:: rsna 2007
Introduction
Maria Lewis of ImPACT attended RSNA 2007 on behalf of the Centre for
Evidence-based Purchasing. You may have seen her photo in RadMag (Jan 2008).
A full report - covering all modalities - is now available on the NHS PASA website: Click here.
Maria's notes focusing on Computed Tomography scanners at RSNA 2007 are below.
Overview
RSNA 2007 proved to be another exciting year for CT, with Toshiba surprising everyone with the launch of their 320-slice scanner, when 256 slices had been expected. Paradoxically, they have named the scanner the Aquilion One. The scanner allows the imaging of 16 cm length in a single 0.35 sec rotation, thereby enabling ‘dynamic volume scanning’ or functional imaging. Currently the main applications of functional imaging are in cardiac studies and organ perfusion studies, particularly brain perfusion. Toshiba currently have five Aquilion Ones installed world-wide, with a few more due to be installed early in the New Year. Commercial shipment of the systems is due to start in summer 2008.
Philips are following Toshiba in the move towards increased volume imaging. They unveiled their Brilliance iCT scanner which will be available in 2009. It has a coverage of 8 cm per rotation at a minimum rotation time of 0.27 seconds. They have adopted the ‘z-sharp’ technology pioneered by Siemens which improves the resolution along the scan axis through double sampling of the detectors with a ‘flying focal spot’. In this way it acquires 256 channels from 128 detector banks. The scanner also features a new x-ray tube with some novel design features for improved focal spot stability as well as a new data acquisition system, TACH2, with lower electronic noise and a double layered detector for dual energy scanning. There is a choice of two generators, the standard 60 kW and a 120 kW option, the largest available on the market.
Siemens have expanded their Somatom Definition range, which originally featured their dual x-ray source system. They have now added two new scanners, the Definition AS and the Definition AS+, both of which are single source systems. The AS has other features similar to the dual source scanner, however, on the AS+ the axial detector bank coverage has been increased to 38.4 mm, with 64 x 0.6 mm detectors. It also has the added advantage of a minimum rotation time of only 0.3 seconds. All the Definition systems feature the z-sharp ‘flying focal spot’ technology, so the AS+ acquires 128 data channels per rotation.
GE were promoting their LightSpeed VCT XT system at RSNA. This has many features similar to the original VCT, but has the capability of prospective ECG-gated cardiac scanning, referred to as ‘Snapshot Plus’. It also enables the extension of organ coverage in perfusion studies from 4 cm to 8 cm, through the use of their ‘volume shuttle’ technology. A big feature on the GE stand was the ‘works in progress’ on their HDCT (High Definition CT) scanner. The scanner will feature a standard 64 channel detector bank, but with a new detector material, GE Gemstone, made from a garnet-based scintillator. The detector allows for faster data acquisition rates and has improved after-glow characteristics. The system will have a new x-ray tube with a range of five focal spot sizes to suit different applications. On the HDCT GE also plan to move from filtered back projection reconstruction methods traditionally used in CT to iterative reconstruction methods which up till now have been computationally too demanding to be practical, but are more accurate and result in fewer artefacts.
The following paragraphs discuss the three new developments in Computed Tomography that are likely to have the biggest impact on improvements in patient care.
Top three in CT
Stroke assessment studies, or “Time is brain…”
Stroke is the third leading cause of death in the industrialised world, but as the title of this section suggests, time is of the essence in the diagnosis and treatment of patients with suspected stroke. The administration of thrombolytic drugs, such as tissue plasminogen reactivator (tPA) can renew blood flow to areas affected by ischemia if administered within a suitable time window. The current NICE guidelines state that this window is three hours from the onset of stroke symptoms.
The first important factor in the diagnosis of suspected stroke is the ruling out of haemorrhage, because for obvious reasons thrombolytic drugs are contra-indicated in this situation. The second stage of patient assessment, if the stroke is found to be due to a thrombus, is to establish whether the tissue in the ischæmic region is still viable. In the presence of viable tissue, often referred to as ‘penumbra’, there will be benefit gained from renewing the blood flow by the injection of tPA. Patients with non viable tissue should be excluded from this treatment as haemorrhage occurs in approximately 15% of cases. The presence of penumbra can be assessed with brain perfusion imaging by injecting iodinated contrast and scanning the same volume repeatedly over a period of time. From this data blood volume, blood flow and mean transit time maps are obtained and the presence of ischæmia and tissue viability can be assessed.
The increased coverage on the newest scanners enables the perfusion of a greater brain volume to be assessed. Toshiba can cover an entire brain in a single 16 cm rotation on the Aquilion One scanner. On the LightSpeed VCT XT, GE can cover a length of 8 cm of brain using their ‘volume shuttle’ technology, where the table move repeatedly between 2 successive table positions 4 cm apart, with a temporal resolution of 3 seconds. In a similar manner, Philips can achieve an 8 cm coverage in ‘jog scan’ mode on their on their Brilliance 64 scanner. On their iCT 8 cm coverage will be achievable with a single rotation. Siemens have taken the approach of using a ‘helical shuttle scan’ where the scanner performs repeated short helical scans in opposite directions over the volume of interest. On the AS+, a length of 10 cm of brain can be covered whilst still achieving an adequate temporal resolution.
