Densitometer

A densitometer is used to study the relationship between the intensity of the exposure of the film and the blackness after processing (sensitometry). In order to do this, two pieces of apparatus are needed: an aluminum step wedge, sometimes called a penetrometer, and the densitometer. The steps involved are:

.First, the film under investigation is exposed through the aluminum step wedge at some standard technique (for example, 70 kVp with 2.5 mm A1 total filtration). When processed, the x-ray film will have areas of increasing density corresponding to sections of the step wedge with decreasing thickness. The step wedge is fabricated so that the intensity of exposure to the film under each step can be determined.

 .The processed film is analyzed in the densitometer, a device that has a light source focused through a pinhole with a light-sensing device positioned on the opposite side of the film. The x-ray film is positioned between the pinhole and the light sensor, and the amount of light transmitted through each segment of the radiograph is measured. These data are recorded and analyzed and when plotted result in a characteristic curve.

.It is not the absolute exposure that is of greatest interest but rather the change in density over each exposure interval.

. The useful range of radiographic densities is approximately 0.25-2.5. However, approximately 75% of all radiographs show image patterns in the range of 0.5-1.25 optical density.

Ear

Ear

Ear is the organ of special sense of hearing. It is also responsible forequilibrium. It is divided into three parts.

Parts of ear are

1. External Ear Lying outside the skull

2. Middle Ear Lying inside the skull

3. Internal ear
External ear : It contains two parts.They are

• Pinna - Funnelshaped organ made of fibroelastic cartilage. It is the organ of collection of sound waves .
• External auditory meatus - small channel of about 3cm length.It is lined with skin and wax creating glands are contained in this part. Hair and wax present in its outer part prevent dust particles.Its inner part is closed by a thin membrane called tympanic membrane or ear drum.This canal is the organ of
conveyance of vibrations of sound to the tympanic membrane.

Middle ear : It is a small cavity in the temporal bone,intrnal to eardrum. i.e.tympanc membrane forms its outer wall.It contains air. It contains

1. Fenestra ovalis (oval window) and fenestrarotundum (round
window).Round window is also called fenestra cochleae.

2. Eustachian tube - which communicates with nasopharynx.It helps in equalisation of pressure on both sides of tympanic membrane.
3. Auditus - Channel connecting middle ear posteriorly with mastoid antrum of temporal bone.

4. Auditory ossicles - Malleus, incus and stapes arranged across middle ear. These are minute bones of middle ear and are bound by ligaments.Theyvibrate as a single unit when sound waves impinge on tympanic membrane.

Internal ear: It contains

1. Bony labyrinth - present in petrous portion of temporal bone.
2. Membranous labyrinth - lyeing with the bony labyrinth

Fluids of Internal ear :  Perilymph is the fluid of bony labyrinth. Endolymph is the fluid of membranous labyrinth. Structures of bony labyrinth :Bony labyrinth contains vestibule, cochlea and semi circular canals. Cochlea is the organ of hearing and semicircular canals for equilibrium.

Vestibule : It is present between vestibule and semicircular canals. Vestibule contains utricle and sacule. Utricle and saccule are parts of membranous labyrinth Cochlea. It is a bony spiral canal. These spirals wind rpund a central bony pillar Modiolus. Basilar membrane is membranous septum dividing cochlea into two parts. Organ of corti is the neuroepithelium of cochlea.It is auditory receptor resting on basilar membrane. Cochlear nerve fibres enter the organ of Corti. Vestibulocochlear nerve collects sensation of equilibrium from vestibular division. It col lects sensation of hearing from cochlear division. Auditory nerve fibres reach special nucleus on the back of thalamus and then cerebral cortex.

Semicircular Canals: Each ear contains three semilarcular canals. They are arranged at right angles to each other. They are superior, posterior and lateral canals. Ampulla is enlarged end of each canal. Vestibular nerve endings are present in ampullae. Ampullae help cerebellum in maintaining equilibrium. Semicircular
canals are for informing dynamic equilibrium and otolithic organ for static equilibrium.

PARTS OF RESPIRATORY SYSTEM

Parts of the Respiratory System
1. Nose
2. Pharynx
3. Larynx
4. Trachea
They lead to the lungs.
5. Bronchi
6. Bronchioles
7. Alveolar ducts
8. Alveoli

They are within the lungs.
 Upper respiratory tract extends from upper nares to the vocal cord. Lower respiratory tract extends from vocal cord to the alveoli

1. Nose
It is the part of respiratory system through which Air is inhaled in and exhaledout.

External nose: It is the visible part of nose. It is formed by the two nasal bones and cartilage. It is covered by skin. There are hairs inside.
Nasal Cavity : It is a large cavity divided by a septum. It is lined withciliated mucous membrane. It is extremely vascular.

