MMed Physics, Task 2

MMed Physics, Task 2

Question 1:

Name the different layers of input phosphor of an image intensifier tube and briefly give the purpose of each layer.

a)      Input Phosphor and Photocathode

i)        Cesium Iodide (CsI).  

(1)    The input fluorescent screen in image intensifiers is CsI.  CsI is deposited on a thin aluminium substrate by a process called “vapour deposition”.

(2)    Image quality is dramatically better with CsI (then with the older Zinc-cadmium sulphide screens). Minimal lateral light diffusion, and minimal light scattering by the “needle-shaped deposition perpendicular to the substrate”.  Three physical characteristics of CsI make it superior:

(a)    The vertical orientation of the crystals

(b)   A greater packing density, and

(c)    A more favourable effective atomic number

(3)    CsI can be vacuum deposited, it requires no inert binder, so more active material can be packed into a given space. The packing density is three times greater than zinc-cadmium sulphide. Phosphor thickness has been reduced from 0.3mm to 0.1mm with CsI. The principle advantage of a thinner phosphor layer combined with needle shaped crystals is improved resolution. The resolution with CsI is 4lp/mm.

(4)    The K-edges closer to the x-ray beam average energy.

(a)    The  mean energy of an x-ray beam is approximately 1/3 of its peak energy, depending on the kVp and filtration. Most fluoroscopy in adults are done between 80 to 120 kVp, which has a mean energy between 30 to 40 keV.

(b)   The k-edges of Cesium (36keV) and Iodide (33.2keV) are almost ideal. CsI absorbs almost 2/3 0f the incident beam, even though CsI is only one third as thick. 

ii)      Photocathode. Photoemissive metal. When light from the fluorescent screen strikes the photocathode, photoelectrons are emitted in numbers proportional to the brightness of the screen. Light from CsI is applied directly to the photocathode.

Question 2:

Name the different layers of output phosphor of an image intensifier tube and briefly give the purpose of each layer

i)        Output Phosphor.

(1)    Silver-activated zinc-cadmium sulphide

(2)     A thin layer of aluminium

ii)      Silver-activated zinc-cadmium sulphide. Diameter of the output image is reduced to 0.5 to 1 inch. Because the electrons are greatly accelerated, they emit more light photons from the output screen then were originally present at the input screen. The number of light photons is increased 50 fold.

A thin layer of aluminium is plated onto the fluorescent screen to prevent light from moving retrograde through the tube and activating the photocathode. The aluminium layer is very thin, and will not prevent high energy electrons from passing through it to the output phosphor

The glass tube image intensifier is 2 to 4 mm thick and is enclosed in a lead lined container to prevent the operator from stray radiation.

The output phosphor image is viewed directly through a series of lenses and mirrors or indirectly through Close Circuit Television

Question 3:

Which factors degrade contrast in an image intensifier tube?

a)      Contrast. Two factors diminish contrast in an image intensifier

i)        The input screen does not absorb all the photons in the x-ray beam. Some are transmitted through the intensifier tube, and few are eventually absorbed by the output screen. These transmitted photons contribute to the illumination of the output phosphor but not to the image formation. They produce background of fog that reduces image contrast.

ii)      Retrograde light flow from the output screen. Most retrograde light flow is blocked by a thin layer of aluminium on the back of the screen. Although the aluminium layer is extremely thin it would absorb electrons that convey the fluoroscopic image. Some light photons penetrate the aluminium and pass back through the image tube to activate the photocathode. These electrons produce “fog” and further reduce image contrast. The contrast tends to deteriorate as an image intensifier ages,   

b)      Lag. Lag is defined as the persistence of luminescence after the x-ray stimulation has been terminated. Older image tubes had long lad times, 40ms. CsI tubes lag times are 1ms.  

c)       Distortion. The center of the image intensifier screen has better resolution, a brighter image, and less geometric distortion.

i)         “Pincushion effect”. Peripheral electrons do not strike the output phosphor where they ideally should. They tend to flare out from their ideal course. The result is unequal magnification, which produces peripheral distortion. The further the electrons from the center the more difficult to control. (this makes it difficult to evaluate straight lines esp when tying to reduce a fracture).

ii)       Vignetting Unequal magnification also causes unequal illumination. The center is brighter than the periphery. A fall-off in brightness at the periphery of an image is called vignetting.

iii)     Unequal focus. Resolution is better in the center of the screen.

Question 4:

What is DAP? Give a typical value

DAP is a descriptor of radiation dose. Dose Area Product. DAP  is a product of the irradiated surface area multiplied by the radiation dose at the surface.

 http://radiographics.rsna.org/content/28/5/1439.full

__________________________________________________________________

Christensen’s Physics of Diagnostic Radiology, 4^th Edition, Thomas, et al.  Image Intensifiers, 166-172

Lesson 12, Fluoroscopic Imaging, MMed Physics

X rays were discovered because of their ability to cause fluorescence, and the first image of a human part was observed fluoroscopically. 

