Grayscale Inversion to aid Diagnosis of Acute Occlusive and Chronic Pulmonary Embolism on CT

Pulmonary embolism (PE) is a potentially life-threatening disease requiring accurate diagnosis and treatment. CT pulmonary angiography (CTPA) has high accuracy for detecting PE.[1] CT is therefore the first-line diagnostic imaging modality in patients suspected of having acute PE. There are several tools described in the literature to improve our accuracy and confidence in diagnosing PE. For example, it is important to select an optimal display window width (W) and level (L) for evaluating the pulmonary arteries.[2] A filling defect representing PE can be masked in standard soft tissue window display setting around W:400, L:40; therefore, radiologists prefer to manually adjust it to a PE-specific window that is unique to each exam. This is done by widening the width and increasing the level, arriving close to the double-half setting (W: 2×, L: 0.5×) where × is the mean attenuation in Hounsfield units within the main pulmonary artery.[3]

Other helpful tools include using multiplanar reconstructions to colocalize the filling defect on coronal and sagittal planes for diagnosis of PE and exclusion of artifact. We observe that when a well-defined pulmonary arterial filling defect is also branching, that supports true PE. Zooming into the image and placing a circle region of interest (ROI) well within the filling defect is another tool. Acute PE demonstrates low attenuation ≤ 78 Hounsfield units (HU) on standard CT at 120 kVp, so if ROI > 78 HU with this protocol, then artifact is more likely than acute PE.[2] CTPA using spectral technique provides monoenergetic low keV datasets and an iodine map as a surrogate for lung perfusion, which can be useful.[4]

Occlusive acute PE may expand the vessel, while chronic PE can contract the vessel.[5] Despite various tools in practice, it can be difficult to diagnose occlusive acute PE in small vessels, particularly if there is suboptimal contrast opacification or parenchymal disease. Recognizing an unopacified contracted vessel as a manifestation of chronic PE can be difficult as well. There is a need for another tool to help diagnose PE with these manifestations that are difficult to detect.

Grayscale inversion is an established tool that can improve the detection of pulmonary nodules on radiograph.[6] Grayscale inversion changes a positive radiographic image into a negative one. Grayscale inversion has been demonstrated to be effective on CT for orthopedic hardware.[7] On a CT image, grayscale inversion of soft tissue window changes the white appearance of bone into black with central dark gray, while it changes the dark gray appearance of soft tissues into very light gray.

The utility of grayscale inversion in diagnosing PE needs validation in clinical studies. Meanwhile, we describe the following observations of acute and chronic PE in a PE-specific window versus a grayscale inverted window and provide corresponding examples ([Figs. 1] and [2]). The expanded vessel as a manifestation of occlusive acute PE is more prominent on inverted lung window, particularly for segmental and subsegmental pulmonary arteries, compared to a PE-specific window where it is difficult to discern ([Fig. 1]). The unopacified contracted vessel representing chronic PE is more visible as very light gray on inverted soft tissue window, while it is a very subtle finding to recognize on a PE-specific window ([Fig. 2]).

We observe that these grayscale inversion tools on CTPA are simple to use and can increase the diagnostic confidence in detection of acute occlusive and chronic PE.

Fig. 1 Axial images from CT pulmonary angiography (CTPA) in a 27-year-old man presenting with acute symptoms. Acute pulmonary embolism (PE) in a subsegmental level pulmonary artery of the left upper lobe on a PE-specific window (A) is subtle and difficult to detect (arrow). The grayscale inverted lung window (B) shows that the corresponding vessel is expanded (arrow), raising suspicion for acute PE.

Fig. 2 Axial images from CTPA in a 69-year-old man with history of chronic PE. An unopacified contracted segmental pulmonary artery in the right middle lobe on PE-specific window (A) is subtle and difficult to recognize. Well-opacified, normal caliber pulmonary arteries are noted in the lower lobes (triangle). The grayscale inverted soft tissue window (B) more conspicuously shows a very light gray contracted vessel (arrow) compared to dark gray normal vessels (triangle).

Publication History

Article published online:
05 June 2024

• References

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