Anterior nares: They are the openings which lead in.

Poserior nares: They are similar openings at the back and lead into pharynx.

Roof : Roof of the nose is formed by ethmoid bone at the base of the skull.

Floor: Floor of the nose is formed by the hard and soft palates at the roof of the mouth.
Paranasal sinuses: They are the hollows in the bones surrrounding the nasal cavity, which are lined with mucous membrane and open into nasal cavity.
Maxillary sinus lies below the orbit and opens through the lateral wall of the nose.

Frontal sinus lies above the orbit towards the midline of the frontal bone.

Ethmoidal sinuses are contained with in the part of the ethmoid bone separating the orbit from the nose. They are numerous .

Sphenoidal sinus lies in the body of the sphenoid bone.

2. Pharynx
It lies between Nasal cavity and larynx.

Pharynx is divided into three parts.
They are.
1. Naso pharynx
2. Oro pharynx &
 3. Laryngo pharynx

Naso pharynx lies between nasal cavity and oro pharynx.It is lined withciliated mucous membrane which is continuous with lining of the nose. Oro pharynx lies in between Naso pharynx and laryngopharynx. Its lateral wall contains collections of lymphoid tissue called tonsils. Laryngopharynx is the lowest part of pharynx. It lies behind larynx.

3. Larynx
It lies below pharynx and above trachea. It is continuous with oropharynx.
Muscles of the neck lie infront of larynx. Laryngopharynx and cervical vertebrae lie behind larynx. Lobes of thyroid gland lie on the either side of larynx.
Larynx is composed of several cartilages. They are joined together by ligaments and membranes.

Cartilages of Larynx are-
1. Thyroid cartilage. 2. Cricoid cartilage.
3. Arytenoid cartilages. 4. Epiglottis

Thyroid cartilage : Thyroid cartilage is formed with two flat pieces of cartilage. It is the largest upper part. Thyroid cartilage is lined with stratified epithelium. Lower part is lined with ciliated epithelium.

Cricoid cartilage: It lies below the thyroid cartilage. Its shape is like signet ring. It is broad at the back. It is lined with ciliated epithelium.

Arytenoid cartilages : They are a pair of smnall pyramids. They are made of hyaline cartilage. They are located on the broad portion of cricoid cartilage

Vocal ligaments are attached to them. Chink is the gap between vocal ligaments.

Epiglottis: Epiglottis is a leaf shaped cartilage. It is attached to the inside of the front wall of thyroid cartilage. During swallowing, larynx moves upwards and forward and its opening is occluded by epiglottis.

4. Trachea
It is also called as wind pipe. It is a cylindrical tube. It is about 11 cm. in length. It begins at the lower end of pharynx. It divides into two bronchi at the level of fifth thoracic vertebra. It is made of sixteen to twenty C-shaped incomplete cartilages. They are connected by fibrous tissue at the back. It is lined by ciliated
epithelium. Ciliated epithelium contains goblet cells which secrete mucus.

5. Bronchi
Trachea divide into right and left bronchi. Trachea and bronchi, combinedly are inverted Y shaped. Right bronchus leads into right lung and left bronchus leads into left lung. Right bronchus is shorter than left bronchus. It is also wider. Bronchi are made up of complete rings of cartilage.

6. Bronchioles
Bronchioles are the finest branches of bronchi. They do not have cartilage. They are lined by cuboidal epithelium. Bronchioles become further smaller to form terminal bronchioles. Terminal bronchioles are a single layer of flattened epithelial cells.

7. Alveolarducts
Terminal bronchioles divide repeatedly to form minute passages. Theseminute passages are called alveolar ducts. Alveolar sacs and alveoli open from alveolar ducts.

8. Alveoli
Alveoli are the final terminations of each bonchi. They cotain a thin layer of epithelial cells. They are surrounded by numerous capillaries. Capillary network is the site of exchange of gases between blood and air in the alveoli.

Lungs
Lungs are the principal organs concerned with repiratory process. They are two in number. They are spongy organs. They lie in the thoracic cavity on either side of heart and great vessels. They extend form roof of the neck to the diaphragm. Ribs, costal cartilages and intercostal muscles lie in front of lungs.
Behind them-ribs, intercostal muscles and transverse processes of thoracic vertebrae lie. Mediastinum is a block of tissue in between the two lungs. With in  mediastinum lie-Heart, great vessels, trachea,  oesophagus,  thoracic duct andthymus gland.
Lungs are conical in shape with apex above and base below. A pex slightly rises over the clavicle. Base is near the diaphragm. Each lung is divided into lobes by means of fissures. Right lung is bigger than left lung. Right lung is divided into three lobes. Left lung is divided into two lobes. Each lobe is divided into number of lobules. Each lobe contains a small bronchial tube. This tube divides and sub divides to end in air sacs.

