into the scanner.
After the RF pulse has been applied, the protons return to their original positions, releasing energy in
the form of a signal, which carries information about specific tissues. The signal is collected by an RF receive coil, a range of which are provided for different areas of the body. The pulse sequences, which describe a pattern of pulses, are designed to provide differing image contrast for different body tissues, such as the brain or the heart. Three further coils built into the scanner, known as the gradient coils, are switched rapidly on and off during the scan. The magnetic fields applied by the gradient coils vary with position, encoding the signal with spatial information so that an image can be constructed. The timing of the RF and gradient pulses, and conversion of the signals into images, is controlled by computer.
Design features
The latest ranges of cylindrical models have been designed with patient comfort in mind, offering wider flared openings to the cylinder, shorter scan lengths, and less acoustic noise.
Higher specifications usually mean better-quality images and/or reduced scan times, plus the ability to perform the most advanced applications. But it might be that
GE Philips Siemens Toshiba
1.5 T Signa HDx Signa HDe Intera Achieva MAGNETOM MAGNETOM Excelart
model Symphony Avanto I Vantage
class & T class
3 T Signa HDx Achieva Allegra Trio, A Tim
model X series System
I class & T class
not all the various pulse sequence packages that come with these models are required. Inevitably, higher-specification machines also tend to be more expensive. However, these options could be worth considering, as it may be easier to add on further
applications as the technology and clinical need develop.
All models come with certain standard features. These include: • A set of core pulse sequence
packages to cover the entire anatomy. The higher-specification models have a range of optional additions for specific
applications, such as spectroscopy (see below). • A suite of RF coils to cover the
entire anatomy: head, body, shoulder, spine coils, etc.
Key purchasing factors to consider
1. Technical features
The strength of the main magnetic fieldwill affect image quality and the number of applications for which the scanner can be used. For instance, advanced neurological imaging techniques, such as diffusion MRI, are normally performed at 1.5 T and above. Low field systems can produce images of excellent diagnostic quality for many applications. However in general, the higher the strength, the better will be the quality of the images produced, because a higher magnetic field provides a higher signal-to-noise ratio. It should be noted, however that the magnetic resonance signal from lung may decay even quicker at higher fields, so any benefit may not be realised unless the imaging time can be reduced further through the use of more powerful gradient coil systems.
The signal-to-noise ratio (SNR) is the most important parameter defining image quality in MRI. A low SNR means that the contrast between different tissues can be
obscured by background noise. On the other hand, a high SNR provides more options. For example, it can be partially traded off to obtain improved spatial resolution (smaller pixels and finer detail) or faster images through parallel imaging. Faster scanning reduces the amount of blurring caused by movement during breathing.
Field uniformity and stabilityare important for a good-quality image. Good magnetic field homogeneity over a large volume, which is reflected in a large maximum field of view, means that image quality is maintained even on the peripheral areas of the body. Sometimes the high signal from fat tissue will need to be suppressed, because it can obscure important detail in the image. This can be achieved using special pulse sequences but it may not be successfully suppressed if magnetic field homogeneity is poor. Homogeneity is normally defined as the variation in magnetic field in parts per million over a defined spherical volume.
Shim coils, which produce a relatively small current, readjust the disturbance to the magnetic field caused by the patient inside the scanner, a process called active shimming. The gradient coils are used for active shimming, but optional dedicated shim coils may also be available. Field uniformity and stability are even more important for spectroscopy, which analyses chemical information at the molecular level and is primarily used in oncology.
The radiofrequency system refers to the number of independent channels that can receive signals from the RF coils. Multichannel RF coils feature several receiving elements, each of which can feed into an independent channel in the system. A higher number of channels boosts the SNR and offers the option of parallel imaging. A higher SNR could be useful for lung imaging, where there is more air than tissue. Multichannel body coils are likely to be the most important component for this application.
Gradient coil systems provide the images in any desired plane (figure 1). The magnetic field gradient applied by the coil is measured in mT·m-1while the rate
at which the gradient can be changed, the slew rate, is
measured in mT·m-1·ms-1.
Higher-amplitude gradients, which can be switched on and off quickly (high slew rate) offer a stronger gradient pulse within a shorter time frame. This provides the same or even better spatial resolution for a shorter scan time. Various imaging parameters can be selected to determine the timings of the RF and gradient pulses. For instance, the repetition time, defined as the time between successive RF pulses, affects image contrast and scan time. Since the MR signal from lung tissue decays rapidly short time-imaging parameters are often essential. High-specification gradients are needed for this. Parallel imaging techniques use information from the RF coils to help spatially encode the magnetic resonance signal, thus reducing the number of times that the gradient coils have to be switched on and off, and speeding up the imaging process. Various commercial packages are available. The factor by which the scan time is reduced is known as the parallel imaging factor. A factor of 2 will suit most applications. This technique is widely used in breath-hold imaging. But higher parallel imaging factors reduce SNR, so higher-order factors have not been used widely in clinical practice to date. Multichannel coils are required for parallel imaging. 2. Ease of use and safety Patient comfort: about one in 10 patients feel claustrophobic inside the closed cylinder and models with wider apertures can help minimise this. Wider, longer tables will accommodate heavier patients, which may become more important if the prevalence of obese patients continues to rise. Large RF coils are quite heavy and some models incorporate spine coils built into the
patient couch. Some also allow all the coils to be plugged in at once, which speeds up the process for both patient and staff.