2)      The fluoroscope

a)      Visual Physiology

Rods, night vision, peripheral or scotopic vision for dim light. Fovea has no rods - Entirely from peripheral vision. Less acute then photopic vision of cones. Most sensitive to blue green light.

Rod vision is improved by motion.

Cones, bright light, central vision or photopic vision for daylight. Cones are concentrated in the fovea.

Dark adaptation is not destroyed by red light so radiologists used to use red goggles before fluoroscopy to protect the dark adaptation.

3)      Image Intensifier Design

The components of an x-ray image intensifier. The tube itself is an evacuated glass envelope, a vacuum tube, which contains four basic elements:

Input phosphor and photocathode

Electrostatic focusing lens

Accelerating anode

Output phosphor

a)      Input Phosphor and Photocathode

i)        Cesium Iodide (CsI).  The input fluorescent screen in image intensifiers is CsI.  CsI is deposited on a thin aluminium substrate by a process called “vapour deposition”. Minimal lateral light diffusion, and minimal light scattering by the “needle-shaped deposition perpendicular to the substrate”.  Image quality is dramatically better with CsI (then with the older Zinc-cadmium sulphide screens).

(1)    Three physical characteristics of CsI make it superior:

(a)    The vertical orientation of the crystals

(b)   A greater packing density, and

(c)    A more favourable effective atomic number

(2)    CsI can be vacuum deposited, it requires no inert binder, so more active material can be packed into a given space. The packing density is three times greater than zinc-cadmium sulphide. Phosphor thickness has been reduced from 0.3mm to 0.1mm with CsI. The principle advantage of a thinner phosphor layer combined with needle shaped crystals is improved resolution. The resolution with CsI is 4lp/mm.

(3)    The K-edges closer to the x-ray beam average energy. The  mean energy of an x-ray beam is approximately 1/3 of its peak energy, depending on the kVp and filtration. Most fluoroscopy in adults are done between 80 to 120 kVp, which has a mean energy between 30 to 40 keV. The k-edges of Cesium (36keV) and Iodide (33.2keV) are almost ideal. CsI absorbs almost 2/3 0f the incident beam, even though CsI is only one third as thick. 

ii)      Photocathode. Photoemissive metal. When light from the fluorescent screen strikes the photocathode, photoelectrons are emitted in numbers proportional to the brightness of the screen. Light from CsI is applied directly to the photocathode.

b)      Electrostatic Focusing Lens. The lens is made up of a series of positively charged electrodes that are usually plated onto the inside surface of the glass envelope. Electron focusing inverts and reverses the image. This is called point inversion. Figure 12-3. The input phosphor is curved to ensure that all electrons travel the same distance.

i)        Accelerating Anode. The anode is located in the neck of the image tube. Its function is to accelerate electrons emitted from the photocathode towards the output screen. The anode has a positive potential of 25 to 35 kV relative to the photocathode and accelerates electrons at a tremendous velocity.

ii)      Output Phosphor. Silver-activated zinc-cadmium sulphide. Diameter of the output image is reduced to 0.5 to 1 inch. Because the electrons are greatly accelerated, they emit more light photons from the output screen then were originally present at the input screen. The number of light photons is increased 50 fold.

A thin layer of aluminium is plated onto the fluorescent screen to prevent light from moving retrograde through the tube and activating the photocathode. The aluminium layer is very thin, and will not prevent high energy electrons from passing through it to the output phosphor

The glass tube image intensifier is 2 to 4 mm thick and is enclosed in a lead lined container to prevent the operator from stray radiation.

The output phosphor image is viewed directly through a series of lenses and mirrors or indirectly through Close Circuit Television

4)      Brightness Gain

The brightness gain tends to deteriorate as an image intensifier ages. Brightness gain is the ratio of two illuminations:

        Brightness gain = intensifier luminescence / Patterson B-2 luminesence

Conversion factor used to enhance reproducibility:

        Conversion factor = (cd/m²) / (mR/s)

a)      Minification Gain. The brightness gain from Minification is produced by a reduction in image size. The quality of the gain depends on the relative areas of the input and output screens.

Minification = ( d₁ / d_o ) ²

d₁  is the diameter of the input screen

 d_o is the diameter of the output screen

b)      Flux Gain. Flux gain increases the brightness of the fluoroscopic image by a factor of 50. For each photon at the input screen, 50 photons are emitted by the output screen.

