Latest CT Technologies in Lung Cancer Screening

Protocols and Radiation Dose Reduction

Marleen Vonder; Monique D. Dorrius; Rozemarijn Vliegenthart

Disclosures

Transl Lung Cancer Res. 2021;10(2):1154-1164. 

In This Article

CT Protocol Settings of Large Lung Cancer Screening Studies

A variety of CT systems and low dose CT acquisition protocols have been used in large lung cancer screening studies, see Table 1. The majority of studies were initiated before 2010, and therefore, the protocol settings (radiation dose and image quality) dates back to systems and standards of ten until twenty years ago. Lung cancer CT screening studies before 2004 used single slice detector systems or the first multi(slice)-detector systems that could acquire 4 slices per rotation, and used collimation of 2.5–5.0 mm.[16–18] From the start, a spiral scan mode was used to cover the entire chest from lung apices to lung base within one end-inspiratory breath hold. Starting from studies initiated in 2004, 16-slice multi-detector CT systems were mainly used in lung cancer screening from then on. This allowed acquiring 16 slices with collimation of 0.75 mm resulting in thinner axial slices.[19,23,24] From 2007 onwards, number of slices in MDCT increased to 128 with collimation as low as 0.625 mm, and scan times ranging from 5 to 10 seconds.[26,27]

The tube voltage used in the major screening studies varied between 80 and 140 kVp. In general, either one fixed tube voltage and/or tube current value was applied on each CT system for every participant or pre-defined values were applied based on a participant's body weight. For instance, a low tube voltage of 80 kVp was used in slim or small participants (up to 50 kg), moderate tube voltage of 120 kVp in medium size participants (between 50–80 kg) and a high tube voltage of 140 kVp in large or tall participants (over 80 kg). Similarly, the tube current was mainly fixed at a low current of maximum 40 mA to achieve a low dose chest CT, or the tube current was adjusted in small, medium and large participants to reach predefined total radiation dose as estimated by the volume CT dose index (CTDIvol) in mGy.[23] As far as results on total mean radiation dose have been published, overall the radiation dose of the low-dose chest protocols varied between 0.8 to 1.5 mSv for medium size participants in the trials initiated between 2001 and 2011.[17,20,21,25,26] To put into context, the radiation dose of conventional high resolution CT of the chest used for clinically indicated scans, could be as high as 5.8 mSv at that time.[28]

The specific reconstruction values of the various study protocols are scarcely published. Information on used field-of-view, reconstruction algorithm or window settings are not published. In general, filtered back projection (FBP) was used to reconstruct the images at a thin slice thickness and a small increment. This resulted in overlapping slice reconstructions to increase the image quality without the need of increasing radiation dose, to acquire thin slices for the detection of small lung nodules. In the process of reconstructing the image with FBP, a convolution filter or 'kernel' is used to enhance or optimize the reconstructed image for specific anatomical areas. In the lung cancer screening studies, a 'soft' kernel and 'hard' or 'sharp' kernel was applied. See Figure 1 for example of soft versus sharp kernel CT image. Soft kernels are optimized for visualizing soft tissues, appear as more smooth images with less noise, and are used to detect nodules and evaluate nodule morphology. Sharper kernels are suitable for visualizing high differences in density (lung-air, bone-soft tissue) structures, appear as sharp images with higher spatial resolution at the expense of higher noise, and can be used for nodule size measurements although segmentation problems may occur. A soft kernel tends to show higher reproducibility for nodule volume measurements but may underestimate nodule volume. There is no consensus yet for the use of soft or sharp kernel,[22,29] in general, a medium-smooth to medium-sharp kernel is advised.[30] It is of particular importance to consistently use the same reconstruction kernel in case of nodule measurements on subsequent CT scans.

Figure 1.

Example of low dose CT protocol with 120 kVp and FBP reconstructed with (A) soft and (B) hard kernel in a patient presenting with an irregular solid nodule of 3,973 mm3 (maximum diameter 26.7 mm) in the left upper lobe.

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