Babeh Edi

MULTISLICE COMPUTED TOMOGRAPHY

MULTISLICE COMPUTED TOMOGRAPHY

Abstract

Computed Tomography (CT) involves the data collection, image reconstruction and image display (image manipulation, storage, recording and communication) of cross-sectional anatomy. (Seeram 2001). Multislice CT (MSCT) scanners are based on the third generation CT system design. The x-ray tube and detectors are coupled and rotate continuously during patient movement through the gantry. Slice thickness is determined by the beam width and the number of rows of detectors. The major difference in the data acquisition geometry of single slice and MSCT scanners is that MSCT scanners utilise a multiple detector array connected to an x-ray tube. The x-ray beam in MSCT is collimated to the entire detector array, which is achieved by utilising fan beam geometry (Seeram 2001). Slice thickness is determined by the beam width and the number of rows of detectors.

Advantages of MSCT Scanners

• Decreased scan time
• Increased image detail
• Identification of small fractures
• Decreased contrast required
• Efficient use of x-ray beam
• Improved accuracy in needle placement in CT fluoroscopy

Disadvantages of MSCT Scanners

• Increased capital cost
• Increased data storage requirements (soft copy)

·        Increased cost: tube

• Increased radiological services
• Potential for higher radiation dose

Improved Applications for MSCT examinations

·        Evaluation of the respiratory system

·        Cardiac procedures

• Brain investigations
• Vascular System integrity

·        Paediatrics

·        Trauma

·        Virtual Endoscopy

• Virtual Labyrinthoscopy

Current development in MSCT indicates that 256 slice systems will be used in the near future (Mather 2006). Subsequent advantages will enable the ability to cover 128mm of anatomy in 0.5 mm slices allowing structures like the heart and liver to be imaged in one gantry rotation without table movement (Mather 2006).

Key Words: multislice, computed tomography, 64 slice.

INTRODUCTION

Computed Tomography (CT) involves the data collection, image reconstruction and image display (image manipulation, storage, recording and communication) of cross-sectional anatomy. (Seeram 2001).

A broad summary of CT operations are as follows: the patient is positioned within the scanner. The x-ray tube and detectors are internal to the scanner gantry and rotate around the patient during scanning. X-rays are attenuated as they pass through the patient and subsequently measured by the detectors. The detectors convert the x-ray photons into electrical signals; these are then converted into digital data for input to the computer. Image reconstruction is performed and then converted back into electrical signals for viewing on a monitor. Figure 1, below, illustrates the image acquisition and processing of CT. The final images can be stored on magnetic tapes, optical disks and/or printed on x-ray film (Seeram 2001).

Figure 1: CT Imaging Process

(Adapted from Figure 1.1 Seeram 2001)

Multislice CT (MSCT) is the most recent development in CT technology. This paper will discuss the history, mechanics, advantages, disadvantages, applications and future of MSCT.

COMPUTED TOMOGRAPHY

History of CT Scanning

In 1967, Godfrey Newbold Hounsfield used the image reconstruction techniques to create the first CT scanner for clinical imaging of the brain (Seeram 2001). CT technology has since developed greatly as summarised below:

First Generation CT Scanners

The first generation of CT scanners used parallel beam geometry and a translate-rotate scanning motion; the attenuated x-ray beams being received by a single detector (See Figure 2: Image A) (Seeram 2001).

Second Generation CT Scanners

The second generation of CT scanners use fan beam geometry and the aforementioned translate-rotate scanning motion; the attenuated x-ray beams being received by a linear detector array of about 30 detectors. (Figure 2: Image B) (Seeram 2001).

Third Generation CT Scanners

The third generation of CT scanners use fan beam geometry and a complete, 360 degree, rotation of both the x-ray tube and the detectors. (Figure 2: Image C) (Seeram 2001).

Fourth Generation CT Scanners

The fourth generation of CT scanners use fan beam geometry and a complete, 360 degree, rotation of the x-ray tube around a stationary ring of detectors. (Figure 2: Image D) (Seeram 2001).

Fifth Generation CT Scanners

The fifth generation of CT scanners are specialised adaptations providing primarily high-speed scanning (Seeram 2001).

FIGURE 2: First to Fourth Generation CT Scanners

(As per Figure 1-12, Seeram 2001)

Conventional Scanners and Spiral/Helical/Volume CT Scanners

Single slice scanning, or conventional CT, is literally the data acquisition taken one slice at a time. The patient remains stationary whilst the x-ray tube and detectors rotate, 360 degrees or less, during scanning.

