In the reticular pattern there are too many lines, either as a result of thickening of the interlobular septa or as a result of fibrosis as in honeycombing.       

Septal thickening
Thickening of the lung interstitium by fluid, fibrous tissue, or infiltration by cells results in a pattern of reticular opacities due to thickening of the interlobular septa.
Although thickening of the interlobular septa is relatively common in patients with interstitial lung disease, it is uncommon as a predominant finding and has a limited differential diagnosis (Table).
Smooth septal thickening is usually seen in interstitial pulmonary edema (Kerley B lines on chest film); lymphangitic spread of carcinoma or lymphoma and alveolar proteinosis.
Nodular or irregular septal thickening occurs in lymphangitic spread of carcinoma or lymphoma; sarcoidosis and silicosis.
c carcinomatosis
On the left we see focal irregular septal thickening in the right upper lobe in a patient with a known malignancy.
This finding is typical for lymphangitic carcinomatosis.
There are also additional findings, that support this diagnosis like mediastinal lymph nodes and a nodular lesion in the left lung, that probably represents a metastasis.

Pulmonary lymphangitic carcinomatosis (PLC)

In 50% of patients the septal thickening is focal or unilateral.
This finding is helpful in distinguishing PLC from other causes of interlobular septal thickening like Sarcoidosis or cardiogenic pulmonary edema.
Hilar lymphadenopathy is visible in 50% and usually there is a history of (adeno)carcinoma.
Identical findings can be seen in patients with Lymphoma and in children with HIV infection, who develop Lymphocytic interstitial pneumonitis (LIP), a rare benign infiltrative lymphocytic disease.
On the left a patient who had a CT to rule out pulmonary embolism.
There is a combination of smooth septal thickening and ground-glass opacity with a gravitational distribution.
The diagnosis based on this CT was cardiogenic pulmonary edema.

Cardiogenic pulmonary edema generally results in a combination of septal thickening and ground-glass opacity. 

There is a tendency for hydrostatic edema to show a perihilar and gravitational distribution.
Thickening of the peribronchovascular interstitium, which is called peribronchial cuffing, and fissural thickening are also common.
Common additional findings are an enlarged heart and pleural fluid.
Usually these patient are not imaged with HRCT as the diagnosis is readily made based on clinical and radiographic findings, but sometimes unsuspected hydrostatic pulmonary edema is found.
On the left a patient with both septal thickening and ground glass opacity in a patchy distribution.
Some lobules are affected and others are not.
This combination of findings is called 'crazy paving'.
Crazy paving was thought to be specific for alveolar proteinosis, but is also seen in many other diseases such as pneumocystis carinii pneumonia, bronchoalveolar carcinoma, sarcoidosis, nonspecific interstitial pneumonia (NSIP), organizing pneumonia (COP), adult respiratory distress syndrome and pulmonary hemorrhage.
Alveolar proteinosis 

is a rare diffuse lung disease of unknown etiology characterized by alveolar and interstitial accumulation of a periodic acid-Schiff (PAS) stain-positive phospholipoprotein derived from surfactant.
is a rare diffuse lung disease of unknown etiology characterized by alveolar and interstitial accumulation of a periodic acid-Schiff (PAS) stain-positive phospholipoprotein derived from surfactant
represents the second reticular pattern recognizable on HRCT.
Because of the cystic appearance, honeycombing is also discussed in the chapter discussing the low attenuation pattern.
Pathologically, honeycombing is defined by the presence of small cystic spaces lined by bronchiolar epithelium with thickened walls composed of dense fibrous tissue.
Honeycombing is the typical feature of usual interstitial pneumonia (UIP).

See my  article on HRCT


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