Pleura is a serous membrane covering the lungs. It contains two layers. Inner layer close to the lungs is called as visceral layer. Outer layer is called as parietal layer. Pleural fluid lies in the space between visceral and parietal layers. Hilum is a triangular shaped depression on the concave medial surface of the lung. It is a vertical slit on each lung through which structures like blood vessels, nerves and lymphatic pass. Root of the lungs (Hilum) is formed by pulmonary arteries, pulmonary veins, bronchial arteries, bronchial veins, bronchi, lymphatic vessels. Pulmonary arteries carry deoxygenated blood to lungs from heart. Pulmonary veins carry oxygenated blood from lungs to the heart. Bronchial arteries are the branches of thoracic aorta carrying arterial blood to lungs. Bronchial veins are the vessels carrying venous blood of lungs to superior vena cava.

URINARY SYSTEM

Urinary System
Urinary system consists of -
KIDNEYS-2
URETERS -2
URINARY BLADDER
URINOGENITAL TRACT/URETHRA

Kidneys
Kidneys are the main organs of urinary system.They are 2 bean shaped
organs lying on the posterior wall of upper abdomen, one on each side of vertebral column. They lie at the level of twelfth thoracic to third lumbar vertebrae. Right kidney is located slightly lower than left kidney. Dimensions of each kidney are 11 x 5 x 3 cm3. Each kidney weighs 150g. approximately.On each kidney, an adrenal gland is present.
                Each kidney is embedded by fat called perirenal fat.Right kidney bears the impression of part of duodenum on its front.Pancreas crosses the left kidney transversely in its front. Each kidney is convex on its outer border and concave in the centre of its inner border. At this point, blood vessels, nerves and ureter enter and leave kidney. This point of kidney is called Hilus. Ureters transport urine formed in kidneys to urinary bladder.From urinary bladder, urine is passed to the exterior through urethra.Urethra in males is also passage for semen.Hence it is also called urinogenital tract in males. In females, it is independent.

Structure of Kidney

Longitudinal section of Kidney :
Kidney is surrounded by a fibrous capsule. It can be stripped off easily.
Portion inside this fibrous capsule can be divided into-
1) Cortex 2) Medulla
Cortex is the outer reddish brown coloured portion and medulla is the inner lighter area.
Medulla is subdivided into 10 to 15 conical areas called renal pyramids.
Pyramids have their broad base towards cortex and apex projecting into lumen of minor calyx. Columns of Bertin are the projections of cortex. They form the boundaries of the pyramids.

Ureters
There are two ureters carrying urine from kidneys to urinary bladder.
They are continuous from renal pelvis to urinary bladder and are about 25-30 cm long. Diameter is about 3 mm.Ureter is slightly constricted at three places.Ureter has three layers and they are-
1. Outer fibrous coat which is continuous with the fibrous coat of kidney.
2. Muscular coat containing outer circular layer and inner longitudinal layer.
3. Lining of mucous membrane continuous with that of bladder.
Urinary bladder
Urinary bladder is sac serving as distensible reservoir of urine evacuating its contents at suitable intervals of time. It lies in pelvic cavity behind symphisispubis. Urinary bladder consists of two portions.
1. Body - sac of detrussor muscle.
2. Trigone - triangular region connecting the three openings of bladder -
two of ureters and one of urethra.

Urinary bladder has two sphincters.They are
1. Internal sphincter
2. External sphincter
Internal sphincter is formed by smooth muscles surrounding the opening of urethra. External sphincter is formed by striated muscle of urogenital diaphragm

Urethra
Urethra is the canal through which urine from bladder is passed to the
exterior. It extends from base of the bladder. It has two sphincters.They are-
1. Internal sphincter - involuntary
2. External sphincter - voluntary
It has two orifices .They are-
1. Internal urethral orifice in the bladder.
2. External urethral orifice.

Male urethra serves as common tract for urinary and reproductive systems and thus it is called urinogenital tract. In females it serves for urinary system only. Male urethra : It is about 20 cm. long. It is divided into three portions.
1. Prostatic portion - surrounded by prostate gland and 3 cm. long. Prostatic and ejaculatory ducts open into this point.
2. Membranous portion - passing through pelvic floor and 1-2 cm. long.
3. Spongy portion - lying within penis and 15 cm. long.
Female Urethra : It is about 4 cm. long. It serves for urinary system only. It extends from the base of the bladder. It passes downwards behind symphisis pubis. It opens to the exterior in front of vaginal orifice.