In 1989, Spiral CT Scanning, also known as Helical CT scanning or Volume CT Scanning, was introduced (Volume Scanning will be used hereafter to describe the process). Volume Scanning is achieved through the continuous movement of the patient through the scanner gantry whilst the x-ray tube and detectors rotate, through 360 degrees, continuously, Figure 3 (Seeram 2001).

Figure 3: Conventional Scanning Detector Array

(Adapted from Fig. 5-27 Seeram 2001)

Spiral/helical CT scanners were developed using slip ring technology removing the linkage between the power cable and the x-ray tube. Spiral scanning has many advantages over conventional CT scanners.

• Shorter scanning time.
• Thinner slice thickness; reduced by up to 1mm, thus smaller identifiable lesions.
• Axial images can be reconstructed into coronal, sagittal and oblique planes.

Figure 4 below demonstrates the difference between conventional and volume scanners.

Figure 4: Comparison of Conventional vs Volume Scanners.

(Adapted from Fig 13-10 Seeram 2001

Single-slice Volume CT Scanner and Multislice CT Scanners

Following the development of the single-slice volume CT scanner described above, in 1992 a dual-slice volume CT scanner was developed.

In 1998, scanners using multidetector technology were introduced enabling the scanning of more than two slices per gantry rotation – these were called Multislice CT scanners, see Figure 5 below (Seeram 2001).

HISTORY OF THE MSCT SCANNER

MSCT is the term used to describe a CT scanner’s ability to scan multiple slices simultaneously, utilising more than one row of detector elements (Lewis 2001). In 1998 the first four slice scanners were introduced. Rapid development followed with more advanced systems; 16 slice scanners in 2001, followed by 32 and 40 slice scanners and the development of 64 slice scanners in 2003 (Nagel 2004).With the advances in increased number of slices, rotation speed was also improved, dropping from 1 second to approximately 0.375 seconds per rotation (Nagel 2004).

The advantages of MSCT can be summed up by the acronym RSVP.

• Resolution: spatial resolution improved along the z-axis
• Speed: scanning time reduced for particular body regions
• Volume: increased volume able to be scanned
• Power: improved power usage (X-ray tube) (Nagel 2004)

While the early dual slice scanners were only capable of improving one of the aspects (R, S, V or P) the newer scanners can improve on all the aspects (Nagel 2004).

The main goal of MSCT is to improve volume coverage speed performance without image quality degradation (Lewis 2001).

MECHANICS OF MULTISLICE CT

Componentry

MSCT scanners are based on the Third Generation CT system design. The gantry houses the x-ray generator, the x-ray tube, and detectors, as well as the detector electronics (Seeram 2001). The x-ray tube and detectors are coupled and rotate continuously during patient movement through the gantry. This method of data acquisition is possible by the use of slip ring technology (Seeram 2001).

Data Acquisition in Multislice CT Scanners

Data acquisition is one of the major influential factors of image quality in CT, as is image reconstruction (Seeram 2001). The major difference in data acquisition geometry of single slice and MSCT scanners is that MSCT scanners utilise a multiple detector array connected to an x-ray tube. The diagram below demonstrates a four slice scanner – depicted by an array of four rows of adjacent detectors.

Figure 5: Data Acquisition Geometry for Multislice CT

(Adapted from Fig 15-6 Seeram 2001)

Collimation and Slice Thickness

In MSCT, the x-ray beam is collimated to the entire detector array. Slice thickness is determined by the beam width and the number of rows of detectors. For example, a 16 row detector array with a precollimator width of 32mm will produce 16 slices each of 2mm thickness (Seeram 2001). The slice thickness is determined by the pre and post patient collimation demonstrated below in Figure 6. Slice thickness in image reconstruction can be different to that of data acquisition, that is, thicker slices can be generated from thin slice acquisition however there are subsequent effects on image quality. Once a slice thickness has been determined, thinner slices cannot be reconstructed at a later stage (Nagel 2004).

Figure 6: Pre and Post Patient Collimation

 (Adapted from Fig 2 Nagel 2004).

A disadvantage of increasing beam width is increased scatter radiation due a larger area of the patient being scanned. Anti scatter collimation can be utilised in the post patient collimation (Seeram 2001).

