INTRODUCTION :

X RAYS : CONVENTIONAL X Rays COMPUTED TOMOGRAPHY ( CT )

CONVENTIONAL RADIOLOGY Production of X- Ray ** X – RAYS are produced by passing very high voltage across two tungsten terminals ( cathode & anode ) within evacuated tube .

** When the cathode heated , it will liberates free electrons .** High voltage is applied ( 50-150 kv ) , the electrons will attracted to words the anode at high speed & once the electrons hit the anode target at high speed the X – RAYS are produced .

Absorption of X-rays The X rays absorbed to variable extent as they pass through the body , & so this differential absorption help to visibility of both normal structures & diseases . ** 4 basic densities seen : Gas , Fat , other soft tissues & Calcified structures . ** Gas : least absorbed ; most black . ** Calcified structures & bone : Most absorbed ; White :** Fat : Fewer absorb ; little blacker .** Soft tissue : ( Muscle , solid viscera , blood , variety of fluid ) : similar absorption : same gray shade .

Images Either on sliver based photographic emulsion . Recorded digitally & seen by computer .

Projection : Described by the path of the X-Ray beam ** PA : Routine CXR .** AP : Plain abdomen . *** AP & PA ( Frontal ) . *** X- Rays are two dimensions , so at least two views are necessary to gain information about the third dimension .*** The views should be are right angles , or oblique .

PORTABLE X –RAY FLIMS ** Patient in bed . ** operation theater . ** Cant move safely .*** poor quality films ; long exposure time , low voltage , in proper positioning of the patient .

COMPUTED TOMOGRAPHY ( CT )

*Relies on the X- ray transmitted through the body , but , difference from conventional radiography , that its more sensitive X-rays detection system is used .•Out standing feature of CT is that very small differences in the X- ray absorption value will be visualized .•Rang of densities recorded is increased by approximately 10 X .•The images consist of sections ( Slices ) taken through the body .•The most frequent cuts or sections are axial .•The slice thickness vary from ( 1-10 mm) .

Spiral ( Helical ) CT & Multislice CT Spiral CT : the patient moved past an array of detectors within the machine , so the data from multiple adjacent levels are collected continuously , during which time the X-ray beam traces a spiral pathway to create ( volume of data ) . Multislice ( multidetector ) : Much faster , more thinner slices , so high quality multiplanar & 3 D reconstructions , as well as CT angiography .

Attenuation values are expressed ( Houns field units ) *** water = zero .*** Air = - 1000 .*** Bone = + 1000 .***** Window width = Rang of densities visualized on particular image .***** Window level ( Center ) = mean level .*** Wide window : all structures are visible , but fine detailes of density difference cant appreciated .*** Narrow window : fine details seen , but much of the image is either totally black or totally white .

CT Angiography & MRA replacing the Dx DSA . **** Artefacts : * Movement & high density objects ( Ba , metal implants , dental filling , surgical clips , shells ) ; produce radiating linear streaks , will result in degradation of the image .

Contrast Agents in conventional radiography & CT

used to visualized structures or disease process that other wise in visible or difficult to be seen . ** Ba ; GIT . ** Iodine in solution to absorb the X – rays ; EU , DSA , EHCT .** High & low osmolality iodinated CM .** low osmolality ; low ionic , low complication .** Allergic reaction : self limiting urticarial rush to bronchospasm , laryngeal odema & hypotension & even life threatening .

*** Previous history of allergic reaction : low osmolar CM , & premedicated with steroids .

**** CM has deleterious effect on renal function , so patient with impaired renal function , should be well hydrated prior to injection .

ULTRASOUND ** Very high frequency sound beam directed to the body from transducer placed in contact to skin , to make good acoustic contact , we use gel , then the sounds travels through the body & reflected at tissue interface , the echoes picked up by the same transducer & converted to electrical signal .**** Air , Bone , & other heavily calcified materials , with absorb nearly all the US beam & so little role in the Dx of lung & bone disease .

*** Since fluid is good conductor of sound , so US is good Dx modality for cystic lesions , fluid content structures ( biliary tree , UB , fetus within the amniotic sac) .

*** Solid structures have different acoustic impedance , that’s help to differentiate tumor from adjacent normal tissue . ** Acoustic enhancement .** Acoustic shadow .

** Small US probes : high detailed images , but , with limited range of few cm & should be placed very near to the site of interest ; rectal probe ( prostate) .*** Tiny US probes , put at the end of endoscopy ( TEUS ) ; esophagus , heart , aorta , pancreas .*** 3 D US .*** US contrast agent : microscopic air bubbles ( liver , renal , CVS ) .***High frequency US : low penetration , high resolution . *** Low frequency US : high penetration , low resolution .

DOPPLER EFFECT :** If sound reflected from mobile structures , then variation in frequency will result & this proportional to the speed of movement & this difference referred to as shift in frequency , which will converted to audible signal = principle of Doppler .**** Doppler frequency shift = difference between the transmitted & received sound . *** Blood cells are the moving subject within the blood , , & if moved to words the transducer , the received signal will be of higher frequency ( Red color) , & if moves away from the transducer , the vice versa & produce ( Blue color) , which reflects venous flow .

