CT choice means more than counting slices

October 1, 2006

We have three CT machines at Addenbrooke's Hospital: a four-slice, a 16-slice, and a 64-slice system. Moving from single-slice to four-slice CT was a huge breakthrough. You could get dramatic coronal pictures, assuming the patient could hold his or her breath. The 16-slice system was yet another step forward in terms of anatomical visualization. In fact, the question became how much detail we wanted to see. You will find a pulmonary embolus, for example, in almost any inpatient 10 days after surgery if you look hard enough. But do you really want to see a 2 by 3-mm PE that is unlikely to be responsible for a patient's chest pain?

Then there is the 64-slice system. Everything happens very quickly on these machines, though, of course, the chance of missing the bolus of intravenous contrast medium also increases. Thankfully, this is less of a problem now that bolus tracking techniques are more robust. It is also terribly tempting to lengthen examinations when using 64-slice CT. Perhaps start with an unenhanced view, then early arterial, late arterial, an early portal, late portal, at which point the patient dose will have risen considerably.

Evidence shows, however, that if you want to see the entire arterial tree or the heart, image quality does improve as you move from four to 16 to 64 slices. If you want to create truly remarkable anatomical images with exquisitely detailed coronal slices, then 64-slice is best. So, yes, there are advantages.

But to what extent does the number of slices or detectors or tubes influence image quality in a practical clinical setting? If you are doing PE screening on a slightly obese young female patient with rapid transit flow, you are not going to get great pictures whatever your CT system. The experience and competence of the radiology staff matter, too. In an ideal world, every CT study would be carried out on perfect equipment and reported by the perfect radiologist. But life doesn't often work out like that.

There are certain applications where you do need state-of-the-art CT. If you are using CT to rule out PE, aortic dissection, and myocardial infarction, you will need the best machine possible. What about CT colonography? If you are no longer performing barium enemas and sending every patient to CT, do you block out your best machine for this bread-and-butter work? Should you put every patient through your best CT system before surgery? And what about biopsies? Is single-slice CT alone okay, or do you need CT-guided fluoroscopy?

Many clinicians say that they want everything done on a 64-slice system. But I don't believe they can tell the difference half the time. Single-slice CT is perfectly satisfactory for high-resolution chest CT or for follow-up after cancer therapy. In terms of cardiac imaging, however, it is only a matter of time before the cardiologists replace their coronary angiography suites with 64-slice CT units. And rightly so. I think that in years to come, we will be replacing chest x-rays with multislice CT, too, though we will have to look at lowering the dose.

Another issue to consider is imaging speed. You will not need to sedate so many patients if you use a fast CT machine. Head-to-toe CT wellness screening requires fast data acquisition, lest patients spend all day on the table. Trauma patients will benefit as well from rapid CT data acquisition.

Patient throughput is not solely dependent on technology. In fact, the number of detectors does not make that much difference. More pertinent parameters arise: How quickly can you change to a new study? How quickly can you change sequences? Do you run into tube heating problems? Can you store the data as fast as you can acquire them? Of course, what really makes the difference is teamwork among the radiographers and radiologists responsible for managing patients.

So I don't think choosing a new CT system is merely an issue of slices. The question is, How many CT machines do we need as a department, and what is the best mix? The answer to this question will be highly dependent on workflow.

This column is based on a presentation at the 2006 U.K. Radiological Congress by Prof. Adrian Dixon, a professor of radiology at Addenbrooke's Hospital, University of Cambridge, U.K.