History Of Mobile Computed Tomography

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					Mobile CT




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                                          Mobile CT

 The invention of computed tomography (CT) is considered to be one of the greatest
innovations in medical imaging since the discovery of x-rays. Computed tomography allows non
superimposed and cross sectional imaging of the human body, thus providing an accurate and
thorough investigation of the human body, including any abnormalities or disease processes that
may be present. The introduction of CT has improved the likelihood of correct diagnosis and
thus an increase in the chances of patient survival as well as improved patient recovery.


 CT and its underlying technologies have come along way in a relatively short time. X-ray
radiation was discovered by physicist Willhelm Conrad Roentgen in 1895, with the first
successful radiograph taken in 1896 by F.H. Williams. Around 1930’s, stratigraphy and
planography were developed, leading to the creation and refining of tomographic technique.
Godfrey Hounsfield developed the foundations for CT in 1972, which allowed for the first time
a digitised scan, which was then reconstructed into an axial slice plan, producing non
superimposed images of a ‘slice’ of an object. This was followed by Siretom releasing the first
head CT system for medical use in 1974, with a whole body CT scanner released in 1976 that
had a scan time of 5 seconds.


 1981-1988 saw a refinement of CT scanners; featuring 3 second scan times per full 360-degree
rotation with 1mm slice width and a selection of five slice thicknesses (1-10mm). The first spiral
CT scanner was in operation 1989 – 1991 which provided 24 sec per 24cm continuous volume
acquisition and a higher resolution. The new and improved spiral CT was built with 4 detectors.
During1994 – 1999, CT scanners were continually advancing, with focus on shorter scan times, less
radiation to the patient, the selection of slice thickness. More patient volume could be scanned in
the same amount of time, thus increasing the resolution. 2000-current – There are manufacturers
constantly competing to design and produce new and improved multi slice CT scanners ranging
from 4 to 64 slices. Other factors to consider are that the innovative CT scanners are patient friendly
by reducing radiation dose and size while increasing cost effectiveness and ease of use as well as an
overall improvement to the quality of diagnostic images.




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 The 1970’s saw the introduction of mobile computed tomography scanners. Mobile computed
tomography scanner were an adaptation of the stationary computed tomography scanners as
diagnostically, CT scanners are very accurate and were becoming more popular and being
incorporated into everyday radiographic technique.


 The term ‘mobile’ is defined as ‘moving readily; movable’ (The Pocket Macquarie dictionary
1989) and can be applied to a mobile CT scanner in the same context as for mobile x-ray machines.
The mobile CT scanner works on the same principles and are of similar construction to that of fixed
scanner.


 The mobile CT scanner is an important tool in producing diagnostic CT images within a variety of
situations and circumstances where a fixed unit is not ideal. One of the more common mobile CT
scanners in use within the hospital environment is the ‘Tomoscan M’, which will be used as the
basis for discussion here. As per mobile x-ray machines, the mobile CT scanner can be run on
battery power. However, due to the limitation of batteries, only examinations of up to 25 slices can
be performed (Seeram 2001). As this is not ideal, the unit may be run on a standard AC power
source with the battery used only as back up.


 The mobile CT unit is comprised of a gantry, patient table and operator’s console, all of which are
mounted on wheels for mobility (Seeram 2001). The general features of the mobile CT gantry
include the aperture (60 cm) and tilt range (+30 to -25 degrees) as compared to 70 cm and +/-12 to
+/-30 degrees for the fixed unit (Seeram 2001) A 48 degree fan angle, 46 cm maximum scan field-
of-view (SFOV) and translation ability of 35 cm are also standard features of the mobile unit
(Seeram 2001).


 The gantry comprises the x-ray tube, generator, detectors and detector electronics (vital
component in the conversion of analogue to digital data and the transmission of the digital data)
(Seeram 2001). The x-ray tube has anode diameter of 102 or 108 mm dependant on tube type with
1.3 x 0.55 mm or 1.7 x 0.7 mm focal spot sizes respectively and a target angle of 12 degrees
(Seeram 2001).