Safety: MRI equipment has a good safety profile. Most adverse incidents are the result of incorrect procedures or are related to the design of the MRI site. Active shielding curbs the extent of the fringe magnetic field around the scanner, while built-in limits in the scanner protect the patient from excessive RF energy.
Ferromagnetic objects, such as oxygen cylinders, can be sucked into the main magnetic field and so present a projectile hazard. It’s also important to check whether patients and staff have implants, metal pins, or pacemakers.
Noise levels can exceed 100 dB, although some models now feature noise-control technology that reduces this to <90 dB for most scans. Hearing protection is essential for patients and for staff remaining in the scan room during imaging. Dimensions:MRI scanners take up a lot of space because of the need to allow for the extent of the magnetic field, and the heavier models may require the floor to be reinforced. Manufacturers specify the minimum installation area, but if space is constrained, large amounts of magnetic field shielding may be required to prevent the fringe field from extending into public areas. Compatibilitywith other existing equipment needs to be checked to ensure it is safe to use near the scanner and that it will not
interfere with the images. Normally there are safe operating conditions that must be followed: for instance, certain equipment may be safe only if kept a certain distance from the magnet.
3. Costs
Capital outlay: you can expect to pay a minimum of £700,000
(€1 million) for a 1.5 T model and from £1 million (€1.5 million) for a 3 T model. Some level of service and on-site training should be included in this price.
Manufacturers will sometimes include accessories in the overall bundle of costs, but these can vary, so check which accessories are included. Manufacturers can offer various financial packages. Accessoriesinclude an independent workstation, which allows you to view images and perform image processing away from the main console. Other accessories you may require include: contrast injectors, cardiac, respiratory and oxygen monitors, additional beds and detachable trolleys. If these are not included in the package, you need to consider where you are going to source them.
Similarly, check what sequence packagesare included, and whether these suit your clinical needs. What RF coils are included and what come as optional extras? Do you need to buy extra RF channels to use some of these coils?
Maintenance: high-specification machines are expensive to buy
and run. The liquid helium in the magnet slowly boils off and it will need to be topped up every few years. What type of service contract will you get? A 6-month service is common and
frequently included in the price, but check. Are there options to extend it? How much will these cost?
Before you buy:as well as reading the brochures and talking to the manufacturers, it is always worth visiting the factory or a site where your selected equipment is already in use before you buy.
4. Customer support
Training:when the equipment is installed by the manufacturer, training on how to use it will automatically be provided. Check how much training will be provided and for whom, and whether this suits your needs. For instance, you may need extra training if you are upgrading from a 1.5 T to a 3 T model. Will all the training be on site?
Operator and reference manuals: will these be provided in electronic or print format, or both?
X Y
Z
DSV: defined spherical volume; RF: radiofrequency; TR: repetition time. Table 2. Technical specifications for 1.5 T equipment.
DSV: defined spherical volume; RF: radiofrequency; TR: repetition time. Table 3. Technical specifications for 3 T equipment.