ANATOMY OF HEART

Anatomy of Heart
Heart lies on the left upper part of thoracic cavity. It lies between the twolungs under sternum. It is broad above and conical below.

Histology of Heart : Heart consists of three layers.They are-
1. Pericardium - outermost layer consisting of
(a) Visceral pericardium
(b) Parietal pericardium
(2) Myocardium - Middle layer made of cardiac muscle cells and interstitial cells.
(3) Endocardium - Innermost layer.
Chambers of Heart
Heart has four chambers. Two of them are upper chambers called atria or auricles. Lower two chambers are called ventricles. The two atria are separated by interatrial septum.The two ventricles are separaed by interventricular septum. Atria are filling chambers and ventricles are pumping chambers. Compared to artia, ventricles are thicker since they are pumping chambers. Of the two ventricles, wall of left ventricle is three times thicker than that of right ventricle since left ventricle pumps oxygenated blood to all parts of body and right ventricle pumps deoxygenated blood to lungs only.

Valves of Heart
Opening between right artium and right ventricle is guarded by tricuspid valve. It prevents back entry of blood into right atrium from right ventricle at the beginning of ventricular systole. Opening between left atrium and left ventricle is guarded by bicuspid or mitral valve. It prevents back entry of blood into left
atrium at the beginning of ventricular systole - Pulmonary trunk is guarded by tricuspid semilunar valve which prevents back flow into right ventricle at the begining of ventricular diastole. Aorta has tricuspid semilunar valve which prevents
back flow of blood into left ventricle at the begining of ventricular diastole. Chordae tendinae and papillary muscles:Papillary muscles arise from ventricular walls. Chordae tendinae attach apical end of valves and papillary muscles. They prevent over distension of valves during diastole.

Blood Vessels attached to Heart

Blood vessels attached to heart are -
1. Superior and inferior venacavae - carrying deoxygenated blood from parts of body to right atrium.

2. Pulmonary artery carrying venous blood to lungs from right ventricle.

3. Pulmonary veins carrying oxygenated blood from lungs to the left atrium of heart.

4. Aorta carrying oxygenated blood to all parts of body from left ventricle of heart.

Blood vessels supplying oxygenated blood to heart :Right and left coronary arteries arising from Aorta supply oxygenated blood to heart. Blood vessels draining heart : Coronary veins bring deoxygenated blood of heart into coronary sinus, which opens directly into right atrium.

Ductus arteriosus : Ductus arteriosus is the vestigeal remnant of cord like structure which existed in foetal life between arch of aorta and pulmonary trunk. In foetal life, it bypasses pulmonary circulation. After birth, it closes, becomes obsolete and atrophies.

Septum ovale : It is crescenteric mark on interatrial setpum. It is closed foramen ovale that existed in foetus.
Foramen ovale : It is the opening in interatrial septum in foetal life. It avoids blood entry into lungs in foetal life. After birth, it closes and forms septum ovale.

Cardiac centres

1. Cardio inhibitory centre is dorsal motor nucleus of vagus in medulla.
2. Cardio accelerator centre is situated in lateral horn cells of upper thoracic segments of spinal cord.

Conducting System of Heart

System of conducting impulses of cardiac contraction consist of-
(1) Sinoatrial node (SA node) (2) Atrioventricular node (AV node)
(3) Bundle of His. (4) Right and left branches of bundle of His. (5) Purkinje fibres.

SA node : It is present at the opening of superior venacava into right atrium. It is called pacemaker of heart. It is made of modified cardiac muscle fibres. It measures about 5x20 mm.

AV node: It is present in the right atrium at the posterior part of inter atrial septum. It is close to the opening of coronary sinus. Cells of AV node are cardiac muscle fibres having a few myofibrils.It measures about 2x5 mm.

Bundle of His : Main trunk of bundle of His is continuous with AV node. It passes through interventricular septum. It is about 20 mm long.

Right and left branches of bundle of His : Bundle of His divides intoright and left branches. Right branch is longer than left branch. Left branch bifurcates into superior and inferior divisions.

Purkinje fibres : They arise from brnaches of bundles of His. They spread from interventricular septum directly to papillary muscle and ultimately end in sub endocardial network.Purkinje fibres have larger diameter than ordinary cardiac muscle fibres. Purkinje fibres have diameter of 50-70  where as cardiac
muscle fibres have diameter of about 15

SURFACE ANATOMY AND SURFACE MARKINGS

Surface anatomy and surface markings

Be able to identify these landmarks on yourself or the patient .
The xiphoid.
The costal margin extends from the 7th costal cartilage at the xiphoid to the tip of the 12th rib (although the latter is often difficult to feel); this margin bears a distinct step, which is the tip of the 9th costal cartilage.