 The mobile CT scanner works on the same principals as the 3rd generation scanner, where the x-
ray tube and detector arrays are coupled and rotate together (Seeram 2001). The detectors are solid




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state with a 400-element array and 16 reference channels (Seeram 2001). The data collected by
them is transmitted by a radiofrequency. The patient table is similar to that of the fixed unit, and
can be locked onto the gantry to prevent it from rolling away.


 The operators console comprises video monitor, audio system, camera interface, network facilities
and storage devices and is connected to the scanner via cables (Seeram 2001). Scan protocols may
also be pre-programmed, that can be selected according to the examination performed. Zoom
reconstruction, multiple image display, image mirroring, annotation, histogram generation,
geometric processing and so on are also available for use (Seeram 2001). Available scan parameters
include scan speed, scan times, slice thickness, detector sampling rate, reconstruction matrix,
reconstruction time and convolution filters, with volume scanning also available (Seeram 2001).
There are currently a couple of mobile CT scanner types out on the market which may vary in
available features, but essentially they all work on the same principals as the fixed CT scanners,
with the same standard components.


 A mobile CT scanner may also be employed in a hospital setting where the patient is unable to
come to the radiography department for one reason or another. This may involve critically ill
patients located within ICU or those within a theatre complex.


 When dealing with extremely ill patients it may be difficult to provide adequate transportation to a
fixed CT unit within the hospital while still providing adequate care for them. Patients in ICU are
often connected to multiple machines and constant monitoring is necessary making it difficult to
move them out of the unit. For these patients, an additional tool called a bedside adaptor may be
implemented to aid in positioning of the patient within the gantry by supporting the head and
shoulders (Butler 2005).




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Fig 1: Diagram demonstrating use of bedside adaptor in ICU (Butler 2005)


 The mobile CT unit can thus reduce discomfort to patients, as they don’t have to be moved
through the hospital and ensures that their condition is not aggravated by a change in surroundings.


 In some complex surgical cases, CT can be extremely beneficial in order to monitor progress. As
transporting a patient during surgery to a fixed CT unit is unrealistic, a mobile CT scanner is an
important tool in aiding the surgeons to perform more competently and confidently. The use of
mobile x-ray machines is still beneficial in most surgical cases, with mobile CT used only for
imaging more complex anatomical areas such as the skull. It has been proven particularly vital in
the treatment of arteriovenous malformations and cervical spine degenerative disorders, as well as
resection of brain tumours and pathologies (Kubota 2004) but may also be used in other situations.


 With the development and implementation of mobile CT, it has shown to have several advantages
and disadvantages. Like any equipment used in medical imaging, the disadvantages of utilising
these modalities are often weighed against the benefit to the patient for purposes of treatment. The
advantages of mobile CT are its versatility, mobility, and cost. Its disadvantages are the time its
takes to scan a patient, and its image quality and radiation dose.


 The versatility of mobile CT is excellent. Its implementation can be as wide reaching as use in
operating theatres for guided biopsy (however these have been rare), trauma use in emergency, ICU,
and has been even used in the SARS outbreaks in 2003-2004 (Parmar et al, 2004). Head trauma
and suspected intracranial bleeds are the most frequently requested CT scans from emergency. As
trauma patients are frequently unstable and require many personnel to monitor the patient (Mayo-
Smith et al, 1999), transport to the department can be risky and time consuming. Using a
transportable scanner, a patient does not even need to be moved from the bed to obtain a head CT
examination, as long as the bed is radiolucent. However this method does not allow a scanogram to
be obtained. If a scanogram is required, the patient can be moved onto the table attached to the
gantry to be scanned using conventional table motion (Matson et al, 1999).


