understanding hrct

UNDERSTANDING HRCT THORAX

LUNG ANATOMY

• Right lung is divided by major and minor fissure into 3 lobes and 10 bronchopulmonary segments

• Left lung is divided by major fissure into 2 lobes with a lingular lobe and 8 bronchopulmonary segments

ANATOMY• The trachea (windpipe) divides into left and the right

mainstem bronchi, at the level of the sternal angle (carina).

• The right main bronchus is wider, shorter, and more vertical than the left main bronchus.

• The right main bronchus subdivides into three lobar bronchi, while the left main bronchus divides into two.

• The lobar bronchi divide into tertiary bronchi, also known as segmentalinic bronchi, each of which supplies a bronchopulmonary segment.

ANATOMY• The segmental bronchi divide into many

primary bronchioles which divide into terminal bronchioles, each of which then gives rise to several respiratory bronchioles, which go on to divide into two to 11 alveolar ducts. There are five or six alveolar sacs associated with each alveolar duct. The alveolus is the basic anatomical unit of gas exchange in the lung.

TRACHEAL ANATOMY• 10-12 cm in length• Extrathoracic (2-4cm) and Intrathoracic(6-9cm beyond

manubrium)• In men, tracheal diameter averages 19.5 mm and in women,

tracheal diameter is slightly less, averaging 17.5 mm• The posterior portion of the tracheal wall is a thin

fibromuscular membrane termed the posterior tracheal membrane

• There is marked variability in the cross-sectional appearance of the trachea, which may appear convex posteriorly, flat, or convex anteriorly

The membranous posterior membrane allows esophageal expansion during expiration

Contains glands, small arteries, nerves, lymph vessels and elastic fibers

Trachealis muscle overlies esophageal muscle and epithelium

BRONCHIAL ANATOMY• Airways divide by dichotomous branching, with

approximately 23 generations of branches from the trachea to the alveoli.

• The wall thickness of conducting bronchi and bronchioles is approximately proportional to their diameter.

• Bronchi with a wall thickness of less than 300 um is not visible on CT or HRCT.

• As a consequence, normal bronchi less than 2 mm in diameter or closer than 2 cm from pleural surfaces equivalent to seventh to ninth order airways are generally below the resolution even of high-resolution CT

BRONCHUS• BLOOD SUPPLY Bronchial Arteries( 2 on left side i.e. superior

and inferior and 1 on right side) Left arises from thoracic aorta Right from either thoracic aorta, sup. lt. bronchial or right 3rd

intercostal artery

• VENOUS DRAINAGE on right- azygous vein on left- left superior

intercostal or accessory hemiazygous vein

• NERVE SUPPLY Pulmonary plexus at hilum (vagus and sympathetic)

BRONCHOARTERIAL RATIO (B/A)

• Internal diameter of both bronchus and accompanying arterial diameter calculated and ratio measured.

• If obliquely cut section seen, then the LEAST diameter is considered.

• Normal ratio is 0.65-0.70

BRONCHIAL WALL THICKNESS (T/D)

• Wall thickness proportionately decreases as the airway divides further as according to the diameter of the airway.

• T/D ratio approximates to 20% at any generation of airway.

The Nomenclature Adopted by the Ad HOC lnternational Committee Meeting at the Time of the lnternational Congress of Otorhinolaryngology in 1949 [I]"

International Nomenclature

Brock Jackson and Huber

Right upper lobe bronchusApical (RB1)Posterior (RB2)Anterior (RB3)Middle lobe bronchusLateral (RB4)Medial (RB5)Right lower lobe bronchusApical (RB6)Medial basal (cardiac) (RB7)Anterior basal (RB8)Lateral basal (RB9)Posterior basal (RB10)

PectoralSubapical

Apical

LateralMedial

ApicalCardiac

Anterior basalMiddle basal

Posterior basal

AnteriorPostenor

Apical

LateralMedial

SuperiorMedial basal

Anterior basalLateral basal

Posterior basal

The Nomenclature Adopted by the Ad HOC lnternational Committee Meeting at the Time of the lnternational Congress of Otorhinolaryngology in 1949 [I]"

International Nomenclature

Brock Jackson and Huber

Left upper lobe bronchusUpper divisionApical (LB1)Apicoposterior LB1 and LB2Posterior (LB2)Anterior (LB3)LingulaSuperior (LB4)Inferior (LB5)Left lower lobe bronchusApical (LB6)Anterior basal (LB8)Lateral basal (LB9)Posterior basal (LB10)

ApicopectoralApical

SubapicalPectoral

UpperLower

ApicalAnterior basalMiddle basal

Posterior basal

ApicalApical-posterior

PosteriorAnterior

SuperiorInferior

SuperiorAnterior medial basal

Lateral basalPosterior basal

MEDIASTINUM

• Broad central portion that separate the two laterally placed pleural cavities.

• Imaginary plane passes through T4 divides it into Superior & Inferior mediastinum

• Inferior mediastinum is further divided-Heart enclosed in pericardium (M)Sternum to anterior pericardium (A)Posterior pericardium to vertebrae (P)

INTERSTITIAL ANATOMY

• Lung is supported by a network of connective tissue called interstitium

• Interstitium not visible on normal HRCT but visible once thickened.

• Interstitium is constituted by AXIAL fibre system (peribronchovascular & centrilobular), PERIPHERAL fibre system (subpleural & interlobular septa) and SEPTAL fibre system (intralobular septa)

Secondary Lobule

• It is the smallest lung unit that is surrounded by connective tissue septa.

• It measures about 1-2 cm and is made up of 5-15 pulmonary acini, that contain the alveoli for gas exchange.

