Computed tomography has established itself as a reliable method for cardiac examinations. But concerns about the possibility of excessive radiation have prevented its routine use for the early detection of coronary artery disease. The latest generation of CT scanners promises to change that. The newest scanner helps to reduce the dose to just 3 to 10 % of the values that were typical only ten years ago.
The Somatom Definition Flash can produce detailed images of the heart with a dose of less than 1 mSv. This is far less than the annual background dose from the environment.
"Less than one millisievert is required for an exam, rather than three to 20 with conventional machines."
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According to the World Health Organization, heart diseases are the leading cause of death in high-income nations. In Germany alone roughly 130,000 people die each year as a result of an insufficient supply of blood to the heart muscle. What can be done to detect these diseases earlier? Health-conscious people who go to a screening with no complaints generally first undergo a cardiac stress test. Details about the patient's heart beat are measured while the patient pedals a stationary bicycle. If irregularities occur, the patient is referred to a specialist for additional tests.
One alternative to this would be an examination with a computed tomography (CT) scanner. Such scans can help to reveal calcifications and constrictions in blood vessels. To perform such an examination, the patient is placed inside a tube in which X ray sources and detectors circle the body at very high speed. Despite their high accuracy and reliability, these devices are barely used for early detection or routine examinations because of their X ray radiation. When it comes to examining the heart and the coronary blood vessels, CT scanners are used primarily in emergency medicine, with at-risk patients such as heavy smokers, or with patients who are exhibiting initial possible, but still unclear, symptoms of heart disease.
That could change, however. For over ten years, researchers at Siemens Healthcare in Forchheim, Germany, have been developing solutions designed to help reduce the radiation associated with CT examinations. They have now set a new record with the latest line of CT scanners, the Somatom Definition Flash.
The scanners can perform a complete scan of an adult human heart in only 0.25 seconds — significantly less time than with conventional machines. In fact, it is so efficient that it can reduce radiation dose to less than 1 mSv. Just a few years ago, the dose for a coronary examination was ten to 30 times higher. One millisievert is substantially less than the global average background dose of 3.1 mSv that a person absorbs from the environment over one year.
In view of this, CT scanning could finally become established as a routine tool for cardiac examinations in the near future. Many experts agree. In a survey of 363 radiologists associated with the dual source CT community (DSCT.com) users were asked what the minimum exposure dose would have to be in order for them to order a computed tomography exam for the early detection of coronary heart diseases. Half indicated values between 3 and 10 mSv. 42 % were even more strict. For them the magic threshold is below 1 mSv.
The Definition Flash is thus the only CT scanner to currently satisfy the strict requirements of particularly radiation-conscious users. Although ideal exposure values may occasionally be exceeded in the case of very large or heavy patients, it is important that reducing the dose below the one millisievert threshold becomes routine rather than being the exception. “The fact is that particularly with young patients, the majority of cardiac scans with the Definition Flash can remain below one millisievert,” says cardiologist Prof. Stephan Achenbach of the University of Erlangen.
Two Sources, Two Detectors. This enormous reduction is achieved through the combination of three technologies. First of all, a particularly important role is played by the dual-source scanner, which Siemens introduced a few years ago. With this technology, two X ray tubes and two detectors offset at 90° from one another revolve around the patient in a single machine. The patient's body is thus photographed from two different angles simultaneously. The patient glides through the tube on the examination table while the two measuring systems revolve. The combination of revolution and forward movement result in a spiral scan pattern.
The two X ray sources complement one another during data acquisition, so that the patient can be moved forward at a rate of up to 45 cm/s. This allows the entire volume of the heart to be scanned much faster than with conventional machines with a single X ray source that has to revolve around the heart several times. “Flash Mode” is the name of the technique that substantially reduces the X ray dose due to its high scan speed and short exposure time. Less than 1 mSv is required to examine the heart, rather than the three to 20 mSv required with conventional machines.