Total Brightness gain = Flux gain x Magnification gain

Total brightness gain is the product of the voltage or the flux gain and magnification gain. 15 cm diameter input image intensifier tube ; output diameter 2,54 cm has a flux gain of 42. The brightness of the tube:

        Magnification gain = (input diameter)² / (Output diameter) ² = 15 ²/2.54² = 34,9

        Total Brightness gain = Flux gain x Magnification gain = 42 x 34,9 = 1466

5)      Imaging Characteristics

a)      Contrast. Two factors diminish contrast in an image intensifier

i)        The input screen does not absorb all the photons in the x-ray beam. Some are transmitted through the intensifier tube, and few are eventually absorbed by the output screen. These transmitted photons contribute to the illumination of the output phosphor but not to the image formation. They produce background of fog  that reduces image contrast.

ii)      Retrograde light flow from the output screen. Most retrograde light flow is blocked by a thin layer of aluminium on the back of the screen. Although the aluminium layer is extremely thin it would absorb electrons that convey the fluoroscopic image. Some light photons penetrate the aluminium and pass back through the image tube to activate the photocathode. These electrons produce “fog” and further reduce image contrast. The contrast tends to deteriorate as an image intensifier ages,   

b)      Lag. Lag is defined as the persistence of luminescence after the x-ray stimulation has been terminated. Older image tubes had long lad times, 40ms. CsI tubes lag times are 1ms.  

c)       Distortion. The center of the image intensifier screen has better resolution, a brighter image, and less geometric distortion.

i)         “Pincushion effect”. Peripheral electrons do not strike the output phosphor where they ideally should. They tend to flare out from their ideal course. The result is unequal magnification, which produces peripheral distortion. The further the electrons from the center the more difficult to control. (this makes it difficult to evaluate straight lines esp when tying to reduce a fracture).

ii)       Vignetting Unequal magnification also causes unequal illumination. The center is brighter than the periphery. A fall-off in brightness at the periphery of an image is called vignetting.

iii)     Unequal focus. Resolution is better in the center of the screen.

6)      Multiple Field Image Intensifiers.

a)      Large Field View Image Intensifier Tubes

Chest, MMed Anatomy

Chest, MMed Anatomy

v  The Chest

v  CXR:

Ø  Normal PA Chest x-ray (with accompanying diagram)

v  Trachea, Oesophagus, Clavicles

v  ii)      Chest wall (rib cage and pleural line)

v  iii)     Aortic Arch, Left main bronchus, Left Pulmonary Artery, Left Pulmonary Vein, Left auricular Appendage, Region of contact of oesophagus and left atrium, Apex of the left ventricle, Left Hemidiaphragm;

v  iv)     Apical right artery, Apical vein right, Superior Vena Cava, Azygos knob (6mm), Right Main Bronchus, Right Pulmonary Artery, Right Pulmonary Veins, Right Intermediate Bronchus, Right Middle lobe arteries and Bronchi, Right Atrium, Right Hemidiaphragm.

v  v)      Other additional markings on Atlas review:

Ø  Anterior Axillary fold, Spine of the Scapula,         

Ø  Normal Lateral Chest x-ray. Trachea

v  Trachea, Left Pulmonary Artery (behind), Right Pulmonary artery, Posterior wall of Bronchus Intermedius.  

v  Three-Dimensional CT Image of Bronchus Tree

v  Pulmonary angiogram

Ø  showing right and left pulmonary arteries

Ø  during venous filling phase

v  Azygos venogram

Ø  Frontal projection

Ø  Lateral projection

Ø  Diagram showing the relationship between the Azygos vein

v  And The right posterior aspect of the oesophagus

v  ii)       Superior Vena Cava

v  Bronchograms

Ø  AP view of the right bronchial tree

Ø  Lateral view if the right bronchial tree

Ø  Oblique view of the left bronchial tree

Ø  Lateral view of the left bronchial tree

Ø  Diagram of AP view of bronchial anatomy

v  RUL: Apical segment, Posterior Segment, Anterior Segment

v  ii)       ML: Lateral segment, Medial Segment

v  iii)     RLL: Superior Segment, Anterior Basal Segment, Lateral Basal Segment, Medial Basal Segment, Posterior Segment,

v  iv)     LUL: Apicoposterior, Anterior segment

v  v)      Superior Segment Lingula

v  vi)     LLL: Superior Segment, Anterior Medial Basal segment, Lateral Basal Segment, Posterior Basal Segment

Ø  Diagram of lateral view of right bronchial tree

v  i)         

Ø  Diagram of lateral view of left bronchial tree

v  Development of the lungs. A bifurcation of the lungs. A bifurcating diverticulum develops from the ventral pharynx.

v  Chest wall

Ø  Axial CT section showing the muscles of chest wall

v  Sternum, Rib,

v  ii)       Transverse thoracis, Pectoralis Major, Pectoralis Minor, Serratus Anterior, Intercostal Muscle, Subcostal Muscle, Rhomboid Muscle, Trapezius Muscle, Erector Spinae

Ø  Coronal MR section. The posterior chest wall muscles are well shown

v  Venous

Ø  Enhanced axial CT study showing the Azygos and hemiazygos veinsin an infant with SVC obstruction

Ø  Diagram illustrating mediastinal venous anatomy

v  Enhanced CT study showing the left superior intercostals vein

v  Coronal MR showing the trachea and the carina

v  Contiguous axial CT images showing bronchial tree.