The iliac crest ends in front at the anterior superior spine from which the inguinal ligament (Poupart’s ligament) passes downwards and medially to the pubic tubercle. Identify this tubercle by direct palpation and also by running the fingers along the adductor longus tendon (tensed byflexing, abducting and externally rotating the thigh) to its origin at the tubercle.

Feel the firm vas deferens between the finger and thumb as it lies within the spermatic cord at the scrotal neck. Trace the vas upwards and note that it passes medially to the pubic tubercle and thence through the external inguinal ring, which can be felt by invaginating the scrotal skin with the fingertip.


Vertebral levels

•◊◊T9—the xiphoid.

•◊◊L1 — the transpyloric plane of Addison lies half-way between the suprasternal notch and the pubis, or approximately a hand’s breadth below the xiphoid. This plane passes through the pylorus, the pancreatic neck, the duodenojejunal flexure, the fundus of the gall-bladder, the tip of the 9th costal cartilage (felt as a distinct ‘step’), and the hila of the kidneys. It also corresponds to the level of termination of the spinal cord.
•◊◊L3 —  the subcostal plane, a line joining the lowest point of the thoracic cage on each side, which is the inferior margin of the 10th rib. It passes through the origin of the inferior mesenteric artery

•◊◊L4—the  plane of the iliac crests. This corresponds to the level of the bifurcation of the aorta. It is also a useful landmark in performing a lumbar puncture, since it is well below the level of the termination of the spinal cord, which is approximately at L1.

•◊◊The umbilicus is an inconstant landmark. In the healthy adult it lies at the junction of L3 and L4 vertebrae. It is lower in the infant and, naturally, when the abdomen is pendulous. It is higher in late pregnancy.

Surface markings

The abdominal viscera are inconstant in their position but the surface markings of the following structures are of clinical value.
X's mark the outline of the liver which reaches from the 5th intercostal space R to the 5th intercostal space L in mid clavicular line, and lower margin 10th rib The aorta bifurcates at L4 which is in line with the iliac crests


Liver
The lower border of the liver extends along a line from the tip of the right 10th rib to the left 5th intercostal space in the mid clavicular line; it may just be palpable in the normal subject, especially on deep inspiration. The upper border follows a line passing through the 5th intercostal space on each side.

Spleen
This underlies the 9th, 10th and 11th ribs posteriorly on the left
side commencing 2 in (5 cm) from the midline. It is about the size of the subject’s cupped hand.

Gall-bladder

The fundus of the gall-bladder corresponds to the point where the lateral border of the rectus abdominis cuts the costal margin; this is at the tip of the 9th costal cartilage, easily detected as a distinct ‘step’ when the fingers are run along the costal margin.

Pancreas
The transpyloric plane defines the level of the neck of the pancreas which overlies the vertebral column. From this landmark, the head can be imagined passing downward and to the right, the body and tail passing upwards and to the left.

Aorta
This terminates just to the left of the midline at the level of the iliac crest at L4; a pulsatile swelling below this level may thus be an iliac, but cannot be an aortic, aneurysm.

Kidneys

The lower pole of the normal right kidney may sometimes be felt in the thin subject on deep inspiration. Anteriorly, the hilum of the kidney lies on the transpyloric plane four finger breadths from the midline. Posteriorly, the upper pole of the kidney lies deep to the 12th rib. The right kidney normally extends about 1 in (2.5 cm) lower than the left. Using these landmarks, the kidney outlines can be projected on to either the anterior or posterior aspects of the abdomen. In some perfectly normal thin people, especially women, it is possible to palpate the lower pole of the right kidney and the sigmoid colon if loaded with faeces; in most of us, only the aorta is palpable.

GENERATOR

                                    GENERATOR

               An x-ray generator is the device that supplies electric power to the x-ray tube, It is not an electrical generator in the strict sense of the word, because by definition a generator converts mechanical energy into electrical energy. The x-ray generator modifies this energy to meet the needs of the x-ray tube. The tube requires electrical energy to meet the needs of the x-ray tube. The tube requires electrical energy for two purposes: to boil electrons from the filament and to accelerate these electrons from the cathode to the anode. The x-ray generator has a circuit for each of these functions, and we will refer to them as the filament and high-voltage circuits. Also, the generator has a timer mechanism, a third circuit, which regulates the length of the xray exposure.

