X-ray CT Scanning Protocols for Lung CT AI Applications

Can X-ray CT Detect and Quantify Pulmonary Emphysema?

Pulmonary emphysema is defined as “abnormal permanent enlargement of air spaces distal to the terminal bronchioles, accompanied by destruction of their walls and without obvious fibrosis.” The workshop pointed out that the increase in the size of the airspace of the lung alveoli and the decrease in tissue mass due to destruction of the alveolar walls would cause a decrease in tissue density, g/mL. As was pointed out in Chapter 2 , the CT numbers assigned to the individual voxels of the lung CT images correspond closely to the density of lung tissue. Therefore, CT scanning should be able to assess the presence and extent of pulmonary emphysema both visually and quantitatively.

Single Versus Multiple Detector Row CT Scanners

At the time of the workshop, multiple-detector spiral CT scanners (MDCT), as described in Chapter 2 , were increasing in number and had advantages over the single-detector spiral CT (SDCT) scanners that preceded them. It was pointed out that the MDCT scanners were preferable to SDCT scanners in the QCT assessment of emphysema. This is because the greater the number of detector rows in the z-axis of the patient, the faster the entire lungs could be scanned. This is especially important in patients that are short of breath due to their underlying emphysema. Four-, 8-, and 16-row MDCT scanners with a detector width of 1.0 to 1.25 mm could obtain high-resolution images of the entire lungs in 25 to 30 seconds, 10 seconds, and 5 seconds, respectively. Sixteen-row MDCT scanners would be preferable, if available.

It was also recognized that for longitudinal studies, the same CT scanner model should be used at the different study time points for a given patient throughout the course of the study. If different CT scanner models were going to be used in a study for the same study patient, CT phantoms need to be used to verify that the different CT scanner models had equivalent spatial and density resolution.

Constant and Optimal X-ray Tube Peak Kilovoltage, mAs, and Radiation Dose

The x-ray tube peak voltage and x-ray tube current time product were discussed in the workshop and, given the MDCT technology at the time, it was agreed that the peak x-ray tube voltage should be between 120 and 140 kV and the same x-ray tube peak voltage should be used for all subjects in a given study. This is because the x-ray tube peak voltage determines the x-ray photon beam spectrum coming from the anode of the x-ray tube. Additional x-ray beam shaping and filtration placed in front of the x-ray tube, but before the patient, were not specifically addressed. The majority of the workshop participants favored setting the tube current time product between 80 and 100 mAs to provide acceptable image-to-noise ratio in the lung, while greater values would increase the ionizing radiation dose to the subject/patient without realizing any additional benefit. Twenty mAs was suggested as the minimum possible for a particular CT scanner model and CT scanning protocol. The recommended dose should follow the ALARA principle, achieving sufficient image quality to assess lung density accurately but no higher, with the effective dose range between 1 and 10 mSv. The 20 mAs study had an effective dose of 0.7 mSv. Phantom studies should be conducted to verify that the kV and mAs for the study would provide sufficient image quality for the chosen quantitative CT metric and keep the effective dose no higher than 10 mSv.

Scan Mode and Pitch

The optimal scan mode was spiral CT scanning, not axial scanning. The pitch settings were discussed and pitch values between 1.0 and 2.0 were acceptable. Pitch values greater than 2.0 were not acceptable because of adverse effects on image quality. Pitch values less than 1.0 increased radiation dose and were not recommended.

Detector Width and Recommended Axial Image Thickness and Spacing

The workshop recommended using an effective detector element z axis thickness of 1 to 1.25 mm, with the ability to reconstruct thin 1- to 1.25-mm axial images and thicker axial images with a slice thickness equal to or greater than 7 mm. The thinner slice thickness would be able to detect small foci of emphysema with greater image noise, and the thicker slices would have less noise but would also average out small foci of emphysema that were much smaller than 7 mm. The 16-row MDCT scanners would be able to accomplish this with scan times less than 12 seconds. The focus of the workshop was on determining abnormally low attenuating areas of lung density (see Chapter 5 ) as a measure of pulmonary emphysema. At the time of this workshop, the assessment of airway geometry, which requires slice thicknesses less than 1.5 mm, and submillimeter is better, was not a consideration. It was recognized that if lung texture was to be assessed, then the detector element thickness would need to be 1.25 mm or less.

Image Reconstruction

At the time of this workshop, filtered back projection (FBP) was the only reconstruction method available for image reconstruction. Iterative reconstruction (IR) was still in the future for commercial clinical x-ray CT scanners. However, it was recognized that the reconstruction kernel used in FBP image reconstruction was very important in determining both image noise and image spatial resolution. It was recommended that a neutral kernel be used, which at the time for the GE CT scanners was the “Standard” FBP reconstruction kernel. This neutral kernel would not edge enhance or smooth the image. It is interesting to note that this recommendation for using a neutral kernel in FBP is still favored in the early 2020 s for lung CT AI.

Optimal Lung Volume—Total Lung Capacity (TLC)

There are two main issues related to lung volume and the scanning of the patient for the assessment of lung disease: lung volume and coaching versus spirometric gating. It was generally agreed that for the assessment of emphysema the recommended lung volume is TLC. It was recommended that careful coaching of the patient to TLC by the x-ray CT technologist was the best approach. The use of spirometric gating was not recommended, as it did not add sufficient value over careful coaching of the patient to a given lung volume (e.g., TLC, RV). It should be noted that careful coaching is the approach in patients who undergo pulmonary physiology testing to assess airflow and lung volumes.