Pulmonary HRCT first appeared in the early 1980s in Japan. This new technique was the successful outcome

of previous radiological, anatomical and pathological correlation (RAP-C). The RAP-C has been pursued continuously

to add new HRCT findings of a wide variety of diffuse and localized pulmonary diseases. The present lecture will

emphasize that a greater understanding of pre-existing structures of the lung enables both clinical and research

activities of image diagnosis to add accuracy and progress further.

(1) Subpleural polygonal mesh

Three-dimensional CT of inflated lung specimens has demonstrated radio-opaque polygonal mesh in the inter-

stitial layer of visceral pleura. The subpleural polygon mesh consists of three major components: pleura-septum junc-

tion, subpleural pulmonary vein and subpleural lymph vessel. The pleuro-septal junction is a V-shaped thick portion of

interstitial tissue of the pleura where the interlobular septum joins the pleura. The interlobular septum is a membrane

that extends from the subpleura to the lung parenchyma 1-2 cm in depth. The interlobular septa are well developed in

the lower portion of the lung and scarce along the interlobar surface. The septa near the anterior and inferior margins

of the lung are abundant and cross the lung to connect different pleural planes. The interlobular septa are accompa-

nied by interlobular pulmonary veins.

Subpleural pulmonary veins run parallel to the pleura and collect blood from visceral pleura and lung

parenchyma beneath the pleura. Small pulmonary veins connecting subpleural pulmonary veins are called intralobular

venules. The subpleural pulmonary veins join interlobular pulmonary veins after passing through the pleura-septum

junction.

Subpleural lymphatic vessels develop more in the lung base. The thick lymphatic vessels can be

differentiated from pulmonary veins by the naked eye, recognizing clear fluid within the lymphatic vessels.

Morphological Bases of HRCT Diagnosis of Diffuse Pulmonary

Diseases

Activity : Lecture

Date : 30 October 2014

Time : 08:00-08:45

Speakers : Harumi Itoh, M.D.Ph.D.

(2) Intersegmental septum

The intersegmental septum has not been described precisely due to difficulty verifying the septum

continuously from the pleura to the hilum through serial lung slices fixed using liquid formalin. The intersegmental

septum has been visualized with serial CT images obtained from inflated lung specimens. The intersegmental septum

and pulmonary veins run together and border adjacent pulmonary segments. The septum is not always flat, particu-

larly near the pulmonary hilum, where the lung parenchyma supplied by a small lateral bronchus gets stuck among

the roots of adjacent pulmonary segments. The intersegmental septum is connected to interlobular septa belonging to

neighboring pulmonary segments.

(3) Pulmonary lobule

The pulmonary lobule or secondary lobule of the lung is a basic lung structure to describe abnormal

patterns of diffuse pulmonary diseases. It has been pointed out that the pulmonary lobule defined by interlobular septa

is quite variable in size, since the interlobular septa are not distributed evenly within the lung. Reid and Matsumoto

have proposed a different pulmonary lobule to solve this problem. This new lobule is about 10 mm in size and

distributed constantly in both outer and inner portions of the lung. Lung parenchyma within the lobule is supplied by

the bronchioles with a constant branching pattern, called a millimeter pattern. In the millimeter pattern, the terminal

bronchioles divide every 1-2 mm from a parent intralobular bronchiole. The lobule is bordered by pulmonary veins,

interlobular septa and thicker broncho-arterial bundles. The lobules located deep in the lung are supplied by a small

lateral bronchus and pulmonary arteries divided from axial broncho-arterial pathways.

The term centrilobular means a portion of the lobule comprising terminal and respiratory bronchioles and

adjacent alveoli. On the other hand, the term paraseptal covers the areas consisting of visceral pleura, interlobular

septum, interlobular pulmonary vein and alveoli adjacent to those structures. The extralobular bronchoarterial bundle

is also paraseptal.

(4) Pulmonary acinus

The pulmonary acinus is a lung unit smaller than a pulmonary lobule. The pulmonary lobule contains 5

pulmonary acini on average. The pulmonary acinus covers the whole lung parenchyma distal to a pair of 1st-order

respiratory bronchioles. The key structure that borders adjacent pulmonary acini is the intralobular venule. Recent

micro-CT study has proven that the pulmonary acinus and subacinus are polyhedral in shape, like the pulmonary

lobule, which enables such small lung units to be packed compactly within the lung. The respiratory bronchiole and

adjacent alveoli are centriacinar structures, while perilobular structures are also periacinar.

(5) Communication between pulmonary and bronchial circulation

Previous study has shown that the bronchial wall, peribronchial interstitial space and lymph node are equipped

with a rich vascular network from bronchial arteries. Although pulmonary veins and broncho-arterial bundles are laid

alternately within lung parenchyma and appear to be isolated structures, postmortem bronchial angiography showed

vascular communication between pulmonary and bronchial circulation. Namely, the bronchial venous plexus, which

surrounds the broncho-arterial bundle, is connected to the nearest pulmonary venules and furthermore to the inter-

lobular veins. The bronchial venous plexus also communicates with alveolar capillaries.

(6) Pulmonary lymphatic vessels

Pulmonary lymphatic vessels are classified into two systems. One is interstitial-venous lymphatics and the

other broncho-arterial lymphatics. Subpleural lymphatics mentioned previously belong to the former system. Since

pulmonary artery and vein are surrounded by the lymphatics, HRCT of those vessels shows their thickening in diseases

extending along the lymphatics.

(7) Arrangement of alveoli

The gas exchange area consists of two compartments. They are air passages (alveolar ducts and alveolar sacs)

and a group of alveoli intervened by the former. The alveoli gather to form a layer where they are arranged to face

in opposite directions. Such a specially arranged structure of the alveoli has been named back-to-back alveoli, since

the dome of the alveoli is shared by the alveoli facing oppositely. Therefore, there is no space between adjacent

alveolar ducts or sacs. The popular 3-D schema of peripheral lung has not been appropriate to express such specific

arrangement of alveoli. The dome of the alveolar wall is equipped with pulmonary arterioles and venules in addition

to alveolar capillaries. The lymphatic vessels are found around those vessels.

(8) Double-faced and single-faced alveolar walls

Gas exchange is usually possible on both sides of alveolar walls. Such walls are called double-faced alveolar

walls, which constitute 90% of the total. On the other hand, alveolar walls attached to the pleura, interlobular septum

and broncho-vascular bundle are called single-faced alveolar walls, where gas exchange is limited to one direction.

Alveolar capillaries are denser in double-faced alveolar walls than in single-faced ones.