64-slice CT passes pediatric safety tests

More and more patients are being referred for CT examinations, thanks to the ability of the latest CT scanners to increase throughput and improve diagnostic accuracy. This increased usage has prompted considerable discussion in the radiological literature about the effects of high-end multislice CT on radiation dose. Radiologists have questioned whether the increasing number of slices and slice overlap will result in greater patient radiation exposure per scan.

This issue is of particular concern to pediatric radiologists. Children face a greater risk of developing cancer following diagnostic imaging tests than adults. Pediatric patients may have a longer life expectancy than adult patients, and growing children's cells are more sensitive to radiation damage.

Installation of a 64-slice CT scanner (Aquilion 64, Toshiba Medical Systems) at the K.K. Women's and Children's Hospital was accompanied by considerable efforts to reduce radiation dose to the lowest acceptable level. Our group of experienced consultant and senior consultant radiologists performed tests on the new 64-slice scanner and our existing single-slice CT scanner (PQ5000, Picker [now Philips]) to find a threshold dose on each machine, below which image quality was deemed too poor for diagnosis.

We found that diagnostic-quality images could be acquired on the 64-slice scanner using much lower dose levels than those for the single-slice CT scanner. This was true for all nonhead CT examinations and head CT scans in the youngest children. Reductions were obtained by minimizing mAs and kV levels and using special features on the scanner known as Sure Exposure and Quantum De-noising Filter. Sure Exposure is a program that modulates mAs during scanning according to patient size and scanning region of interest. The Quantum De-noising Filter allows mAs to be reduced by half of its original setting, without altering image quality. Patient dose can be reduced by up to 40% using this feature. The features can be applied to all nonhead CT examinations.

We have additionally compared x-ray radiation doses produced by the two different CT scanners. Exposure data were acquired in October 2005 using the low-dose CT parameters acquired as described above.

Our 64-slice CT scanner has a CT dose display column. We can simply key in the CT parameter and record the CT dose index volume (CTDIvol) and dose length product (DLP) readings displayed. DLP is equal to CTDIvol multiplied by the scanning range.

Our single-slice CT scanner does not have this feature. Instead, we used a CT dosimeter to measure CT dose index air for all typical settings and age groups. We then calculated the test value of CTDI air, along with the CT dose application, to get a comparable value of CTDIvol and DLP.

Technicians from the Radiation Protection Board Inspectorate of Singapore verified all test values.

We found that the x-ray radiation dose was significantly reduced when using the 64-slice CT scanner compared with single-slice CT for all studies apart from head CT examinations in children aged over five years. The unavailability of Sure Exposure and Quantum De-noising Filter features for head examinations is almost certainly the cause of these results. We are continuing to work at reducing the dose further on our 64-slice CT unit. Now we only have the 64-slice CT unit.

Using a 64-slice CT scanner has enabled us to increase our throughput. Faster scanning has also allowed us to lower the number of sedations performed by 66%. Where sedation is still required, we can rely on mild sedation (chloral) rather than moderate sedation (midazolam), to keep the risk to patients as low as possible.

Because our institution is a 750-bed women's and children's hospital with over 350 pediatric beds and serving a population of 4.4 million, the most difficult pediatric cases are often referred to us. Many of these cases are neurological, and some patients suffer from poorly controlled movements. The worst cases still require sedation. On average, we used to sedate 30 children per month, but this number fell to 10 children during the first month of 64-slice CT scanning. The figure has dropped further since then.

In summary, 64-slice CT offers some specific advantages to pediatric radiology. The x-ray dose can be reduced in many cases, and far fewer patients will require sedation during imaging. These are both significant safety features.

Prof. Stringer is head of the diagnostic imaging department at K.K. Women's and Children's Hospital, Singapore. Mr. Qing Long is senior radiographer in charge of CT at the same hospital.