Nodules (pulmonary nodules) are usually defined as round-like lesions in the lung up to 3 cm in diameter, or small nodules up to 2 cm in diameter, of which no more than 1 cm can be called micronodules (schedule). The diagnosis and differential diagnosis of small nodular lesions has always been a difficult problem in radiology, and how to improve the details of small nodules is one of the keys. As the saying goes, “a clever woman cannot cook without rice”, it is difficult for the most skilled physician to draw conclusions about a nodule with no features, while even a first-time examiner can have an equally accurate answer about a nodule with detailed features. With the advent of spiral CT, the rate of detailed display and accurate diagnosis of small nodules has improved significantly.
For a peripheral pulmonary nodule, the characteristic display includes three aspects: morphologic features, node-lung interface and adjacent changes; vascular changes are present throughout all three of these aspects.
The morphological features of DD are direct and fundamental signs, and the principle of display is “multi-level and multi-faceted”.
1, morphology (morphology)
(1) Round mass sign, which shows that the lesion tends to be round (round-like), reflecting its growth mode of cell accumulation, in order to distinguish from triangular, rectangular and lamellar lesions. Technically, post-processing 3D-SSD is preferable and allows a three-dimensional, direct and effective evaluation, while MPR is also valuable, while conventional cross-sectional evaluation requires some experience in “reconstructing” the three-dimensional image of the lesion in the brain.
The lobulation sign refers to the fact that the contour of the mass is not purely round or oval, but often has an uneven surface with multiple arcs, resembling the fusion of multiple nodules, and can usually be divided into deep lobulation and superficial lobulation, with the curvature of the lobulated portion as the criterion: the ratio of chord to distance length >2/5 is considered deep lobulation. The pathological basis is firstly related to the different degree of differentiation and growth rate of tumor cells in each part of the tumor margin. Secondly, the connective tissue interval of the lung, the blood vessels entering the tumor, bronchial branches, blood vessels and connective tissue growing outward from inside the tumor can cause tumor growth restriction and produce depression, thus forming a lobulated form. Deep lobar sign is important in the diagnosis of lung cancer. Technically, target scans are preferred, overlapping reconstructions are often necessary, combining cross-sectional and non-cross-sectional 2D images can be effectively differentiated, and 3D reconstructions can provide visual but not very accurate images.
③Spiculate protuberance (spiculate protuberance), meaning different from burr, pathologically has direct tumor invasion, imaging refers to a thicker and blunt structure between lobulation and burr, sometimes called a special kind of lobulation, many studies believe that spiculate protuberance is only seen in lung cancer, thus, the importance of its detection is evident, but the difficulty is accurate recognition; target The target scan is able to show the edges of the mass and the spines well, and 3D is also able to show this “pestle-like” structure well.
The former refers to multiple round nodule-like lesions, i.e., consisting of multiple nodules, and is seen in early-stage lung cancer, which is pathologically based on the multifocal origin of lung cancer and has not yet fused; the latter refers to a 1-2 mm nodule within the lesion.
(6) Hollow sign, which refers to a large translucent shadow without tubular morphology within the lesion, pathologically refers to necrotic liquefaction within the nodule and caused by drainage. It is defined as an intra-focal translucent shadow larger than 2 times of the corresponding bronchial meridian and discontinuous with the upper and lower bronchial level, or a round or round-like air-like low-density shadow larger than 5 mm, with various forms, including thin/thick-walled cavity, central/eccentric cavity, smooth or irregular wall, and with or without wall nodules. The tumor cavities are mostly central thick-walled cavities with irregular walls and may have wall nodules. The wall thickness ≤4mm tends to be benign and ≥15mm tends to be malignant. Same as the above signs, this sign is also better to be shown by thin layer or target scan.
2.Density
Density is one of the most important parameters used to evaluate the internal tissue characteristics of pulmonary nodules. One of the popular and better methods is to establish TDC (line graph), which can be replaced by at least three time points in daily work: before enhancement, early after enhancement (30-40s), and delay (about 4-5min). Technical requirements for density measurement: ① use regional values (ROI), with regional pixels no less than the minimum value, as mentioned before for homogeneous nodules use 60% area or diameter to take ROI, or include the whole nodule as far as possible but avoid the part of the edge with partial volume effect; ② zoning measurement, for inhomogeneous nodules, different density areas should be measured and evaluated separately to avoid mutual influence; ③ calcification, necrosis and cavities should be avoided. necrosis and cavities should be avoided.
The study of dynamic enhancement has been more popular in recent years, and the results of Swensen et al [5,9,10] were that the enhancement value in the malignant tumor group (mean 40 Hu) was significantly higher than that in the benign group (mean 11 Hu), and that a threshold of 20 Hu could effectively identify benign and malignant nodules, and the results of Yama***a et al [11] were similar; there are also many studies in China, which divided nodules into three categories and proposed Intensification values ≤20Hu are highly suggestive of benign, 20-60Hu are suggestive of malignant, and >60Hu with inflammatory nodules are likely to be large, and our results are consistent with this. The study also showed that the degree of nodule enhancement was independent of diameter. In recent years, enhancement studies have delved from whole nodules to regional evaluation of nodules, and density values were measured separately for each part of the nodule and then analyzed comprehensively, with a view to improving diagnostic accuracy.