Ø  Diagram illustrating anteroposterior view of the bronchial tree.

v  Mediastinal Vessels: Axial MR

v  Hilum: Representative plain tomogram

Ø  Azygos vein

Ø  Right Lower lobe Artery

Ø  Tributary of inferior pulmonary vein

v  Bronchus: PA Chest

Ø  Bronchus end on – anterior segment artery

Ø  Accompanying bronchus

v  Bronchogram showing terminal bronchodilator filling

Ø  Terminal Bronchiole

Ø  Acinar Filling

v  Diagram demonstrating bronchopulmonary segments

v  HRCT

Ø  Normal

Ø  Lymphangitis carcinomatosa

v  Anterior Junction line PA CXR, Posterior Junction Line

v  Anterior and posterior Junction line on CT

v  Oblique fissures and Horizontal fissures

Ø  Axial CT

v  Azygos fissure

Ø  CXR

Ø  CT

v  Inferior accessory fissure separates the medial basal segment from the anterior basal segment of the lower lobe. Consolidation in the anterior basal segment is sharply demarcated by the inferior accessory fissure.

v  Superior accessory fissure.  Horizontal fissure. Superior Accessory Fissure.

v  Inferior Pulmonary Ligament

v  Bronchial Arteriogram

v  Mediastinal contents, Coronal Images

v  Digital Subtraction Angiography demonstrating vessels and background

v  Mediastinal Spaces

Ø  PTS: Pretrachial Space

Ø  APW: aortopulmonary window

Ø  SCS: Subcarinal space

Ø  RPS: Right paratracheal space

Ø  PPTS: posterior tracheal space

Ø  A: Azygos Vein

Ø  O: Espohagus

v  Azygo-oesophageal line

v  American Thoracic Society lymph node mapping scheme

v  Thymus

v  Thymus in an infant

v  Pleuro-oesophageal line

v  Phrenic Nerve Arrow

v  Systemic review of a CXR:

v  Blind area review: Apices, Hila, Outer ends of the clavicles, Supraclavicular region, Retro-cardiac region, Lung below the dome of the diaphragm, and Strip inside the chest wall

v  Mediastenal and connective tissue compartments in the mediastinum are divided into different spaces:

Ø  Pretrachial space,

Ø  Aorticopulmonary window,

Ø  Subcarinal Space, and

Ø  Right pretrachial space

Imaging Methods

v  CXR frontal and lateral projections

v  CT

Ø  CT Assessment of the mediastinum, lung and parenchymia and the thoracic wall. 10mm is standard

Ø  Spiral CT – ensures completeness

Ø  HRCT- High resolution CT uses slice thickness of 1-2 mm, fine assessment of the lung parenchymia, pleura and tracheobronchial tree

Ø  MRI, non-ionizing radiation imaging method

Ø  Angiography is replaced by MRI and CT

Development of the Respiratory tract

v  Development of the trachea and the lungs

Ø  Tracheobronchial groove: Slit on the ventral end of the caudal oesophagus

Ø  Downward growing diverticulum

v  Bifurcates to form the primary bronchial branches

v  ii)      Respiratory alveoli is lined by cuboidal epithelium in embryonic life, transfer to pavement epithelium of the alveoli is accompanied when respiration commences at birth.

Ø  Persistent tracheo-oesophageal fistula , indicating the close relationship between the respiratory tract and the foregut. Associated with atresia of the oesophagus, the fistula situated below the atresia.

Ø  3/40 lung buds (ventral outpunching of the endodermal foregut). Bifurcation into the right and left primary bronchial buds. Progressive branching produces the pulmonary respiratory tree. The stem produces the trachea and the larynx.

Ø  Alveoli surrounded by a dense network of capillaries. Pulmonary Agenesis or hypoplasia

Anatomy

The chest wall

v  Blood Supply to the Chest wall

Ø  Posterior Intercostal and anterior intercostal arteries running in the subcostal groove. Each posterior intercostal artery gives off a spinal branch for the vertebrae and the spine.

§   Lower Nine pairs of posterior intercostal arteries are derived from the thoracic aorta for the lower nine intercostals spaces.

§  First and the second intercostals spaces are supplied by the superior intercostal artery derived from the subclavian artery via its costocervical trunk.

