Helical CT is also known as spiral CT; the two terms are interchangeable (Kalender, 1994). Both MRI and helical CT have been introduced into clinical practice ahead of any evidence for cost-effective improvement in clinical care. Both technologies are still evolving. For instance vascular 3D imaging is a newly expanding indication within CT. Although helical CT is replacing conventional CT the question arises as to whether it will replace MRI.
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Helical CT began in the 1990’s. It is a fast technique; data is collected continuously at less than one second for a 10 mm slice. It is called helical because the patient moves continuously though the machine whilst the X-ray tube rotates around them. Slip ring technology enables the scanner, mounted on a gantry, to continue rotating in the same direction around the patient yet still maintain its power supply and x-Ray capability. Because it is so quick breathing does not affect the quality of the final image and it is an excellent way to view the lungs and liver. Because of the continuous rotation helical CT enables patient translation and the acquisition of data to take place at the same time. Helical CT requires completely different equipment to convention CT necessitating the replacement of the entire unit not just an upgrade. A multislice CT scanner is along the same principle as a helical scanner but is even faster still and contains more detection elements. Although the actual data acquisition is so much faster with multislice the time required to process the image is lengthy (so patient through put will be no faster). The amount of data storage space required for multislice images is incredibly vast and may overload the capability of the existing PACS system within the hospital.
The equipment for MRI consists of a large, heavy magnet which creates the magnetic field. Magnetic shielding of the room is necessary together with stringent safety precautions to avoid accidents for instance with flying metal objects within the room. The scanning tube where the patient must lie is relatively enclosed and this can create problems with claustrophobia. The equipment is also very noisy which may unnerve the patient. MRI requires more extensive software for viewing the images than does CT. Some MRI machinery is more open permitting greater patient access even to the extent of allowing simultaneous surgery (Gould and Darzi, 1997).
MRI involves the person being placed in a large magnet the magnetic field of which causes all the protons (the nuclei of hydrogen atoms) in the body to line up and oscillate at a certain frequency (precision frequency). Radiofrequency pulses are emitted from the machinery at the same frequency as the precision frequency causing the protons to come out of alignment for a brief time and subsequently realign emitting energy in the process. The radiofrequency of these emissions is specific to the type of issue (since it reflects the hydrogen content) and is then computed to form an image. Patient movement is a major problem with the MRI technique since data acquisition is quite slow and so it is not as good as helical CT for moving organs such as the lungs and liver. MRI scans are more expensive to produce that helical CT. The major advantages of MRI over helical CT are that MRI involves no x-Ray exposure and certain structures provide better images with MRI such as the brain and musculoskeletal system. MRI is definitely the best test for acoustic neuroma (Renowden and Anslow 1993). CT is better than MRI for imaging brain trauma and is better in the abdomen for the bowel (on account of it being a moving structure) whereas MRI is better in the pelvis. Helical CT is finding a place in the diagnosis of pulmonary embolism (Roy 2005). The disadvantages of CT are the x-Ray dose and the nephrotoxicity of some contrast agents.
In 1993 the Royal College of Radiologist guidelines recommended MRI be used for investigations on the brain, musculoskeletal system, oncology and paediatrics, the 1995 version of the guidelines recommended back pain beyond six weeks be investigated by MRI. The Royal College of Radiologists document on oncology (1999) provides graded evidence based recommendation of which scanning modality to use according to tumour site.
Staff training is necessary for both modalities of scanning. MRI staffing costs are higher than with CT. Because MRI scans are in such demand and scanning time long it is often necessary to run the machines in the evenings and at weekends (Moore & Golding, 1992). Multislice CT can involve increased radiologist workload.
Patients with metal implants or pacemakers or who are claustrophobic are unsuitable for MRI. Mechanical ventilation is a relative contraindication. Patients with acute major trauma including head injury are unlikely to be suitable for MRI because of the duration of scanning. The increased x-Ray dose to patients (and to the community) of the later generation CT scanners is of concern (National Radiological Protection Board, 1990). For this reason MRI is the preferred modality for children and fetuses (Duncan 1996). Patients requiring interventional procedures may be suitable for a CT fluoroscopy (Wagner 2001).
5) Quality of results
MRI is preferred for the brain and spine (where it is of overriding advantage), orthopaedics and the pelvis. MRI produces very accurate images of soft tissues but imaging time is longer and artefacts are caused by patient movement. It is likely it has reduced the number of knee arthroscopies (Stoner, 1995) and it is anticipated to reduce the number of invasive radiological investigations such as angiograms. MRI may develop a clinical role as investigating the actual function of the brain in neuropsychiatry (Callicott and Weinberger1999). CT is preferable for bone. In brain trauma, subarachnoid haemorrhage and acute cerebrovascular disease MRI is not as good as CT.
Spiral CT is used for the lungs and abdomen and pelvis. It is valuable in detecting small lesions. It is helpful in trauma patients since the procedure is so quick. Spiral CT does lose a bit of resolution as compared with conventional CT and so for structures that are not moving conventional CT or MRI has the advantage.
Cost considerations include those of initial purchase (or lease) set up and also running costs. Assistance in the procurement process is available from the Diagnostic Medical Equipment team which is working closely with the Department of Health in the optimising of value for money in the replacement of all MRI and CT scanners that are pre-1997. A 16 multislice CT scanner costs approximately £500 000 whereas an MRI scanner is more at £800 000; running costs are also more with MRI (Frank, 2003). Bowens and Smith (writing in 1999) state the costs of an MRI scanner are from £400 000 for a 0.5T and £750 000 for a 1.5T. They state the service contracts are around £50 000 per year and that to lease a machine costs about £120 000 per year. MRI may be more expensive to install since the magnet is large and heavy. The site may be unsuitable with regard to load bearing or access. In any case expense will be incurred in magnetic shielding. MRI is a relatively expensive imaging modality. Fletcher (1999) has analysed costs of acquiring and operating MRI in the NHS over a seven-year machine lifespan. Its staffing, upgrade, maintenance and running costs are all high. The cost of an MRI scan varies from £30 to £180 (Bowens and Smith, 1999).
In evaluating costs it is necessary to look at the whole picture. The running costs of isolated MRI machines will be higher than where machines are grouped together. Smaller MRI scanners just for joint scanning use may prove cost effective (Marti-Bonmati & Kormano, 1997). If a more expensive scanning modality saves on the costs of surgery then overall there may be economic gain. For instance MRI may avoid knee joint surgery (Bui-Mansfield 1997). It is important to ensure that it is actually replacing other investigations or surgery and not just adding to them (Hailey & Marshall, 1995). Overall the cost effectiveness will depend on how appropriately the imaging modality is used.
Regarding CT the X- Ray tubes are expensive. A helical scanner is likely to need one x-Ray tube replacement per year (possibly more frequently in the case of a multislice scanner) and this will cost approximately £30000-40000 (Conall and Hanlon, 2002). Berry (1999) performed a systematic review finding little clinical or economic impact of spiral CT.
Although there has been away from MRI to helical CT in some clinical situations units will need access to both types of scan. Cooperation between different units is important in order to provide a comprehensive service to the population. It is likely that some patients such as orthopaedic outpatients should move to another unit for the scan. Computerised reporting makes off site scanning realistically closer. Choice of scanning modality is likely to ultimately depend upon collaboration with local units to develop a hub and spoke approach to providing cost effective services which are also effective and convenient for patients.
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