Beam Geometry

As previously stated the beam covers the width of the detector array, in addition it must also cover the length of the detector array. This is achieved by utilising fan beam geometry (Seeram 2001) as illustrated in Figure 7. A wider angled beam covers a longer array of detectors, and a greater number of rows in the array results in a increase in beam width. As a result cone beam geometry is used as there is more beam divergence along the z axis.

Figure 7: Fan Beam Geometry and Cone Beam Geometry

 (Adapted from Fig 15-9 Seeram 2001)

Pitch

The definition of pitch in MSCT scanners is varied however in general it is based on the “table speed per rotation and either slice thickness, detector row collimation of the beam width at the centre of rotation” (Seeram  2001 p253). Pitch affects the density, as pitch values of <1 produce greater density, additionally, pitch values <1 reduce the amount of spiral artefacts at the expense of reduced volume coverage per unit time (Nagel 2004).

Image Reconstruction

Key advantages of MSCT with regard to image reconstruction are the abilty to retrospectively create thinner or thicker sections from the same raw data and improved production of three-dimensional images. The single slice systems were unable to accurately match the transverse and axial data – that is unable to create an isotropic three dimensional voxel as can be achieved by MSCT. The result of MSCT is an increase in imaging possibilities - post data acquisition, with enhanced image quality due to reducing slice thicknesses that have become possible without negative impact on the speed of examinations (Kopp et al 2000).

ADVANTAGES AND DISADVANTAGES OF MSCT SCANNERS

Advantages of MSCT Scanners

Decreased scan time. MSCT scanners have built in faster image acquisition due to multiple active rows of detectors and an increased tube rotation speed (Lewis 2001). These factors combine to provide faster coverage of a larger volume of tissue in a shorter period of time. The increased speed is advantageous for non-cooperative, breathless or trauma patients. Large volume coverage reduces the chance of artefacts due to patient motion; see Appendix A: Wagga Wagga Base Hospital Interview, for scan time comparison. A direct benefit of decreased scan times is an increase in patient throughput.

Increased image detail. MSCT scanners provide more detailed images in less time. Due to the use of thin slices, MSCT has better isotropic resolution. Axial, vertical and horizontal sides of the voxels in the slice have equal dimensions compared with single slice. This advantage is applicable to multiplanar reconstructions and three-dimensional images with reduced artefacts. (Seeram 2001)

Identification of small fractures. Decreased slice thickness enables smaller pathologies to be identified (Rowarth 2006).

Decreased contrast required. Less contrast can be used and administered at a faster rate. As an example of reduced contrast use in the latest 64-slice as versus the previous 16-slice scanner see Appendix A: GE Interview.

Efficient use of x-ray beam. Using fan beam geometry, the x-ray beam width is opened to fall on a two dimensional array. The entire beam is used to acquire the slice number determined by the operator per 360 degree rotation without wasting any area of the x-ray beam unlike the single slice scanner, 360degree rotation, where part of the x-ray beam is wasted (Seeram 2001).

Lewis (2001) stated that MSCT has improved the utilization efficiency of the x-ray beam in the z-direction; the same tube loading results in up to four times the volume coverage, allowing increased volumes to be scanned without tube cooling restrictions.

Improved accuracy in needle placement in CT fluoroscopy. The multiple images obtained using MSCT fluoroscopy are able to provide a better understanding of needle position compared to the single CT scanner (Seeram 2001).

Disadvantages of MSCT Scanners

Increased cost: capital. The greater the number of slices and imaging reconstruction capabilities the greater the cost of the MSCT hardware and software (see below for data storage and tube cost information).

Increased cost: data storage. MSCT scans generate an alarming number of images per study. A scan of the chest/abdomen produces 500 – 600 images. Such an enormous data acquisition needs a compatible workstation providing increased processing power with optimised software packaging and a high speed network for image transfer from CT system to the work station. Large storage facilities are required to accommodate the data (Indrajit, Shreeram, & D’souza 2004).

Increased cost: tube. MSCT scanners require high heat capacity x-ray tubes, generators, ceramic detectors, on board computers - all utilise advanced technology and are inherently expensive. MSCT x-ray tubes operate at 7MHU in contrast with conventional CT tubes that operate at 1-2 MHU - increasing the cost of hardware (Indrajit, Shreeram, & D’souza 2004).

Increased cost: radiological services. More time is required for data analysis due to the increased number of images produced - radiological workload is increased and subsequently an increase in examination expenses.