 Again, this versatility can be seen in ICU, where most patients requiring CT scans are critically ill.
These patients are also at a great degree of risk whilst in transit to radiology, requiring a large
number of staff for supervision and transport. The approximate cost (with regard to personnel




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needed, and others to cover ICU whilst out of the department) of transferring a patient from ICU to
a radiology department can be about $452 USD (White et al, 1999). Mobile CT reduces this cost,
meaning that staff can remain in ICU to monitor the patient, and also gives a greater sense of
security for personnel, being in a situation where all equipment is available in case of emergency.
Use of the Mobile CT scanner in ICU also ensures that the fixed CT scanner in radiology is not tied
up waiting for the ICU patient. This decreases patient waiting time in the radiology department and
prevents backlog of work.


 The cost of a transportable CT unit is roughly the same as a fixed scanner. One then needs to
determine if there is a need for a dedicated mobile CT unit in the hospital, based on the number of
CT scans needed for critically ill patients. As long as the demand for mobile CT is warranted, the
expense of mobile CT is relatively small in comparison to costs of personnel and time required
without such a modality.


 The major disadvantage of using mobile CT is its image quality. In the majority of comparative
scans, the transportable unit produced images of a poorer spatial resolution then that of the fixed
scanner. The mobile scanner also displayed a greater chance of artefact then the fixed scanner
(White et al, 1999). The main reason for this was beam hardening, especially in the posterior fossa
(Mayo-Smith et al, 1999).
         Mobile Scanner                    Fixed Scanner




Fig 2: 46yr old women 15 days after right aneurysmal clipping (Mayo-Smith et al, 1999)




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Comparison of Fig.2 above, the two images shows the difference in signal to noise ratio, and
contrast resolution. However, despite the reduction in image quality the images generated by the
mobile scanner are still diagnostic (Mayo-Smith et al, 1999).


               Mobile Scanner            Fixed Scanner




Fig. 3: 50yr old male with left frontal haemorrhagic contusion, shown by arrow (Mayo-Smith
et al, 1999)


 The use of the mobile scanner in Fig. 3 still demonstrates pathology adequately.
One can see that the fixed scanner image quality shown in Fig.’s 2 & 3 is superior to the mobile
scanner. It is not surprising to see this, considering the way mobile CT operates. The average
current produced with this particular mobile CT was 110mAs, whereas the fixed scanner used
340mAs (Mayo-Smith et al, 1999).


 White et al, 2004 concluded that the soft tissue windows of mobile CT are comparable to fixed
CT scanners, and that lung windows were inferior to fixed CT but still diagnostic.


 The time needed to scan a patient is another disadvantage to mobile CT. The mobile scanner
uses a 2 second or 4 second scanning time for each section in a head CT exam, and this time
interval can increase the possibility of a motion artefact. However reports have shown that
motion artefact has not greatly affected the diagnostic quality of the images produced (Parmar et
al, 2004). It can take on average 30 minutes to scan a patient and print hardcopies, which is
comparable to the time needed for transporting the patient to radiology, scan time, and returning
to the ward (Mayo-Smith et al,1999). Parmar et al, 2004, reported only being able to scan 6




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patients in an 8 hour shift, however this can also be contributed to the need for observing
quarantine procedures in isolation.


 Dose produced by the mobile CT is comparable to a fixed scanner, but it must be remembered
that there is nowhere near the same level of radiation protection and shielding on the ward as
there is in the department. Therefore the radiographer, and other patients can be at risk of
radiation exposure, and the limits for recommended dose for yearly public radiation exposure
must be carefully observed. Use of shielding, distance and reduction of personnel in the vicinity
should be adhered to in order to prevent overexposure of other patients and staff on the ward
(Matson et al, 1999). However, not all hospitals have the funds or the number of patients to
warrant the purchase of a CT scanner, fixed or portable. This has provided a market for a
different kind of mobile CT


 One area where mobile CT has been implemented is that of a rural/isolated community. Here,
the cost of the unit combined with the lack of necessity and/or qualified/fully trained CT
technicians may result in a fully functioning CT scanner set up not being a viable option. Having
a CT unit housed within a truck which can then be rotated between communities within a health
system within a certain time frame based on demand, is an essential tool in many isolated/rural
communities. In this design, instead of the CT scanner itself being mobile, the CT scanner is
fixed unit, except it is located in a mobile object. This allows patients who may be in need of a
CT but happen to be located in an isolated area access to this technology, instead of having to
travel what is often vast distances in order to have a simple examination performed.




