Physicians often need to recognize abnormalities going on in body tissues and organs to diagnose any specific diseases or not at all.These abnormalities are seen in the form of images,there are different imaging techniques used today,the most common one is a radiograph(X ray film) which provides a two dimensional image of the interior of the body.

To produce such a radiograph a single barrage of X ray is passed through the body and exposes the film on the radiograph(or flouroscent screen) placed on the opposite side.Tissues having different densities show up as images of differing densities on the radiograph i.e. x rays have their nature of being absorbed to various degrees as through body tissues depending on the density of them.There are 4 fundamental radiographic densities which produce natural radiograph contrast on an X ray film.See how they appear on radiograph(or called flouroscent screen). Radiographic contrast

• Air and gases appear black-------radiolucent

• Fat appears gray_black

• Muscles and water appear gray

• Bones and Calcium appear white-------radiopaque

A tissue that is relatively dense absorbs(stops) more xrays than tissuesthat are less dense.A very dense tissue is said to be radiopaque and a less dense tissue is said to be radiolucent.Bone is very dense tissue and fat is moderately dense while other tissues are least dense.

Note: Its important to differentiate between two types of densities that are physical density(refers to no. of grams per cubic centimeter of the material) and radiographic density(refers to the degree of blackness of a film).The radiographic density of a substance is related to its physical density.

The most common X-ray examination is Chest x-ray,and the musculoskeletal x-ray exam. comes second to it.

Computed tomography(CT) scanning also called computer axial tomography(CAT) scanning.permits the study of tissue slices so that tissues having minor differences can be recognized.

In CT scanning a source of x-ray moves in an arc around the part of body being observed and sends out a beam of x rays.The beam of x ray having passed through the region of the body, are collected by a special detector which converts the x-rays into electronic impulses that produce readings of the density of the tissue in a 1cm slice of the body.From these readings a computer is assemble a picture of the body called CT scan that can be viewed on a flouroscent screen and then photographed for later examination.

Magnetic rsonance imaging MRI is quite safe to the patient and its use can be more revealing than a CT scan since it provides better differentiation between different tissues.

MRI uses the magnetic properties of the hydrogen nucleus excited by radiofrequency radiation transmitted by a coil surrounded by the body part.MRI gives a better image than CT scan as some tissues contain more hydrogen in the form of water than do other tissues.

MRI is a non invasive techinque that does not use ionizing radiation.In the parameters used for medical MR imaging its without significant health hazard.

• The CT scans are highly sensitive in detecting disease in soft body tissues and can actually provide images of internal organs which are impossible to visualize with an x-ray technique.

• Using CT guidance, biopsy,excision or ablation procedurs could safely be performed.

• The imaging effects or biological effects of x-rays can cause tissue cell injured or even celldeath.So X-ray is doubled edged sword; in x-ray therapy it can help kill cancerous cells,at the same time it can damage normal body cells.Thus we mmust use x-rays in aproper way and pay attention to protect patient as well as ourselves from high doses of x-rays exposure. X-Rays used in CT scans usually have no harms.

   

• CT scan is a more developed version of x-ray to mainly focus on specific parts of body. CT scan has additional advantage of being able to produce imaging virtually in any orientation.

   

• CT scans provide better images for bones structures,inner ear as it can easily detect tumors in the auditory canal and cochlea.

   

• CTscans help diagnose bone tumors,bone fractures,internal injuries, internal bleeding & blood clot and to monitor cancer and heart diseases.

• In CT scans images are created by using x-rays while MRI uses magnetic waves to create images.

• CT scans cann't help much in seeing clearly very fine soft tissue details as in the knee or shoulder compared to MRI.Also MRI provide more detais of bony structures compared to CT scan.

   

• MRI scanners are best suited for imaging soft tissue.

   

• CT scan is more costy and needs long time(30min) to be completed as compared to MRI that is almost done in 5min.

   

• Pregnant women and patients having surgical clips,metallic fragments,cardiac monitors and pace makers cannot have CT scan.

   

• MRI machines can produce images in any plane without moving the patient and has got the abilit to change the contrast of the images making them more clear than CT scan.

The first recorded x-ray diagnostic use of x-ray was in 1896.Before 1970, the diagnostic imaging study primarily relied on radiographs or x-ray films that were often supplemented by various contrast examinations for clinical problem solving.

A revolution in diagnostic imaging began in the early 1970s with the development of computed tomography and soon rapid improvements in technology afforded us the ability to directly image areas of the body that previously were only accessible to surgeons knife.

MRI joined the diagnostic armamentarium in the early 1980s and added a new modality for the diagnosis of disorders of whole body especially in the CNS and musculoskeletal system.

Now diagnostic imaging includes Plain film radiography,Contrast-enhanced radiography,Computed tomography(CTscan),Digital subtraction angiography(DSA),Nuclear imaging,Magnetic resonance imaging(MRI),and diagnostic Ultrasound.

The first 4 of these imaging form use x-rays plain film radiography also known as ordinary radiography or plain film.Plain film is the standard method of producing an image on an x-ray film by passing a beam of x-rays through being the examined area of the patient.

The term "plain film" means that no contrast material is used to enhance various body structues.In plain film examinations the natural contrast between the basic 4 radiographic densities--air,soft tissue(or water),fat, and bone is relied on to define abnormalities.Examples of plain film studies with which you are familiar with include chest,abdominal & skeletal radiographs.