§  Anterior intercostal artery. The internal thoracic artery from the subclavian artery for the upper six intercostal spaces.  The internal intercostal artery terminates in the musculophrenic artery and the superior epigastric artery.

Ø  The Intercostal Venous System

§  Anterior veins drain into the musculophrenic vein and the internal thoracic veins.

§  Posterior veins drain into the Brachiocephalic vein and the Azygos system.

§  Azygos system ascends to the level of the fourth thoracic vertebrae. It arches forward above the root of the right lung and ends in the posterior SVC just before it pierces the pericardium, approximately 1cm below the junction of the right and the left Brachiocephalic Veins.

·         Usually the Azygos remains in the right mediastinum and occupies the right tracheobroncial angle.

·         Azygos Lobe: 1% the Azygos transverses the lung before  entering the SVC. The SVC appears distorted on plain films and CT.

·         The Azygos lies:

¨       Anterior to the:

Ø  bodies of the lower eight thoracic vertebra;

Ø  The anterior longitudinal ligament

·         The right posterior intercostal arteries just right to the midline.

¨       On the right are:

Ø  The great splanchnic nerves;

Ø  Lung and pleura

·         On its left,

¨       Thoracic duct and Aorta, throughout the greater part of its course

¨       Oesophagus, Trachea and the Right Vagus, where it arches forward above the root of the lung.

¨       Drains the:

Ø  posterior intercostal veins on the right side, with the exception of the first, the second, third and the fourth intercostal spaces usually drain via a common stem called the right superior intercostal vein.

Ø  Hemiazygos veins and the accessory hemiazygos veins, several oesophageal, mediastinal and pericardial veins, and near its termination, the right bronchial veins.

¨       “AZYGOS CONTINUATION OF THE IVC”: Occasionally the IVC does not develop in the usual fashion, and the Azygos vein then forms the venous conduit draining the IVC blood back to the blood back to the heart. Don’t confuse the Azygos continuation of the IVC with mediastinal mass or lymphadenopathy on imaging studies.

¨       Grants p75 to p87

Ø   Hemiazygos

¨       Lower Hemiazygos lies to the left of the midline, anterior to the vertebrae to the 8^th thoracic vertebrae

¨       Passes across vertebral column, anterior to the vertebrae  and behind the aorta, oesophagus, and thoracic duct.

¨       The ascending lumber vein and the subcostal veins of the left side, and some oesophageal and mediastinal veins.

Ø   Accessory Hemiazygos vein lies from the forth to the eight intercostal spaces on the left side

¨       Crosses the vertebrae at T7 that opens with the Hemiazygos veins into the Azygos vein via a common trunk.

Ø  Left superior intercostal vein

Ø   Anterior intercostal veins drain to the Internal thoracic artery that drain into the Brachiocephalic veins. In SVC Obstruction the chest wall collaterals, including those from the lateral thoracic veins, can be observed draining to the IVC via the musculophrenic branches.    

The Nerves to the Chest wall

v  12 Thoracic nerves on each side

v  The Sympathetic Chain

v  Cardiac, oesophageal, and pulmonary plexus

Ø   Greater splanchnic nerves formed by filaments from the Fifth to the Ninth thoracic vertebrae. Branches to the descending aorta.

Ø  Lesser Splanchnic nerve formed by the ninth and tenth thoracic ganglia. Pierce the crus diaphragm to reach the abdominal ganglia.  

v  The lungs and the airways

Ø  10 tertiary segments on the right and 8 tertiary segments on the left. On the left the apical and the posterior segment have a single stem, as do the anterior basal and medial basal.

§  The right bronchus is shorter and wider and more vertical then the left. Therefore more susceptible to aspirated foreign objects.

§  Right Superior lobar bronchus (eparterial bronchus), before the hilus

§  Intermediate Bronchus, divides after entering the hilus, into right middle and inferior bronchus

Ø  Left main bronchus divides into left superior and left inferior bronchus, further divide into segments  

v  The Pulmonary Hila

Ø  Composed mainly of blood vessels and major bronchi. Lymph nodes, nerves and connective tissue can’t be seen. The right main bronchus and bronchus Intermedius is outlined posteriorly by lung that appears as a stripe posteriorly.

v  The Lungs beyond the Hila

Ø  Segmental bronchi do not have cartilage after progressive 6 to 20 divisions. Invisible except if seen as end-on ring shadows but are clearly seen on CT or bronchography.  The lung exchange units, Acini lie beyond these.

v  The Brochopulmonary segments

Ø  Variable, identified on bronchography and CT.

v  Lobules of the Lungs (secondary pulmonary lobes)

Ø  Secondary pulmonary lobes are composed of upto 10 acini. Acini composed of respiratory bronchioles, alveolar ducts and alveoli. Surrounded by connective tissue septa that contain veins and lymphatic vessels.