Potential for higher radiation dose. Relatively high doses of radiation are used in MSCT images. For example a chest x-ray delivers an effective dose of 0.02mSv, equivalent to about three days exposure to natural sources of radiation in the environment. CT scans of the abdomen/pelvis deliver about 10mSv, the equivalent of 500 chest x-rays, or 4.5 years of background radiation exposure (Allan & Williams 2004). Overall, one study described CT as comprising approximately 11% of all imaging examinations however, contributing to 67% of collective dose (Golding 2005).

The individual patient doses have increased as:

• volume scanning has replaced discrete slice by slice scanning
• thinner slices and overlapped sections are used
• there is a tendency for liberal inclusion of wider areas of the body
• demand for shorter scanning times and higher resolution CT
• increased demand for better quality images results in higher exposure factors and the frequent repeat studies that are done on the same patient eg. non-contrast then contrast (Rehani 2000).

Golding (2005) states that “multislice CT (MSCT) has become radiology’s major radiation protection challenge”. Readings state contributions of CT to examinations range from 2.5% to 3%, but radiation doses from CT contribute to as much as 40% collective radiation dose arising (Yates, Pike & Goldstone 2004).

Yates, Pike & Goldstone (2004) state that to avoid the problem of beam penumbra it is necessary to irradiate more of the patient than is actually imaged to ensure uniform intensity across the detectors, as demonstrated in the diagram below:

Figure 8: Beam Collimation in Single-slice and Multislice CT

(Adapted from Fig 1. Yates, Pike & Goldstone 2004)

A study was conducted by Yates, Pike & Goldstone (2004) into the effect of MSCT scanners on patient dose in East Anglia. The results showed that since the use of MSCT (2002) mean effective doses were 34% higher than that in 1999 with the use of single-slice CT. Relative effective doses were calculated for all scanners, for each examination category. The average relative effective dose for all single-slice scanners was reported at 0.85±0.10. The average effective dose for all MSCT scanners was reported at 1.15±0.06, on average 35% greater. Yates, Pike & Goldstone (2004) does state that this increase was not uniform across all examinations. It was identified that examinations that utilised narrow slices had the greatest effective dose and is explained by the beam collimation effect in MSCT.

Lewis (2001) states that at the time of writing, MSCT produced an increase in dose by approximately 10% than that of single-slice CT, however for 1mm slices the dose is approximately 40% higher. It is believed that MSCT will lead to a decrease in patient dose due to a reduction in repeat examinations caused by patient movement due to shorter examination times, or incorrectly timed contrast studies (Lewis 2001).

APPLICATIONS OF MULTISLICE CT SCANNERS

Respiratory System

Multislice CT scanners are used to evaluate lungs, peripheral airways and parenchyma. Virtual Bronchoscopy depicts airways from the endoluminal perspective, by using shaded surface display or volume rendering with increased opacity settings (Indrajit, Shreeram, & D’souza 2004). MSCT is an effective technique to evaluate lung disease such as emphysema, bronchiectasis, central airways and lung cancer. It is minimally invasive, well- tolerated, safe procedure for the evaluation of the endolunimal and tracheobronchial tree (Yates, Pike & Goldstone 2004).

Cardiac

MSCT is used to detect blood clots in the pulmonary arteries, locate aneurysms in blood vessels due to accurate delineation of narrow vessels which is achieved by the improved z axis resolution (Indrajit, Shreeram, & D’souza 2004). MSCT is also used to measure calcification in the coronary arteries. Cardiac calcification scoring,  a technique that utilises numerical quantification of calcium in coronary arteries,  provides an indicator of cardiac blood vessel blockage or coronary heart disease, facilitated by MSCT. MSCT has been also utilised for Coronary angiography and as an assessment for cardiac output (Indrajit, Shreeram, & D’souza 2004

Brain

Imaging of the brain provides a quick method for assessing perfusion disturbances in acute strokes. Images are a reliable identification of stroke and identify the vascular origin of the ischemic insult. MSCT enables mapping the extent of the stroke size and haemorrhagic risk (Indrajit, Shreeram, & D’souza 2004).

Vascular System

Multislice CT is effectively used to detect blood clots in the pulmonary arteries, locate aneurysms in blood vessels and can be used to measure calcification in the coronary arteries (early sign of cardiac blood vessel blockage). MSCT is now also used for angiograms; no sedation, less time required for scan, less contrast media is used, resulting in a reduction in examination expensives (Indrajit, Shreeram, & D’souza 2004).