Han Yucheng et al. classified pulmonary nodules into three types based on the ratio of reduction in the area of the largest dimension of the lesion at the transition from the lung window to the mediastinal window (called “mediastinal window reduction rate” by the authors) in terms of lung density; Intermediate type: shrinkage rate is greater than 50%, in between. This is a practical tool, especially for the primary level.
The technique to complete the densitometric analysis is best with target scanning. The smaller the layer thickness, the more detailed the structure, the more valuable the density value, but the layer thickness is too small, the volume of pixels contained is too small and affects the representativeness of the density value, so the appropriate value should be taken, usually 1/3 of the nodule diameter as the layer thickness is better. HRCT uses a high-resolution or high-frequency algorithm, which increases the noise and has an edge enhancement effect, overestimating the pixel information; the soft algorithm, on the contrary, underestimates the pixel information, and only the standard algorithm, which has no increase or decrease in information, is close to the original face of the nodule and is suitable for densitometry. It is suitable for densitometry.
3.CalcificationrnV
Obvious calcifications can be recognized on normal CT, but subtle calcifications are often missed. Foreign authors define calcification as structures with CT values greater than 164 HU, and the positive rate with this criterion is low, so some domestic authors have revised the threshold to 100 HU, but in our experience, the positive rate is still low. In contrast, calcification plays an important role in the characterization of small nodules, and thus it is necessary to make further detailed display.
(1) Book-layer scanning: on the one hand, it improves the resolution (spatial resolution at high tissue contrast), on the other hand, it reduces some of the volume effects, increases the density contrast, and significantly improves the detection of calcifications, better in combination with small FOV. Spiral CT is the best technique to achieve this approach.
②HRCT: Many authors believe that this is a good technique to show calcifications and internal structures. Generally, this technique is good for detecting calcifications inside nodules, but it should be used with caution because of the edge enhancement effect of the high-resolution algorithm, which can easily produce false images and increase the false-positive rate of calcifications.
③Dual-energy CT scan: Bhalla et al [6] reported 27 cases of dual-energy CT scan: 10 cases of benign among 11 nodules with elevated density; Yunqing Li et al reported 15 scans: 8 cases of benign among 9 nodules with elevated density and 4 cases of malignant among 6 nodules with decreased density. a multicenter study by Swensen et al [12] rejected this technique.
④phantom CT: Some foreign authors [15] proposed to use the window technique by adjusting the window width 1-2HU and the window center 164HU, and anything that can be dense above 164HU can be shown, suggesting benign nodules. In our experience, the window center can be set to different values as needed, because small nodules with a mean density above 60 HU on plain scan are rarely malignant, which improves sensitivity.
The distribution, morphology, and content of calcification are also important. 395 cases analyzed by Mahoney et al. found that: dense, central, laminar, popcorn-like, and scattered calcifications are mostly benign, while punctate, reticulated, and indeterminate calcifications are mostly malignant; Li Yunqing et al. found 8 cases of benign nodules with elevated density, whose calcification distribution was dense, central, or scattered, and 1 case of squamous carcinoma with punctate calcification.
4.bronchus signrr
Small translucent shadows in the upper and lower layers that are continuous, long or branching, associated with bronchi or vascular accompaniment are defined as bronchial air bronchogram (AB sign). This intra-nodular bronchial shadow manifests differently in different pathological conditions, and some investigators have suggested that the analysis of this bronchial morphology can help in the qualitative diagnosis. The bronchi in benign lesions are structurally intact and undamaged, their inner walls are smooth and thus the lumen is mostly normal. On the one hand, the bronchi in malignant lesions are invaded, resulting in luminal narrowing, truncation, non-glossy inner wall, thickened and stiffened walls, etc. On the other hand, tumors often have mucous secretions, which can lead to bronchial dilatation, with the distal end thicker than the proximal end, and such bronchial dilatation, although also seen in benign lesions, is rare.
When it is difficult to determine whether the intra-nodular translucent shadow is a vacuolar sign, a cavernous sign, or a bronchial air phase, it may be useful to use purely morphologic typing. Kiyomi Furuya et al [13] classified intra-nodular translucent shadows into tubular (in the form of elongated tubular or continuous multifaceted small circular shadows) and cystic (round-like or multiform cystic), and analyzed 60 cases of 163 SPN with translucent shadows (lung cancer 41, metastasis 2, tuberculosis 4, inflammation 13) and found that: tubular 45 = lung cancer 31 + inflammation and tuberculosis 14, mostly 1-2 tubular; cystic 33 = lung cancer 29 + inflammation4, and multicystic (≥3)22 were all lung cancers (including 2 cases of metastasis). The authors concluded that the multicystic translucent shadow suggested a malignant tendency.
The technical key to show the bronchial sign in the nodule is still the thin layer scan, and the continuous image display is also critical, thus target scan is preferred, sometimes HRCT is also used, but the correlation analysis between the upper and lower levels is often lacking; it is sometimes more helpful if combined with the enhanced scan; post-processing such as MPR reconstruction helps to show the oblique bronchial sign, but its practical significance is not significant due to the resolution, and the latest The isotropic imaging achievable with MultiCT can theoretically be helpful.