Ø  CXR, upright position, there is a gradual increase in the diameter of both the arteries and the veins from the apex to base.

v  The Lymphatic network

Ø  Invisible radiologically

Ø  Grant p43. The subpleural lymphatic vessels are found just beneath the pleura where they interconnect with each other and the interlobular septa. Connect to the helium by way of lymphatic channels that run peribronchially and in the deep septa.

v  The Pleura

Ø  The visceral and parietal pleura are continuous with each other at the Hilum of the lung and the inferior pulmonary ligament.

Ø  Junctional Lines: where the two lungs come into contact with one another

Ø  Parietal pleura is supplied by the intercostal arteries and the branches of somatic intercostal and phrenic nerves. Sensitive to pain.

Ø  Visceral pleura, is supplied by the pulmonary and bronchial arteries. The visceral pleura is insensitive to pain because it only receives autonomic innervations.

Ø  Azygos fissure and lobe

v  Bronchial Arterial Supply

Pulmonary Arteries and veins

v  The Mediastinum

The normal Mediastinum

v  CT and MRT

Ø   Heart and blood vessels, the major airways, and the oesophagus

Ø   The oesophagus

§  From the neck to the oesophageal hiatus in the diaphragm

Ø  1cm at its narrowest diameter

Ø  The Thoracic Duct and its tributaries

§  Invisible on CT unless lymphangiographic contrast is administered

§  2 to 8 mm in diameter

§  Transports all the lymph except that from the lungs and the right upper quadrant of the body.

§  From the cysterna chyli behind the medial arcuate ligament and ascend between the Azygos vein and aorta. At T6 the thoracic duct crosses to the left of the spine and asends to the lateral aspect of the oesophagus behind the aorta and the left subclavian artery. It arches across the subclavian artery to insert in the large central vein within 1cm of the junction of the internal jugular and subclavian veins.   

v  Mediastinal Blood Vessels

Ø  Ascending aorta diameter is 3.5cm, Descending aorta diameter is 2.5cm

Ø  3 major aortic branches arranged in a curve: Brachiocephalic, Left Common carotid Artery, and left Subclavian Artery. The Brachiocephalic artery is larger than the others.

Ø  Aberrant Right Subclavian artery: 0.5% the Right subclavian lies distal to the left subclavian artery as a fourth major branch of the aorta. It arises from behind the oesophagus from left to right, or at just above the level of the aortic arch, to lie against the right side of the vertebral bodies before entering the root of the neck. Here the right common carotid artery will be smaller and will equal in diameter to the left common carotid artery.  Grant p71 C.

Ø  Descending Aorta travels through the chest from the left of the vertebral bodies to the midline where it exits through the aortic hiatus of the diaphragm.

v  Brachiocephalic Veins on both side are formed by union of the ipsilateral internal jugular vein and the subclavian veins.

Ø  The left Brachiocephalic vein oblique behind the manubrium sterni to the sterna end of the right first costal cartilage where it unites with the right Brachiocephalic vein to form the SVC.

§  It crosses the front of the pleura over the left lung apex, the left internal thoracic artery, the left subclavian and common carotid artery,

§  the left phrenic and vagus nerves

§  the trachea and

§  the Brachiocephalic artery

§  It has tributaries from the left vertebral, internal thoracic, inferior thyroid, and superior intercostals veins, and sometimes from the left posterior intercostals vein and the thymus and pericardial veins

§  OVAL appearance on axial CT because of its oblique course.

Ø  The right Brachiocephalic vein behind the sternal end of the right clavicle, and passes almost vertically downward to join the left Brachiocephalic vein to form the SVC below the lower boarder of the first right costal cartilage, close to the right boarder of the sternum

§  Anteriolateral to the Brachiocephalic artery and the right vagus nerve

§   Tributaries from the right vertebral, internal thoracic and inferior thyroid veins, and

§  Sometimes the first posterior intercostals vein

§  CT most lateral vessel, larger than the arteries and has an oval shape

v  SVC

Ø  Collects blood from the upper half of the body.

Ø  Formed by the junction of the two Brachiocephalic veins, Behind the lower boarder of the first right costal cartilage, close to the sternum

Ø  The lower half of the vessel in within the pericardium

Ø  In front, the right lung and its pleura cover the SVC and separate it from the Internal thoracic artery and the 2^nd and 3^rd costal cartilage

Ø  Trachea and the right vagal nerve are posteriomedial

Ø  Phrenic nerve to the right of the SVC with lung pleura

Ø  Left: Ascending aorta and the Brachiocephalic artery

Ø  Tributaries: ascending azygos vein and small pericardial veins.

