Dual Energy- Single Source vs Dual Source

The following question has been sent by Daniel Newman:

I am asked frequently about the difference between dual energy and dual source.  It seems that for most people the terms are interchangeble.  However, not all CT systems utilize 2 tubes.  Can you explain to me the pros and cons of each type of dual energy system?

Pal Suranyi, MD, Medical University of South Carolina:

Dear Dr Newman,

Thank you for your interest in our website and this great question. An article published earlier this year in Diagnostic Imaging gives a nice basic overview of dual energy techniques. This article by Paula Gould describes the basic differences as follows:

“The earliest attempts to gather information at different energy levels required patients to be scanned twice with a single-slice CT unit. Results were limited by the low photon output at low energy levels and by problems coregistering the separate images. Technology has since moved on. A dual-source CT system that has two separate x-ray tubes and corresponding detector elements is available commercially from Siemens, and alternative ways of acquiring dual-energy CT images, including multilayer detectors, are being explored by other vendors”
(see link below for full article)
http://www.diagnosticimaging.com/viewpoints/young-researchers-spotlight/article/113619/1384627

In other words if you have to scan a patient twice (“sequential scanning”) which inevitably will cause misregistrations due to patient motion (voluntary or involuntary), changes in the depth of breathhold, motion of the viscera etc.

More modern dual-energy CT approaches are either scan with different X-ray spectra (dual source or fast-kVp switching) or utilize energy selective detector technologies.

With dual source CT, two separate, orthogonally oriented x-ray tubes generate two x-ray beams with different energies, and two projections are acquired simultaneously, circumventing the above problems of motion and misregistration.

Multilayer detectors also eliminate issues of misregistration, because two “projections” are simultaneously detected, one detecting the x-ray projection of lower-energy x-rays and another detector-layer detects high-energy x-ray projections. (For this, the conventional CT detection pixel has been reconfigured by some vendors, to consist of two scintillator layers, read simultaneously, by a double-layer, side-looking photodiode.) The raw data of each of the detector layers is reconstructed separately, resulting in a Low-Energy and a High-Energy Hounsfield Unit images.  Where do the differential x-ray energies come from in these systems? The x-ray we are using in CT  is actually a collection of x-rays of various energies (a spectrum) which contains more or less of the low-energy x-rays depending on the x-ray tube settings and material itself and on filtering.

So in summary, both a dual-source system and a dual-layer detector system may acquire differential x-ray-energy projections simultaneously. Dual source has a disadvantage of cross-scatter radiation, meaning that a small fraction of high energy x-rays from one tube actually ends up hitting the low-energy detector row and vice versa. If one is interested in further details, the objective way to compare the two methods is looking at image noise and Iodine-bone separation power at given image sharpness and dose exposure.

When it comes to scanning fast,  however, which is crucial in cardiac imaging or in imaging patients who cannot hold their breaths, a dual-source system offers significant advantages due to its superior temporal resolution.

I hope this answers the question. Please fell free to let us know if you would like us to elaborate more on this subject.

Best regards,
Pal Suranyi