** Artery & vein , each has wave form flow & characteristic shape .

**** Application : DVT , Arterial stenosis & occlusion , ( carotid ) . Abd. tumour , obstetrics ( umbilical artery ) , Doppler echocardiography ( regurgitation flow & pressure gradiant )

Radionuclide Imaging Radioactive isotope used in Dx imaging .•Emit Gamma rays : electro magnatic radiation similar to X-ray , produced by radioactive decay of the nucleus . •Natural radioactive isotope have long half life , so not suitable for Dx imaging . • Artificial radioactive isotope have short half life ( few hours or days).• 99m – Tc ( 6 hr ) , iodine 123 , thalium 201 , gallium 67 .•Used in either free ionic form ( 99mTc pertechnetate ) gastric mucosa in Mickle`s divertriulum or with radiopharmaceutical as organic phosphate ; skeletal survey .

* Special particles ( albumin macroaggregates ) ( 10-70 micro meter ) trapped within the pulmonary capillaries so when labelled with 99m- Tc , the Blood flow to the lung can be visualized . * Radioactive gas ; Xenon 133 , xenon 127 , krypton -81m; these can be inhaled to assess the pulmonary ventilation .* RBC can be labelled with 99m-Tc to assess cardiac function or WBC labelled with 99m-Tc or indium -111 detect abscess . * The nuclear medicine techniques are used to measure function & to produce anatomical images , but has poorer spatial resolution as compared to other imaging modalities .

POSITRON EMISSION TOMOGRAPHY (PET) ** Uses short- live positron emission isotopes which are produced by cyclotron immediately before use .** Two gamma rays are produced by annihilation of each positron which can be detected by a specialized gamma camera . ** So the resulting image will reflect the distribution of the isotope .*** By using isotopes of biologically important elements such as carbon , oxygen , PET can be used to assess the physiological processes such as blood perfusion of tissues , metabolism of substances such as glucose , as well as complex biochemical pathways such as neurotransmitter storage & bindings .

***** 18 F- FDG ( fluorodeoxy glucose) ; most sensitive technique for staging solid tumor ( CA bronchus) & follow up of lymphoma ( active disease from residual fibrosis) .

*** PET is also used for evaluation of IHD , brain disorder as dementia , epilepsy & Parkinsons` disease .

*** PET demonstrate the biological activity , while CT gives anatomical information , so that leads modern equipments allows both PET CT or PET MRI to performed sequentially .

Magnatic resonance imaging ( MRI) * The basic principle depend on the fact the nuclei of certain elements align with the magnetic force when placed in strong magnatic field .

•At the field strength that currently used in medical imaging , hydrogen nuclei ( protons ) in water & lipid are responsible for producing imaging .

•Radiofrequency Pulse is applied at resonant frequency of hydrogen , then proportion of protons will change their alignment , flipping through preset angle & rotate in phase with one another .

*** Following this radio frequency pulse , the protons return ( Realign = relax ) to their original positions , while mean they will induce a radio signal which will detected & localized by antenna coils placed around the patient , then the intensity & source of theses signals can be calculated & displayed as images . i.e. the image that produced representing the distribution of hydrogen protons .

•The strength of signal depends on proton density , & two relaxation times ( T1 & T2 ) .

•T1 depends on the time taken by protons to return to the axis of the magnatic field , while T2 depends on the time the protons take to dephase . & thus T1 & T2 weighted images contrast between tissues depend on their T1 or T2 relaxation properties .

•Most pathological processes have long T1 & T2 relaxation time so appear with low ( blacker ) T1 & high ( whiter ) T2 signal intensity .

•One advantage of MRI over CT is that ; direct multiplanar imaging . While disadvantage is that the patient should kept still for longer time scanning than CT as well as un avoidable movement from cardiac pulsation , breathing , peristalsis often degrade image .• MRI Is establishing technique for brain , spine , bone , joints , pelvic organs , biliary system , urinary tract & heart .•MRI provide valuable information in skeletal disease . •Signal void ; fast flowing blood , calcification , air .•MRI Contra indication : absolute C.I. in cardiac pacemaker , intraocular FB , cochlear implant , & certain types of aneurysmal clip .

Contrast Agents / MRI :•Gadolinium ( Gd ) ; Crosses only the breaked BBB or high vascular supply .•Tissue specific media ex. Iron oxide agents ( Retriculoendothelial system )

Radiation Hazards :•Limited the ionizing radiation examination ( x rays , gamma –rays , other radionuclide emission ) to conditions where the benefits clearly outstanding the risks . • ALARA ( as low as reasonably achievable ) ; appropriate equipment , good technique – limiting the size of x-ray beam , limiting No . of films , keep repetition to minimum , & make sure that the examination is not done .• Avoidance of unnecessary requests of X – rays examination esp . High dose radiation as ( Ba enema , lumbar spine & CT ) .•Radiation is particularly harmful to the dividing cells , then adverse mutation occurred following radiation of the gonads .•No safe radiation dose .•Fetus ; congenital malformation , leukemia , & other malignancy during the first 10 y increased by 40% .• Standard CT examination increased the risk of cancer by 1/2000 .