Paediatric

The medical imaging of paediatric patients has always struggled with the inability to keep children immobile for the duration of the imaging process to avoid patient motion artefacts. The reduction of scanning times using MCST means that there is a greater chance of data acquisition before the child moves. Ms. Roughy (GE 2006) indicated that this provides the advantage of the reduced need for sedation of the paediatric patient.

Trauma

High image quality and fast scanning capabilities are significantly beneficial in diagnosing trauma patients. The MSCT scanner provides more precise images of bones, organs and internal bleeding which can be life saving for trauma patients. Furthermore high quality images allow accurate diagnosis therefore potentially providing a better outcome for those with chest pain or stroke, or acutely ill patients (McLeod Health 2006).

Virtual Endoscopy

Colonography is another application of volume imaging of MSCT, generating three dimensional images of the entire colon, increasing the speed, and ease of locating and analysing colonic polyps and cancer (Indrajit, Shreeram, & D’souza 2004)

Virtual Labyrinthoscopy

MSCT allows detailed visualisation of the tiny and complex structures of the inner ear, mastoid and labyrinthine segments of the facial canal by using high quality three dimensional reconstructions (Indrajit, Shreeram, & D’souza 2004).

THE FUTURE OF MSCT

Current developments in MSCT indicate that 256 slice systems will be used in the near future (Mather 2006). The advantage of a 256 slice system is its ability to cover 128mm of anatomy in 0.5 mm slices allowing structures such as the heart and liver to be imaged in one gantry rotation without table movement. This will mean that dynamic scans of these structures will be easily obtained. Currently phasic scans or volume scans can be performed but not at the same time. 256 slice scanners will make this possible (Mather 2006).

Figure 9 illustrates the advances in MSCT in the imaging of a heart. The 256 MSCT is able to scan the heart in one rotation whereas the 4, 16 and 64 slice scanners require multiple rotations.

Figure 9: Image Using 256 Slice MCST

(Mather 2006 p5)

CONCLUSION

 MSCT is the latest breakthrough in CT technology. It provides:

• an increase in spatial resolution along the z-axis
• increased image detail
• more efficient use of the x-ray tube 
• shorter scanning times which have greatly enhanced the diagnostic imaging capabilities

The advantages of shorter scanning times and faster data acquisition can be highlighted through the application of MSCT in cardiac, vascular and musculoskeletal imaging as well as the use in paediatric and emergency wards.

These advantages of MSCT have associated disadvantages, such as:

• increased costs of CT hardware
• increased costs of CT examinations
• an enormous increase of data collection
• increased radiation dose to the patient

This paper highlights some of the ways in which MSCT has dramatically changed the capabilities and applications of computed tomography in medical imaging.

REFERENCES

Allan, PL & Williams JR 2004, Full Body CT Scans: Are they worth the cost in money and radiation exposure? Copyright 2004, Royal College of Physicians of Edinburgh, viewed 16 Mar 06, <http://www.behindthemedicalheadlines.com/articles/ct_scans.shtml>

Golding, SJ 2005, Multislice Computed Tomography (MSCT): The dose challenge of the new revolution, Radiation Protection Dosimetry, vol 114, Nos 1-3, pp303-307.

Indrajit, K, Shreeram, MN & D’souza, JD, Multislice CT: A Quantum Leap in whole body imaging, electronic version, Indian Journal of Radiological Imaging 2004, vol 14 no.2 pp209-216, viewed 2 May 2006, <http://www.ijri.org/articles/ARCHIEVES/ 2004-14-2/netech_physics209.html>

Kopp, AR, Klingenbeck-Regan, Heuschmid, M, Kuttner, A, Ohnesorge, B, Flohr, T, Schaller, S, & Claussen, C.D 2000, Multislice Computed Tomography: Basic Principles and Clinical Applications, electronic version vol 68, no.2, viewed 11 May 2006, <http://healthcare.siemens.com/medroot/en/news/electro/issues/pdf/heft_2_00/english/05KOPPEN.PDF>  

Lewis, MA 2001, Multislice CT: opportunities and challenges, electronic version,

British Journal of Radiology, vol. 74, pp.779-781, viewed 20 March 2006,

<http://bjr.birjournals.org/cgi/content/full/74/885/779>

Mather, R 2006, The next revolution: 256-slice CT, updated February 2006, Toshiba

