Several technical parameters and settings contribute to obtaining high‑quality MIP and MinIP images. As with any CT of the lungs, the most crucial factor for image quality is minimizing motion artifacts, particularly respiratory artifacts. Therefore, adjusting the duration of the scan to the breath‑hold capacity of the patient is essential (e.g., faster imaging in ventilated patients). Secondly, it is essential to use thin slices, preferably submillimetric. Thirdly, MIP and MinIP images generated using a soft or standard kernel are more appealing and diagnostic than those created with hard kernels such as lung or bone. Also, radiation dose can affect the quality of these postprocessing images. However, with potential artifacts taken into account, MIP images can significantly enhance diagnostic capability even with ultra‑low‑dose CT.
MIP ‑ A common application of MIP in the lungs is the detection of pulmonary nodules [2]. A less well‑known use is the contribution of sagittal MIP to the classification of (micro)nodular patterns as perilymphatic, random, or centrilobular [3]. Sagittal 5 mm MIP may sometimes help quickly identify the correct pattern (Figure 1).
Figure 1
Sagittal MIP images acquired from a soft tissue kernel in three different patients with micronodular lung disease. The fissures are indicated with blue arrows.
A. Perilymphatic distribution. The fissures are affected by the micronodules, more so than many parts of the parenchyma. Also note the patchy distribution of the nodules in the lungs.
B. Centrilobular distribution. The fissures are not affected by the micronodules, as the nodules are located in the center of the secondary pulmonary lobule, away from the fissure.
C. Random distribution. The fissures are affected by the microndules, but not more than the lung parenchyma. The nodules are evenly distribution throughout the lung and the fissure.
Perilymphatic nodules are located in the peribronchovascular interstitium (both perihilar/central and centrilobular), in the subpleural interstitium, and in the interlobular septum. Typically, these nodules have a patchy distribution throughout both lungs, with some areas disproportionately affected. The most frequent pathology is sarcoidosis.
Random micronodules result from hematogenous spread to the lungs. The nodules are uniformly distributed across both lungs without the patchy areas, unlike perilymphatic disease. Hematogenous metastases may be more pronounced in the lower lung regions owing to the higher vascularization. Metastases sometimes exhibit temporal heterogeneity, visible as nodules of varying sizes—older nodules appear larger than newer metastases, which have had less time to grow. Miliary spread of mycobacterial or fungal infection typically presents as micronodules of equal size throughout the lungs. Miliary tuberculosis may show upper lobe predominance.
Centrilobular micronodules are located at the center of the lobule, away from the septa of the SPL. There may be a single nodule per lobule, multiple nodules, or an entire cluster. Occasionally, a large nodule can occupy the entire lobule. Tree‑in‑bud is a specific morphology that, if identified correctly, is very suggestive of either transient pathology of an unknown cause, infection, or aspiration.
MinIP ‑ MinIP can be used to detect areas of varying lung attenuation, called mosaic attenuation [4]. To determine whether the pathology is situated in the areas of higher or lower attenuation, it is important to realize that a significant part of the lung attenuation is due to its vascularization. Smaller and fewer vessels in the areas of lower lung attenuation both point toward hypoperfusion in this region; that is, the areas of low attenuation are abnormal. This can be due directly to vascular disease or indirectly due to (small) airway disease. In airway diseases, blood will be shunted toward areas with normal ventilation to allow for an exchange between gases in the lung and the blood. This will result in the areas with abnormal ventilation receiving less blood, i.e., having a lower attenuation and smaller vessels. When the vessels in the areas of low attenuation are normal, the mosaic attenuation is due to ground‑glass opacities.
Competing Interest
The author has no competing interests to declare.