                          The mechanism of an x-ray generator is usually continued in two separate compartments: a control panel or console and a transformer assembly. Control panels may be very simple or quite complex, and any attempt to describe a single panel or console would be of little value. The controls allow the operator to select the ap-propriate kVp, rnA, and exposure time for a particular radiographic examination. Meters measure the actual rnA and kVp during the exposure. One exposure button (standby) readies the x-ray tube for exposure by heating the filament and rotating the anode, and the other button starts the exposure. The timing mechanism terminates the exposure
The second component of the x-ray generator, the transformer assembly , is a grounded metal box filled with oil. It contains a low-voltage transformer for the filament circuit and a high-voltage transformer and a group of rectifiers for the high-voltage circuit. The potential differences in these circuits may be as high as 150,000 V, so the transformers and rectifiers are immersed in oil. The oil serves as an insulator and prevents sparking between
the various components. By definition, a transformer is a device that either increases or decreases the voltage in a circuit. A rectifier changes alternating current into direct current

RECTIFICATION

RECTIFICATION

Rectification is the process of changing alternating current into direct current. The device that produces the change is called a rectifier.A rectifier allows an electrical current to flow in one direction but does not allow current to flow in the other direction. Rectifiers are connected into the X-ray circuit in series. They are mainly divided into half wave and full wave rectifiers. If alternating voltage is applied directly to the X-ray tube, the anode will emit electrons, whenever it is negative with respect to cathode.These electrons will travel towards the cathode and bombard the filament and destroy the filament. This is called back projection which is avoided by the supply of rectified DC voltage. Thus, rectifiers play an important role in X-ray production.

HALF WAVE RECTIFIER

Vacuum tube diodes or solid state (semiconductor) diodes can be used for rectification. In a half wave rectifier, a single diode is used. An alternating voltage is applied to the

diode as input. The output is obtained across the resistance R. When the plate is positive, the diode will allow the current to flow. When the plate is negative, the diode will not allow the current. Therefore, the diode will allow the current only during those half cycles when the plate is positive. Hence, the output current is always in one direction. This circuit is known as half wave rectifier and it is mainly used in mobile and dental X-ray units. A single solid state diode cannot prevent reverse current at higher voltages. Hence, many diodes are placed in series in a stick to do rectification.

FULL WAVE RECTIFIER

In the half wave rectifier, the input voltage is used only in one half of the cycle. The other half of the cycle is not used. Therefore, there is a need for a rectifier, which will use the full cycle of the input. This is possible by having two or more number of diodes. The alternating voltage is applied between A and The output is obtained across the resistance R.When end A is positive, D1 and D4 will conduct and a current flows through R. During the next half of the cycle end A is negative, and end B is positive. Now, the diodes D2 and D3 will conduct and a current flows through R. Thus, the current flows through the resistance R during full cycle of the input voltage, in the same direction. X-rays are produced in two pulses per cycle, irrespective of the polarity of the transformer. Three phase generator employ multiple rectifiers in the secondary circuit. Full wave rectifiers are used in high end B. X-ray tubes which employ rotating anode X-ray tubes.

HIGH-TENSION TRANSFORMER

The high voltage transformer is used to transfer low voltage into high voltages required to operate X-ray tubes. This is known as high tension generator, which provide voltage from 20–150 kV and current up to 1000 mA for the X-ray tubes. It is a step up transformer with two windings and a shell type core. The number of turns in the secondary is higher than that of primary, and it is decided by the voltage ratio. If 400 volt is to be transformed into 80,000 volts, then the voltage ratio is 80,000/400, requiring 200 turns in secondary per primary turn. Thus primary winding consists of a few hundred turns of thick copper wire, which is well insulated and wound on a cylinder. A thin copper sheet is fitted over the primary winding and it is earthed. This is known as stress shield, which protects the primary circuit during breakdown of the secondary insulation . The secondary winding consists of about 100,000 or more turns of thin copper wire coated with insulated varnish. This is wound in an insulating cylinder, which is placed over the primary winding. The layers are separated from each other by thin paper prepared with wax for insulation. The voltage difference between any two layis about only 200–300 volts. This method of design reduces the risk due insulation breakdowns. The core of the high tension transformer is rectangular in shape, which is earthed and well laminated. Usually, the secondary is wound in two equal multiple parts and the center of the winding is earthed through the core. This means that instead of running from 0 to + 150 kV, the two secondary cables run from – 75 kV to + 75 kV. This is to reduce the insulation, size and cost. Since the current measurement in the primary coil is not an accurate representation of the current in the secondary coil, the current must be measured on the secondary side.