Medical systems, viewed 20 March 2006, <www.medical.toshiba.com>

McLeod Health 2006, McLeod Health Brings One of the Most Advanced Multislice CT Systems to the Pee Dee, updated 20 January 2006, electronic version

<http://www.mcleodhealth.org/Resources/news.cfm?newsID=247>

Nagel, HD 2004, Multislice CT Technology, version 2, updated 01 June 2004, electronic version, www.multislice-ct.com, viewed 16 March 2006, <http://www.multislicect.com/www/media/introduction/msct_technology_2004_06_01_v02.pdf>

Rehani, M M. CT: Caution on Radiation Dose, electronic version, Indian Journal of Radiology and Imaging 2000, vol 10 no 1, p19-20, viewed 22 March 2006, <http://www.ijri.org/articles/archives/20001001/physics_ct.html>

Seeram, E 2001, Computed Tomography: Physical Principles, Clinical Applications, and Quality Control, 2^nd edn, Saunders, Philadelphia, pp248-252.

Yates, S.J, Pike, L.C, & Goldstone, K.E 2004, Effect of multislice scanners on patient dose from routine CT examinations in East Anglia, The British Journal of Radiology, vol 77, pp 472-478.

LIST OF FIGURES AND TABLES

Figure 1            CT Imaging Process                                                     Page     3

Figure 2            First to Fourth Generation CT Scanners                                    4

Figure 3            Conventional Scanning Detector Array                          5

Figure 4            Comparison of Conventional vs Helical Scanners                       5

Figure 5            Data Acquisition Geometry for Multislice CT                 7

Figure 6            Pre and Post Patient Collimation                                               8

Figure 7            Fan Beam Geometry and Cone Beam Geometry                       8

Figure 8            Beam Collimation in Single Slice and Multislice CT                    11

Figure 9            Image Using 256-slice MCST                                       13

APPENDIX A

Interview conducted with Evan Rowarth, Radiographer, Wagga Base Hospital, 21 Mar 06.

Why did the department upgrade from the previous scanner to a 64 slice CT scanner – why not 16 or 32?

It is common practice in the Government sector to upgrade scanners every 10 years – therefore most hospitals will chose to upgrade to the latest, most technologically advanced, equipment, in this case the 64-slice scanner. The hospital’s main employment of the CT scanner is abdominal aortic aneurysm (AAA) ruptures and trauma, thus minimal scanning time is required. The 64-slice scanner can scan the entire body in less than 10 seconds. The hospital purchased a General Electric (GE) VCT (Volumetric CT) Lightspeed 64-slice scanner with GE Advantage Workstation.

The previous scanner was a GE Prospeed single-slice helical scanner.

What were the priorities when researching what type of scanner was to be purchased?

            Slice thickness, table weight limitations, bore size, staff familiarity with the scanner and software and flexibility of software packages. Toshiba was the scanner of choice with the following specifications – primarily chosen because of current staff members’ familiarity with the equipment:

·        Slice thickness 0.5mm

·        80cm bore

·        increased table weight

·        flexibility with algorithms

Although the Toshiba was preferred the GE product was purchased by the hospital management.

How much did the cost of the scanner affect what type of scanner was chosen?

            The actual cost of the scanner was unknown – however the GE 64-slice scanner that was purchased had similar costs to the Toshiba 64-slice scanner.  

In what areas has the new scanner been most beneficial, ie. In which examinations, if any?

Orthopaedic – identification of small fractures that were not identified on the previous scanner in particular fractures of the cervical spine.

            AAA – decreased speed.

            CT Angiograms            – in particular Circle of Willis examinations.

                                                -  leg run offs – now able to identify occlusions.

Scanning pelvis, femur with hip replacements – better algorithms eliminate scatter, therefore less noise on the image.

The higher mA possible, 800mA, is also an advantage for larger patients.

Additional benefits are:

• The old scanner was unable to reconstruct images in the coronal or sagittal planes. The new scanner can reconstruct in all planes as well as provide three dimensional images.
• The software is able to deduce appropriate mA changes with regard to patient body size thus assisting in the reduction of patient dose.
• The table is able to carry greater weight (227kg)
• The injector has an automatic cut-off switch “if the tube comes out of its stopper”.

What effects, if any, has the new scanner had on contrast examinations (due to increased speed)?

            Increase the amount of contrast and use a faster rate eg. for normal adult abdomen examinations now use 75ml of contrast instead of 60ml at 2.5ml/s rather than the previous 2ml/s. This has been found to improve the image quality.