Ø  CT: diameter 2/3 of the ascending aorta oval or round

Ø  Persistent left SVC: 0.5% of the population.

§  Failure of obliteration of the left cardinal vein of fetal development.

§  A right SVC vein and an interconnecting Brachiocephalic vein persists

§  Joins from the left jugular and subclavian veins and travels vertically through the left mediastinum along the left side of the aortic arch passing anterior to the left main bronchus.

§  Drains into the coronary sinus on the posterior surface of the heart (obviously enlarged because of the increased blood flow), the blood flows from the coronary sinus to the Right atrium (where else could it have gone)

§ 

The Inferior Vena Cava

v  Seen before it enters the right atrium.

v  Azygos continuation of the inferior vena cava

The ‘mediastinal Spaces’ http://radiographics.rsna.org/content/27/1/33.full

v  Central spaces

Ø  Pretracheal space

Ø  The aortapulmonary window

Ø  Subcarinal space

Ø  Right paratracheal space

v  Junctional areas , where the lungs approximate each other

Ø  Anterior juctional line. Anterior to the aorta and pulmonary artery

Ø  Posterior junctional line. Posterior to the trachea and oesophagus.

Ø  Paraspinal lines

Ø  Retrocrural lines

v  Pretracheal space

Ø  CT: Triangle with the trachea or carina posteriorly, the SCV or right Brachiocephalic Artery on the Right, and the Aortic Arch on the left with its Superior Pericardial Recess is a small pocket of the pericardium investing the aorta.

Ø  Fluid in the pericardial recess may be confused with lymphadenopathy on CT and MRI

v  The aortopulmonary window

Ø  Under the aortic arch but above the pulmonary artery.

Ø  Bound Medially by the trachea and oesophagus and laterally by the lung pleura

Ø  Contains: Ligamentum arteriosum and the Recurrent laryngeal nerve. The ligamentum arteriosum may be calcified and seen on plain radiographs or on CT

v  The Subcarinal space

Ø  Bronchi on either side and esophagus posteriorly

Ø  Azygo-oesphageal recess of the right lung lies behind the subcarinal space

v  The right paratracheal space and the posterior tracheal space

Ø  The right paratracheal stripe. 1to 4 mm thick.

Ø  The right lung is separated from the trachea by a thin layer of fat. The only exception is the tracheobronchial angle where the azygos vein lies between the lung and the airway.

v  The left paratracheal stripe

Ø  Seen in 3 to 5%. Left Common Carotid Artery and Left Subclavian Artery lie against the left wall and trachea preventing lung from touching the left tracheal wall.

v  The anterior junction

Ø  Anterior to the pulmonary artery, the ascending aorta and the tree major branches of the aortic arch.

Ø  Represents the contact of the right and left lungs against the mediastinum. 4 layers of pleura together with some intervening fat.

Ø  The anterior junction line never extends above the clavicles

Ø  Obscured by the sternum, heart, vertebrae, vessels or thymus

Ø  At an oblique angle to x-rays (http://radiographics.rsna.org/content/27/1/33/F2.large.jpg)

Ø  Superiorly is the Left Brachiocephalic vein. Left is the internal thoracic vascular bundle

Ø  Contains the phrenic nerve, thymus, fat and the lymph nodes. Lymph nodes are:

§   prevascular,

§  along internal thoracic artery,

§  anterior cardiophrenic angle

v  The posterior junction and the paraspinal areas http://radiographics.rsna.org/content/27/1/33/F7.large.jpg

Ø  Formed by the supra azygos and supra aortic lung coming into contact behind the oesophagus

Ø  Extends from the thoracic inlet (above the clavicles) to the level of the ascending aorta

Ø  Higher more vertical to anterior junctional line and may reconstitute below the Ascending

Ø  Posterior to the trachea and the heart

Ø  The right lung invaginates behind the hilar structures and heart to contact the pleura overlying the azygos vein and the oesophagus, forming the azygo-oesophageal recess.

Ø  On the left the lung interfaces with the descending aorta till it nearly reaches the midline.

v  The retrocrural Space

Ø  Structures that pass with the aorta through the aortic hiatus, which is bound by the diaphragmatic crura and the spine.

                                       i.      Azygos and hemiazygos veins,

                                      ii.      The commencement of the thoracic duct and the cisterna chylii

v  Mediastinal Lymph nodes

Ø  Right and left paratracheal lymph nodes,

§  Upper and Lower groups in relation to the aortic arch

Ø  Aortapulmonary window nodes lateral to ligamentum arteriosum

Ø  Subcarinal nodes , beneath the carina and the main bronchi

Ø  Tracheobronchial nodes and hilar nodes, adjacent to the right and left main bronchi

Ø  Paraoesphageal and pulmonary ligament nodes

v  Normal Lymph node size

Ø  Mediastinal Lymph nodes: 95% are < 10mm in the short axis and 5% < 15mm

Ø  Brachiocephalic Vein nodes are <5mm

v  The Thymus

Ø  Anterior to the aorta and the right pulmonary artery.