                             A milliameter (mA meter) is connected between the inner ends of the two secondary windings at which the transformer isgrounded, which is also the center of the coil. This minimizes the risk of electrical shock to the operator. Though the mA meter is connected at this point, it is placed at remote distance at the control console. The entire unit is immersed in an earthed metal tank, which is filled with oil. The metal tank is closed with a tight lid. In the case of dental and mobile X-rays, the heat production is very low and hence, oil is not used. Instead of oil, plastic is used as an insulator. The transformer is immersed in plastic when it is in fluid state. Later on, the plastic solidifies and acts like a solid insulatorers 

BEAM RESTRICTORS OR COLLIMATORS

An X-ray beam restrictor is a device that is attached to the X-ray tube housing, to regulate the size and shape of an X-ray beam. They can be classified into three categories, namely, (i) aperture diaphragms (ii) cones and cylinders  and (iii) collimators. Aperture diaphragms consist of a sheet of lead with a hole in the center. The size of the hole determine the size and shape of the X-ray beam. It is simple and the aperture can be altered to any size and shape. The disadvantage of an aperture diaphragm is that it produces large penumbra. The penumbra can be reduced by keeping aperture diaphragm far away from the X-ray target. Aperture diaphragms are used in dental radiography with rectangular collimation. In addition, it is used in trauma and chest radiography. The use of cones and cylinders will reduce the penumbra considerably. Both have extended metal structures that restrict the useful circular beam to the required size. The position and size of the distal end determine the field size. If the X-ray source, cone and film are not aligned properly, then, one side of the film may not be exposed, which is called cone cutting. Cone is a ideal beam restrictor, but the flare of the cone is greater than the flare of the X-ray beam. These systems provide only limited number of field sizes. The collimator is the best X-ray beam restrictor. It defines the size and shape of the X-ray field that emerges from the X-ray tube.The collimator assembly is attached to the tube housing at the tube port. A collimator consists of two sets of shutters, which can be moved independently. Each shutter consists of four or more lead plates of 3 mm thick, which can absorb X-rays completely, to provide a well defined X-ray field. When the shutters are closed, they meet at the center of the X-ray field. The collimator also has a light and mirror arrangement, to illuminate the X-ray field. The light bulb is positioned laterally and the mirror is mounted in the path of the X-ray beam at an angle 45o. The target and the light bulb should be kept at equal distance from the center of the mirror. The collimator provides variety of rectangular X-ray fields and the light beam shows the center of the X-ray field. The light field and radiation field should match exactly with each other. The variation must be with in 4% of TFD. Thealignment of light beam and X-ray beam should be checked periodically. A well collimated beam covers lesser area of the patient, giving less patient dose. Also it generates less scatter radiation, which improves the image quality.Collimators that automatically limit the X-ray field size to the useful area of the detector is also available. These are called positive beam limitation (PBL) collimators. A sensor in the cassette holder, adjust the collimator opening, equal to the cassette dimensions. Thus, PBL collimators limit the irradiated volume and reduce the patient dose.

ROTATING ANODE X-RAY TUBE

In 1933, the rotating anode X-ray tube was invented, in which the anode is made to rotate before the electron is emitted. It was developed to increase the heat loading with higher X-ray output. In these tubes, the electrons transfer their energy over a large area of a rotating target. Rotating anode tubes are larger in size, but the principle and function are similar to that of stationary X-ray tube.

Principle

Consider a rotating anode of radius R and circumference L. The electrons bombard a region of height ab and

width cd. The length may range up to the circumference (L 2pR) depending on the exposure time. But, the X-rays always appear to come from a focal spot of area cd × cd.

Let us consider a stationary anode of actual focal area 7.3 mm × 2 mm, and rotating anode of R = 30 mm and length 7.3 mm, then

Actual focal area of rotating anode

Loading gain = ————————————————————

Actual focal area of stationary anode

2 p × 30 × 7.3

= ——————————— = 94.2

7.3 × 2

Thus, the rotating anode arrangement helps to increase the loading to a greater extent of the order 100. The construction of such a rotating anode is a remarkable technological development. The diameter of the tungsten disk determines the total length of the target track, and obviously affects the maximum permissible loading of the anode.

Cathode

Rotating anode X-ray tube consists of a cathode and an anode which are kept in a glass bulb . The cathode is a tungsten filament which is offset from the long axis of the X-ray tube to face the target near the periphery of the anode disk. Usually, rotating anode tubes are fitted with two filaments , one larger than the other set side by side in the cathode assembly. One filament is designed to focus the electrons on a larger area of the anode, which require heavytube loading. The other filament is used to focus the electrons on a smaller area of the target. This type is used when high resolution is required. Both filaments should focus the electron on the same part of the anode, so that focal spot is at the same point for both modes of operation. Some tubes provide two target angles for two filaments, so that each filament will have a separate focal spot. Smaller angle is used with the smaller focal spot.