For CT of pulmonary arteries  (CTPA) – the operator watches the pulmonary artery and when the contrast is seen the operator can trigger the scan to start. Other scanners are able to self trigger once a locater has been set up.

What effect has the new scanner had on time taken for examinations, thus, throughput?

            Previously the department would normally complete 10 patients per day, now the average is 20 patients per day. A comparison of scan times is detailed below:

Examination	Old scanner	New scanner
Abdomen	10mm slices 30sec -1 min scan time 15-20 mins exam time	1.25mm slice 5 sec scan time 5mins exam time
Brain	5 and10mm slices 30 sec scan time 5-10mins exam time	0.625mm slices 13sec scan time <5min exam time
Knee	5 mins scan time 1 hr reconstruction time	3 sec scan time 5 mins reconstruction time

What effect has the new scanner had on the training required by staff for operation?

            Two weeks of training was conducted by GE for 2 radiographers. Then the department has conducted internal training for other staff. GE provides a 24 hour technical support network, however there is no support on the weekends.

Was there any limitation by the room size on the type of scanner selected?

            Although no serious modifications were required to the room, the new gantry sits at a different angle to the previous scanner due to the increased table length in order to fit in the room. There is now only single side access to the table. The air conditioning system required modifications as there is more heat produced from the new scanner. Larger electrical cables were required due to the increase in power required. More cables were passed through the roof and the basement to the room.

            In addition there was a three day transition time for the new scanner. 

Have there been any effects on storage requirements, demands by the new scanner?

            Storage is currently on a mini Picture Archiving and Communication system (PACs) which has a capability of 6-7 terabytes. Data is required to be archived for 7 years.

            Axial images are still printed, so until the hospital implements a PACs hardcopy images are still an issue with the requirement for storage space.

Is the scanner used to its potential ie. Do you make full use of the capabilities available through the software packages supplied? (or are you pretty much providing the same output that you did with the previous scanner?)

            The scanner is able to do cardiac examinations, however due to there being no cardiologist at the hospital these examinations are not conducted. The scanner is also able to do CT colonography, however the department did not purchase the applicable software package and at this time is unable to conduct such examinations.

What do you consider as the future for the scanner’s involvement within the department/hospital?

            The CT scanner may replace plain film examinations for major trauma accidents.

Interview conducted with Ms. Bronwyn Roughy, GE CT Applications Specialist, Wollongong Base Hospital, 30 Mar 06.

The Wollongong Hospital was in the process of upgrading from a 16-slice CT scanner to a 64-slice CT scanner during March/April 2006.

Advantages of 64-slice CT Scanner?

Coverage:

The 64-slice has a 40mmm detector at 0.625mm slices

The 16-slice had a 10mm detector at 0.625mm slices

This means that the 64-slice scanner has four times faster scanning time with the scanning operating at sub millimetre slices for the whole scan.

Whole organ scanning:

Whole organs are able to be scanned in less than 5 seconds.

For example:

• heart – able to be imaged in a single heartbeat therefore no motion artefact
• Circle of Willis is able to be imaged in one rotation – able to provide a pure arterial phase etc
• Liver – able to image each phase separately (arterial, venous etc)

‘Triple Rule Out’ examination – at present a patient with chest pain could have

• myocardial infarction (MI); or
• Pulmonary embolism (PE); or
• Thoracic dissection. The 64-slice is able to rule out all three alternative pathologies in one scan.

Less Contrast

Less contrast can be used and administered at a faster rate

For example:

For PE examination, the contrast volume has decreased from 90ml to 50ml. To explain the reduction in contrast consider that at 4 mls/sec, 90mls will take 18secs to administer

With the 16-slice – inject at 10secs, image for 10secs

With the 64-slice – inject at 10secs, image for 2.5secs therefore there is 5.5secs of unneeded contrast

Compact Machinery

The data acquisition system (DAS) has been miniaturised, is now an eighth of its original size – this provides better signal-to-noise ratio as there is less image manipulation between various interfaces.

Paediatric Examinations

The quicker scanning time means less sedation required to reduce patient motion artefact

Tube Load

The tube is able to go up to 800mA (the 16-slice tube was capable of a maximum of 440mA). Able to throughput more examinations due to less tube cooling time required.

Table Load

The 16-slice scanner bed was able to carry 180kg load – the 64-slice bed is able to carry 227kg load.