Ø  Inferior to the Left Brachiocephalic Vein on CT

Ø  Seen best at section trough aortic arch

Ø  Projects upward as far as the thyroid gland

Ø  Children under 5 years, sharp angular boarder, a sail sign on plain films

v  Paracardial area:

Ø  Heart, mediastinal fat, pericardium, praracardial lymph nodes

v  Hilar point:

Ø  Left hilar point is 1 to 2 cm higher than the right hilar point

Normal Mediastinal contours on plain chest radiographs

Frontal Projection

v  The left mediastinal boarder

Ø  Left  Subclavian artery or the adjacent fat (sometimes the left carotid artery). Above the aortic arch , the usual appearance is a greatly curving boarder which fades out at the interface with the neck

v  Below the aortic arch the border is formed by the aortic-pulmonary pleural stripe, the main pulmonary artery and the heart.

v  Children the thymus may form the left mediastinal boarder

v  Small “nipple” projecting laterally from aortic arch due to left superior intercostals vein

v  The left boarder of the descending aorta can be traced through the shadow of the main pulmonary artery and heart as a continuous boarder of the aortic hiatus in the diaphragm

v   The right mediastinal boarder

Ø  Formed by the right Brachiocephalic vein, the SVC and the Right Atrium

Ø  Thymus is prominent in babies and young children, sail shape is characteristic

Ø  Right Paratracheal Stripe: is seen through the right Brachiocephalic vein and the SVC because the lung contacts the trachea from the clavicles down to the arch of the azygos vein (? At T4). The stripe is of uniform thickness of 3mm and consists of the wall of the trachea and the paratracheal fat.

Ø  The Azygos vein: is outlined by air at the lower end of this stripe. The diameter of the azygos vein is 8mm.

v   The anterior junction line

Ø  Anterior to the aorta, where two lungs are adjacent to each other. The line fades out superiorly as it reaches the clavicles. It cannot extend to the point where the two lungs separate to envelope the right ventricle. 

v   The posterior junction line and azygoespohageal recess

Ø  The lungs touch each other behind the oesophagus to form the posterior junction line. The line envelops the azygos and aortic arch. Extends to the lung apices where it diverges and disappears in the root of the neck, well above the clavicles.

Ø  Width depends on the amount of mediastinal fat.  

Ø  Pleuro-oesohageal stripe / line:

§  Above the arch of the aorta

§  Right extends from the pulmonary apex to the right main bronchus where it deviates to the right over the azygos vein. Visible through the trachea in 10% adults and 50% kids.

§  Left pleura-oesophageal stripe: identified by the aortic knuckle and left main bronchus. Less frequently seen because of teh great vessels that obliterate it.  

Ø  Azygooesophgeal Line / reflection:

§  Below the arch of the aorta where it makes contact with the azygos vein and the esophagus.

§  Upper part should always be straight or concave towards the lung.

§  Convex mass suggest a subcarinal lymph node mass.

v   The paraspinal lines

Ø  The right and left paraspinal lines are normally less than 1cm wide.

Ø  Aortic unfolding contributes to the left paraspinal line. The left is thicker (<1cm) than the right (4mm) as the descending aorta strips the pleura from the tight association of the spine.

ii)      Lateral View

v  The mediastinum above the aortic arch

Ø  Brachiocephalic artery lies anterior to the tracheal air column, its posterior boarder forms an S-shape across the trachea

Ø  The braciocephalic vein forms an extrapleural bulge behind the manubrium.

v  The trachea and retrotracheal area

v  The retrosternal line

v  Supra-azygos and supra aortic area

Ø  Azygos vein in the right tracheobronchial angle <1cm in erect view

Ø  Axygos node Above and medial to the vein. Distinguished from the vein  as vein changes in size in supine position

v  Infra Azygos and Infraaortic Region

Ø  Aorticopulmonary window

Ø  Azygooesophgeal reflection

v  The diaphragm

Ø  Hiatus

§  Aortic hiatus: Aorta, azygos vein, hemiazygos veinand thoracic duct.

§  Oesophageal Hiatus: anterior to the aortic hiatus. Oesophgus, the Vagus Nerves, the oesophageal Vessels.

§  IVC hiatus: Most anterior, IVC

Ø  Right Hemidiaphragm: Full inspiration the diaphragm is found at the level of the 6^th Rib. Higher in women and in those above 40 years. 1.5 to 2.5 cm higher then the left hemidiaphram.

Ø  A linear density is seen arising from the IVC across the surface of the right diaphragm, this is the right phrenic nerve.