Focusing Cup

The focusing cup (cathode block) surrounds the filament, shapes the electron beam width. It is used to focus the electrons on a small area (focal spot) in the anode. There are two ways by which the focusing cup is energized, namely, unbiased and biased . In unbiased setup, same voltage is applied to both focusing cup and filament. In this type, the electron spread is wider and the focal spot width is larger. In biased X-ray tubes, insulated focusing cups are used and it is given more negative supply (–100 V) than the filament. This creates a tighter electric field around the electron, reduces the electron spread and gives smaller focal spot width. Thus, the focusing cup width determines focal spot width and the filament length determines the focal spot length

.Anode

The anode is made in the form of large disk of tungsten, or an alloy of tungsten, which is saucer shaped. The target track is near the periphery of the disk, to maximize the length. The track is a mixture of 90% tungsten and 10% rhenium (Z = 75), which reduces the crazing effect caused by thermal stresses. Modern rotating anodes are made from  solid molybdenum onto which a thin tungsten-rhenium focal track is coated. The specific heat capacity of molybdenum is higher than that of tungsten (250 vs 130 J Kg–1K–1). The mass of molybdenum anode is also lower due its lower density. Some high output X-ray tubes have radial slots cut into the anode disc to reduce thermal stresses caused by repeated heating and cooling. Heavy duty X-ray tubes have graphite layer (Carbon) in the back of the anode disk. The anode disk has a beveled edge and the angle of the bevel may vary from 6 to 20o. The bevel is used to achieve the line focus principle.

Anode Stem

The anode disk is mounted on a stem, which is attached to the rotor. The anode assembly rotates with the help of bearings. The stem is made of molybdenum, which is having high melting point (2620°C) and poor heat conduction. The molybdenum stem prevents the flow of heat from the tungsten to the bearings of anode assembly, due its small cross section. Thus, the bearings are protected from heat, which may cause them to expand and bind. Higher the length of the stem, higher the inertia of the tungsten disk, more will be the load on the bearings. Hence, it is desirable to keep the stem as short as possible.

Rotor

The anode disk is connected to a rotor, which is made up of copper bars arranged around a cylindrical iron core. There are electromagnets surrounding the rotor, outside the glass envelope is called stator . Both the stator and rotor is called as an induction motor.

When the stator coils are energized, a rotating magnetic field is produced, that induces current in the copper bars of the rotor. This induced current produces an opposing magnetic field that causes the rotor to spin. The rotor rotates at a speed of about 3000–9000 revolutions per minute (rpm). This will facilitate the electrons to bombard a constantly changing area of the target. Since the electrical conductivity of copperis higher, it will facilitate induction of strong currents from the induction coil supply. The surface of the rotor is blackened to enhance heat dissipation by radiation process. The rotor support is made of steel and the positive high tension supply is made at the end of the rotor support outside the glass envelope. Low speed rotor operates from 60 Hz power (single phase) and gives rotation of about 3000 rpm. This speed is too slow for short exposure of the order of mill-seconds. High speed rotor operates from 180 Hz power (3 phase) and gives a rotation speed of 9000 rpm. If the speed of rotation is increased, the heat generated at the focus will be spread over a larger area. Modern X-ray tubes employs higher rotor speeds by increasing the frequency of the stator supply with frequency multiplying circuits. The X-ray machines are designed so that the tube cannot be energized until the anode attains its full speed. This delay time (1–2 s) is incorporated in the exposure buttons. Thepower supplied to the induction coils, produce eddy current which causes heating of the rotor.

Bearings

The anode assembly is rotated with the help of bearings, which are made of steel ball races. Bearings are in the high vacuum environment and require special heat insensitive, nonvolatile lubricants. The bearings are coated with lead or silver to act as (metallic lubricants) lubricant. Commonly available lubricants such as oil and grease cannot be used,

since it will vaporize while heating and destroy the vacuum. Even dry lubricant (graphite) would wear off as a powder and destroy the vacuum. When the X-ray unit is turned on, the motor alone is energized first for a few seconds until the rotor reaches its operating speed. Then, a high voltage is applied to the tube for the required exposure. After the exposure, the rotor is slowed down quickly by dynamic braking to avoid the wear in the bearings. Since the electrons are striking the whole circumference of the anode, no part of the anode attains very high temperature. The heat from the tungsten disk is dissipated by radiating through the vacuum to the wall of the tube, and then into the surrounding oil and tube housing. The life of a rotating anode X-ray tube is limited, because of pitting due to continuous electron bombardment on the anode surface.These changes are due to thermal stress. This pitting reduces the X-ray yield and changes the spectral distribution. The decreased output, results in excessive scattering of X-rays and increased absorption of X-rays by the target itself. A pitted anode will affect the electric field between cathode and anode, so alter the size of the focal spot.