• The secondary lobule is supplied by a small bronchiole (terminal bronchiole) in the center, that is parallelled by the centrilobular artery.

• Pulmonary veins and lymphatics run in the periphery of the lobule within the interlobular septa.

• Every CT scan starts with a scout view, a projection image that looks like a second rate X-ray.

• A line on scout view tells you the level of axial cut.

HOUNSFIELD UNIT (HU)

• HU scale is a linear transformation of the original linear attenuation coefficient measurement into one in which radiodensity of distilled water at STP is defined as zero HU, while radiodensity of air at STP is defined as -1000 HU.

• Fat -50 to -100 HU• Blood +30 to 45 HU• Bone >+400 HU• Muscle +40HU• Contrast +130 HU

APPEARANCE ON CT SCAN

• AIR JET BLACK• FAT MODERATELY BLACK• WATER GRAY• MUSCLES SLIGHT WHITE• BONES WHITE• CALICIFICATION DENSE WHITE

LUNG WINDOW

MEDIASTINAL WINDOW

BONE WINDOW

LOBAR AND BRONCHIAL ANATOMY ON HRCT THORAX

RIGHT APICAL SEGMENT

LEFT APICAL SEGMENT

TRACHEA

ESOPHAGUS

RB1 LB1

CARINA

LEFT MAIN BRONCHUSRIGHT MAIN

BRONCHUS

RB2 RB1

RB3LB3

LB1,2

BRONCHUS INTERMEDIUS LEFT UL

BRONCHUS

RIGHT ML BRONCHUS

RB5

LUL

LLLRLL

RML

RB5 LB4

LB5

RLL BRONCHUS

LLL BRONCHUS

LB6

LINGULAR BRONCHUS

RB6

RB7LB6

RLL BRONCHUS LLL BRONCHUS

MAJOR FISSURE

RB8

RB9RB10 LB10

LB9

LB8

RLL

LML

LLL

RML

LB2

LB6

LB10

RB7

RB10

RB9

RB6

RB2

RB1

RB2

LB9

LB1,2

UL

LL

UL

LLML

RB1 joining RUL bronchus LB1,2 joining LUL bronchus

LB8RB8

CARINA

RC2

LC2

RB1

RB3

RB8

ML Bronchus

LB4

LB3

RB3

RB5RB4

LB5

LB3

VASCULAR ANATOMY ON CT THORAX

Rt. CCA

Rt. IJV

Rt. EJV

Lt. IJV

Lt. EJV

Lt. CCA

Rt. BCVRt. SCV joining Rt. BCV

Rt. CCA

Lt. CCALt. SCV

Rt. SCALt. SCA

Lt. BCV joining Rt. BCVRt. BCV

Rt. BCA

Lt. CCA Lt. SCA

SUPRA AORTIC LEVEL

Formation of SVC

Branching from Aortic Arch

AORTIC ARCH

SVC

AORTIC ARCH LEVEL

Ascending AORTA

Main Pulmonary TrunkSVC

Right Pulmonary Trunk Left Pulmonary Trunk

Descending AORTA

Aorta arising from Left Ventricle

Pulmonary Trunk arising from Right Ventricle

SVC draining into Right Atrium

Pulmonary Veins draining into Left Atrium

Pulmonary Veins

RV

LV

LA

RA

RV

LVIVS

DA

IVC

RV

LV

DA

LYMPH NODE STATIONS ON CT THORAX

LYMPH NODES STATION IN THORAX

1. SUPRACLAVICULAR NODES

• LOW CERVICAL • SUPRACLAVICULAR• STERNAL NOTCH

Extends from the lower margin of the cricoid cartilage to the clavicles and the upper border of the manubrium.

The midline of the trachea serves as border between 1R and 1L.

2. UPPER PARATRACHEAL NODES

2R. Upper Right ParatrachealExtends to the left lateral border of the trachea. From upper border of manubrium to the intersection of caudal margin of innominate (left brachiocephalic) vein with the trachea.2L. Upper Left ParatrachealFrom the upper border of manubrium to the superior border of aortic arch.2L nodes are located to the left of the left lateral border of the trachea.

3A. Pre-vascularThese nodes are not adjacent to the trachea like the nodes in station 2, but they are anterior to the vessels.

3P. Pre-vertebralThese nodes are not adjacent to the trachea like the nodes in station 2, but behind the esophagus, which is prevertebral.

4. LOWER PARATRACHEAL NODES

4R. Lower Right Paratracheal From the intersection of the caudal margin of innominate (left brachiocephalic) vein with the trachea to the lower border of the azygos vein.4R nodes extend from the right to the left lateral border of the trachea.

4L. Lower Left Paratracheal From the upper margin of the aortic arch to the upper rim of the left main pulmonary artery.

5-6. AORTIC NODES

5. Subaortic nodesThese nodes are located in the AP window lateral to the ligamentum arteriosum. These nodes are not located between the aorta and the pulmonary trunk but lateral to these vessels.

6. Para-aortic nodesThese are ascending aorta or phrenic nodes lying anterior and lateral to the ascending aorta and the aortic arch.

Inferior Mediastinal Nodes 7-9

7. Subcarinal nodes Nodes below carina

8. Paraesophageal nodes Nodes lateral to esophagus

9. Pulmonary Ligament nodes Nodes lying within the pulmonary ligaments.

Hilar, Lobar and (sub)segmental Nodes 10-14

These are all N1-nodes.10. Hilar nodesThese include nodes adjacent to the main stem bronchus and hilar vessels.

On the right they extend from the lower rim of the azygos vein to the interlobar region. On the left from the upper rim of the pulmonary artery to the interlobar region.

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