On most multi-slice scanners, a full stroke assessment is performed as three examinations. The first study is an un-enhanced brain scan to determine if the stroke is due to thrombus or haemorrhage, and also to locate its position in the brain. This is followed by a contrast-enhanced angiography scan that provides additional information on the vasculature. Finally, the perfusion brain scan determines the presence of viable tissue. The advantage of the Aquilion One scanner in stroke assessment is that this 3 stage procedure can be performed in a single examination, thereby saving valuable time. Additional information may also be obtained by studying the venous and arterial flow of contrast through the vessels. The capabilities of dynamic volume scanning open up many possibilities for functional imaging in a diagnostic imaging area that was previously limited mainly to anatomical information.
Dual energy scanning, or “Escaping the Hounsfield cage…”
One of the fundamental principles of CT is that materials or tissues are characterised by their CT, or Hounsfield, number. The CT number is proportional to the total linear attenuation coefficient of the material in the path of the x-ray beam. At the x-ray energies used in CT, the total linear attenuation coefficient is a sum of the photoelectric and Compton attenuation coefficients. Elements such as calcium and iodine have similar CT numbers, and are often difficult to differentiate. This can be a particular problem when performing automatic bone removal or differentiating between calcified plaque and contrast material in angiographic studies. By performing scans at two different x-ray tube potentials, such as 80 kVp and 140 kVp, differences in materials with similar CT numbers but different atomic numbers (Z) can be enhanced.
The technique of dual energy scanning is now being promoted by three of the four main CT manufacturers, although each uses a different approach. Siemens achieve dual energy scanning on their dual source system by operating the two x-ray tubes at different kilovoltage potentials. The information is acquired almost simultaneously, at a 90 phase difference. GE are currently proposing a technique where half scan information is acquired alternately at the two different tube potentials. In the future, on the HDCT scanner, they plan to switch the tube potential at intervals of 0.8 milliseconds throughout a rotation. Philips have approached the problem from a different angle. They use a single kVp, but have developed a dual layer detector, where the top layer is optimised to detect lower energy x-rays and the bottom layer, the high energy ones. With this method the information is acquired simultaneously, but with a reduced energy discrimination compared to the other methods.
At present this dual energy technology is being promoted for improved automatic bone removal algorithms and differentiation of calcified plaque and contrast media. This will improve quality, and speed up the time of diagnosis in angiographic studies. Moreover, the technique has further potential in any area where improved tissue differentiation would be of benefit, such as improved discrimination between tumours and cysts.
Dose reduction methods, or “Less is more…”
The diagnostic potential of CT scanning is undisputed, however, there is an ever increasing concern over the radiation doses delivered from CT examinations. As more scanners become available and new diagnostic areas open up to CT, more patients are being referred for CT examinations. In addition the new scanners have potential to scan longer lengths of patients and at higher tube currents. There have been a number of peer-reviewed papers published in recent years highlighting the radiation risks of CT scanning. The most recent of these, by David Brenner and Eric Hall, was published in the New England Journal of Medicine during this year’s RSNA and lead to a front page article in the popular American daily newspaper, USA Today, entitled ‘Unnecessary CT scans exposing patients to excessive radiation’. Although the findings of the study are contentious, both legally and ethically radiation doses must be kept ‘as low as reasonably achievable’.
One of the highest dose techniques in CT scanning has been that of coronary CT angiography (CCTA). This technique is increasingly used in place of conventional coronary angiography due to its non-invasive nature, although, because of the high associated doses, there has been some caution in its adoption. Data from these studies is reconstructed using retrospective ECG-gating, which requires scanning at low pitches giving rise to high patient doses. Typically effective doses from this procedure are in the order of 10 - 20 mSv. More recently, methods for performing CCTA with prospective ECG-gating have become available. This technique is said to reduce doses from CCTA examinations by around 80%, resulting in effective dose values of 2 - 4 mSv. GE and Philips propose performing prospectively gated CCTA studies with axial scans. On Toshiba’s Aquilion One a CCTA exam can theoretically be performed in a single prospectively gated rotation, which is claimed to reduce doses even further.
Another dose reduction technique launched at this year’s RSNA was that of the ‘adaptive collimation’ concept. Helical scanning has many clinical benefits, however, it results in increased doses due to unnecessary irradiation at either end of the irradiated volume, so-called ‘over-beaming’. This ‘wasted dose’ is particularly significant for short scan lengths. Siemens and Philips have now developed collimators which open and close asymmetrically during the course of the helical scan, so reducing the amount of unnecessary irradiation at the scan extremities. The extent of dose saving is dependent on the length of scan and the table feed. Dose savings of up to 25% can be obtained for short scan lengths at low table feeds.
In the scientific sessions, work was presented on optimisation of tube kilovoltage (kV) used in scanning protocols. It was shown that in many cases, particularly in contrast studied, improved contrast to noise ratios for the same dose can be obtained at lower kV settings than are used conventionally. In the past use of lower kVs was limited to children due to tube current limitations. It is now practical to use lower kVs more generally due to the higher power capabilities of modern CT scanners.
The paragraphs above describe a selection of the dose reduction approaches being investigated and implemented on the latest CT scanners. Their use maintains the excellent diagnostic capabilities of CT whilst at the same time minimising the radiation risk to the patient.