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Fig 4: Basic Floor-plan of CT scanner in Truck (General Electric Company 2004)


 The introduction and development of CT has bought about a new age in medical imaging.
While the radiation dose received by patients and staff is greater than that of general x-rays, the
benefits outweigh the risks in most cases as the information that can be gathered from these
examinations contains highly detailed images and can easily incorporate large areas of the body.
Mobile CT has allowed this modality to reach more patients in a cost effective way, only helping
to improve the standards of health care in the communities with these facilities available.




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References

Bernard, JRL 1989, The Pocket Macquarie Dictionary, 2nd edn, Jacaranda Wiley Ltd, Milton,
QLD.

Butler, WE, Piaggio, CM, Constantinou, C, Nicklason, L, Gonzalez, RG, Cosgrove, GR &
Zervas, NT 2005, A Mobile CT Scanner with Intraoperative and ICU Applications,
Massachusetts General Hospital Neurosurgical Service, USA, viewed 4 May 2006,
<http://neurosurgery.mgh.harvard.edu/NeuroScience/mobileCT.htm>

General Electric Company 2004, Mobile Systems CT, General Electric Company, viewed 4 May
2006, < http://www.gehealthcare.com/rad/mobilesys/ct_index.html>.

Matson, M. B., Jarosz, J. M., Gallacher, D., Malcolm, P. N., Holemans, J. A., Leong, C., Seed, P.
T., Ayers, A. B., Rankin, S. C., 1999, Evaluation of Head Examinations Produced with a Mobile
CT Unit, The British Journal of Radiology, Vol 72, p 631-636

Mayo-Smith, W. W., Davis, L. M., Clements, N. C., Cobb, C. M., Smith, W. J., Tung, G. A.,
1999, CT of the Brain: A Comparison of Transportable and Fixed Platform Scanners, American
Journal of Roentgenology, Vol 173, p 1481-1484

Parmar, H. A., Lim, T. C. C., Goh, J. S-K., Tan, J. T., Sitoh, Y. Y., Hui, F., 2004, Providing Optimal
Radiology Service in the Severe Acute Respiratory Syndrome Outbreak: Use of Mobile CT, American
Journal of Roentgenology, Vol 182, p 57-60

Seeram, E 2001, Computed Tomography: Physical Principles, Clinical Applications, and
Quality Control, 2nd edn, Saunders, USA

Toshihiko, K, Hiroaki, T, Yuji, H & Kazufumi, S 2004, Application of mobile CT for
neurosurgical operation and sterotactic radiotherapy, International Congress Series, vol. 1259,
viewed 1 May 2006, <http://www.sciencedirect.com/science?_ob=ArticleURL&_aset=V-WA-
A-W-A-MsSAYVA-UUW-U-AAVEZCZUZE-AAVDWBZYZE-YCWCUVDWV-A-
U&_rdoc=1&_fmt=summary&_udi=B7581-4BH9CCF-
2B&_coverDate=02%2F29%2F2004&_cdi=12913&_orig=search&_st=13&_sort=d&view=c&
_acct=C000053903&_version=1&_urlVersion=0&_userid=1588505&md5=0cc7ad80b9334ee6
90be82196a4f8e55>.

White, C. S., Meyer, C. A., Wu, J., Mirvis, S. E., 1999, Portable CT: Assessing Thoracic
Disease in the Intensive Care Unit, American Journal of Roentgenology, Vol 173 p 1351-1356




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