GE Signa HDx Philips Achieva Siemens Allegra Siemens Trio X series
Main magnetic field 0.25 0.5 0.1 0.1 homogeneity (40 cm DSV ppm)
Helium refill 4 yrs NA 7 months 9 months Minimum installed area m2 36 19 28 <33
RF system HDx Freewave iPAT/IPAT plus 102x8/102x18/102x32 No of independent RF 8 /16/32 8 /16/32 iPAT=4 102x8=81/102x18=18/ receive channels iPAT plus=8 102x32=32
Max amplitude mT·m-1X, Y, Z 50 Standard mode = 40 40 40 (45 for z direction)
Enhanced mode = 80
Slew rate mT·m-1·ms-1X, Y, Z 150 Standard mode = 100 400 200
Enhanced mode = 200
Min TR ms 3D gradient echo 1.2 1.1 1.72 1.6
Max field of view mm 450 (x & y directions) 530 (isotropic) 220 (isotropic) 500 (isotropic) 480 (z directions)
Min field of view mm 10 5 5 5
GE Signa HDx Philips Achieva Siemens Allegra Siemens Trio X series
GE Signa GE Signa Philips Philips Siemens Siemens Toshiba Excelart HDx HDe Intera Achieva Symphony Avanto Vantage Main magnetic <0.27 <0.27 0.35 0.2 0.4 0.2 <1.0 field homogeneity (40 cm DSV ppm) Max field of 480 480 530 530 500 500 500 view mm (isotropic) (isotropic) (isotropic) (isotropic) (isotropic) (isotropic) (isotropic)
For Atlas ZGV: 550 for x & y directions and 500 for z direction Min field of 10 10 5 5 5 5 5 view mm
Helium refill >3 yrs >3 yrs 3 yrs 3 yrs 2 yrs 10 yrs 2–3 yrs
Min installed <33 <22 30 30 30 ≤30 28.8 area m2
RF system EXCITE HDx EXCITE HDe Synergy Freewave Advanced/ TM AGV/XGV/ Whole body 32x8/76x18/ ZGV/Atlas Array 76x32
No of 8/16/32 4/8 4/6 8/16/32 Advanced=4 8/18/32 AGV=4 independent Whole body=8 XGV=4,8
RF receiver ZGV=8
channels Atlas=16
Gradient HDx Twin HDe Pulsar Pulsar Nova Nova Quantum Q SQ AGV XGV ZGV system HP HP Dual engine engine
HP Max amplitude 33 50 23 33 33 33 33,66 30 33 40 30 30 33 mT·m-1X, Y, Z (45 Z) Slew rate 120 150 50 80 100 180 180, 90 125 125 200 50 130 200 mT·m-1·ms-1 X, Y, Z Min TR ms 3D 1.2 1.2 1.7 2.25 1.07 0.83 0.83 1.8 1.8 1.5 3.5 3.5 3.5 gradient echo
GE Signa GE Signa Philips Philips Siemens Siemens Toshiba Excelart HDx HDe Intera Achieva Symphony Avanto Vantage
Service contract: find out if there are enough engineers to fix any potential problems and how soon the manufacturer guarantees to rectify them. How widely available are parts? How quickly can they be shipped in? Does the manufacturer guarantee a certain number of working days in the year (guaranteed ‘uptime’)? Remote diagnostics:some manufacturers operate remote diagnostics, whereby the system is automatically linked to the factory. This can speed up detection and resolution of problems. Check whether this is available. All CEP reports since 2002 are available to download from the organisation's website. An email alert service is also available, by contacting cep@pasa.nhs.uk ■
GE Signa HDx
GE Signa HDe
Philips Intera Philips Achieva Siemens Symphony Siemens Avanto Toshiba Excelart Vantage Patient aperture at narrowest cm Width x height (couch to pole) 60xx46.5 60xx46.5 60xx42 60xx42 60xx45.2 60xx45.5 60xx48.3 Total length cm 195 195 167 167 160 160 149.5 Couch max/min height cm 97/69 97/69 89/52 89/52 100/45 89/47 87.5/42 Body mass limit kg 159 159 150 150 200 200 200
DSV: defined spherical volume; RF: radiofrequency; TR: repetition time. Table 4. Patient comfort: 1.5 T models.
1. NHS Purchasing and Supply Agency. Report 06006 3T MRI Systems. Issue 4. May 2007.
2. NHS Purchasing and Supply Agency. Report 06005 1.5T MRI Systems. Issue 7. May 2007.
3. Heidemann RM, Griswold MA, Kiefer B, et al.Resolution enhancement in lung 1H
imaging using parallel imaging methods. Magn Reson Med2003; 49: 391–394. 4. Kluge A, Gerriets T, Lange U, Bachman G. MRI for short-term follow-up of acute
pulmonary embolism. Assessment of thrombus appearance and pulmonary perfusion: a feasibility study. Eur Radiol2005; 15: 1969–1977.
5. Mills GH, Wild JM, Eberle B, Van Beek EJR. Functional magnetic resonance imaging of the lung. Br J Anaesth2003; 91: 16–30.
6. Fain SB, Korosec FR, Holmes JH, et al.Functional lung imaging using hyperpolarized gas MRI. J Magn Reson Imaging2007; 25: 910–923.
REFERENCES
Philips Medical Systems www.medical.philips.com Siemens Medical Solutions www.medical.siemens.com GE Healthcare www.gehealthcare.com
Toshiba Medical Systems (Europe) www.toshiba-europe.com/medical/
MANUFACTURERS’ WEBSITES
Table 5. Patient comfort: 3T models.
GE Signa HDx Philips Achieva X series
Siemens Allegra Siemens Trio
Patient aperture at narrowest cm Width x height (couch to pole) 60xx105 60xx42 35xx35 60xx45.5 Total length cm 189 167 142 213
Couch max/min height cm 69/97 52/89 46/80 57/100
