radiology basics & principles

7/23/2019 Radiology Basics & Principles

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APPENDIX J

Basic Radiology & Radiographic Principles

I. Radiology in Veterinary Medicine

Although there are a myriad of places where anatomy is used in the daily practice of

veterinary medicine, there are 3 major ways that anatomy is used in the practice of veterinarymedicine:

A. In the physical examination of patients (must be able to distinguish normal versusabnormal).

B. In surgical procedures (which consists of evaluating, rearranging, removing orremodeling anatomic structures).

C. In the interpretation of diagnostic imaging techniques (such as radiographs, ultrasound,MRI, CT scans, etc).

Although there are a growing number of diagnostic imaging techniques and an increasingavailability to the practitioner, radiographic imaging is still the bread and butter for the

veterinarian. Since radiographic interpretation is one of the major ways that anatomy is used inpractice, and since anatomy plays a role in essentially every clinical case from vaccination tothoracic surgery, the importance of radiographic anatomy is self-evident. Although a radiologycourse is part of the professional curriculum, it deals primarily with radiation physics andradiographic interpretation. You will be expected to already know fundamental radiographicanatomy as a prerequisite. Consequently, normal radiographic anatomy makes up a significant partof your gross anatomy courses.

II. X-rays

A radiograph is a 2-dimensional representation of a 3-dimensional object. It is aphotographic record made on a specialized type of film. X-rays, on the other hand, are a form ofelectromagnetic radiation similar to visible light, but with a much shorter wavelength. X-rays obey

most of the same physical laws as visible light (e.g., they travel at the speed of light), but theirshorter wavelength allows them to penetrate materials that light cannot.

To summarize the difference between radiographs and x-rays, one should keep in mind that aradiograph consists of an image on film whereas x-rays are invisible electromagnetic radiation. Thevalue of radiographs is that they allow indirect assessment of organs and structures that wouldotherwise require extensive invasive procedures for direct evaluation.

A. X-rays versus radiographs

1. X-rays Form of electromagnetic radiation which are capable of penetrating

objects to varying degrees

They are produced when electrons, moving at high speeds, impact certainheavy metals, such as tungsten

X-ray photons travel at the speed of light and obey most of the same laws

of light, except that they penetrate objects


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2. Radiographs A radiograph, on the other hand, is a 2-dimensional photographic image of

a 3-dimensional object which is produced when x-rays differentially passthrough that object

B. Generation of x-rays

An X-ray machine is basically a direct current generator capable of producing apotential of several thousand volts. When a large voltage potential is applied to the poles of avacuum tube, electrons stream from the negative pole (cathode) to the positive pole (anode).The high speed impact of the electrons with the anode results in the production of heat (about99% of the energy) and x-rays (1% of the energy). In the specialized vacuum tubes used inx-ray machines, the anode is called the target and is formed from a tungsten alloy that has avery high melting point to resist the tremendous heat production. Because x-rays travel instraight lines and stream off of the target in all directions, the x-ray tube is housed in a metalcasing (which absorbs x-rays) with an opening only on one side. This opening (aperture)allows the emerging x-ray beam to be focused on an area of interest without simultaneouslyirradiating everything in the room!


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III. Radiographic Film and Image Production

X-ray film is really nothing more than a modified form of photographic film. It consists of aclear plastic sheet (structural base or backing) which has been coated with an emulsion of silverbromide crystals (usually on both sides of the base even though the diagram below only shows oneside). When an x-ray photon (or light for that matter) contacts a silver bromide crystal, the energy ofthe photon is transferred to the silver bromide molecule causing the silver to become sensitized orexcited. When the film is developed, any silver molecules which are sensitized become reduced toatomic silver which is black in color and remains on the film. Wherever there are silver moleculeswhich havent been sensitized, they are washed away during the developing and washing process,leaving the clear plastic base and thus appears white on the radiographic viewer.

Therefore the degree of blackness depends on the number of x-rays which reach the film andultimately on the number of silver bromide crystals affected. Film that is not exposed to x-rays (orlight) remains transparent (white) after development. Film exposed to a great deal of radiation (orlight) appears black after development. Film exposed to intermediate amounts of radiation appearsvarious shades of gray. This is NOT because the silver is actually gray; it just appears gray becauseof the mixture of black silver granules and intervening transparent (white) areas.


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IV. Radiographic Opacity (Radiopacity)

The subject intentionally radiographed in veterinary medicine is typically some animal orportion thereof. Animals are composed of organs and substances that absorb x-rays to varyingdegrees. It should be kept in mind that absorbed x-rays do not reach the film and thus do notultimately (after development) turn it black. Substances that readily absorb x-rays (i.e., x-rays donot readily penetrate them) are said to be radiodense.

A. Definition

Radiodensity is simply the property of being resistant to the passage of x-rays. Themore radiodense an object or substance is, the more x-rays it absorbs and thus the film willappear white. The less radiodense an object or substance is, more x-rays penetrate it (i.e.,does not absorb as many x-rays) and thus reach the film which will appear black. Since wecould talk about optical density (how black the film is), subject density (what the patient ismade of), and physical density (grams matter per cubic centimeter), the term density canbecome confusing. Therefore, the profession has moved toward using the term opacity orradiopacity to describe the degree of blackness or whiteness of any structure on theradiographic image. (See below)

B. Radiopaque and radiolucent

These terms are relative descriptive terms for how a structure appears on theradiographic film. The radiopacity and radiolucency of the patients tissues describeshow white or black they appear with respect to one another. For example: Bone is moreradiopaque (whiter) than soft tissue on radiographs. Air is more radiolucent (blacker) thansoft tissue. The more radiopaque an object is, the more resistant it is to the passage of x-rays.Therefore it absorbs the x-rays, preventing them from penetrating the object and passing ontothe film. If the x-rays do not reach the film, that area will appear white on the film and thestructure is thus said to appear relatively radiopaque. The less radiopaque an object is, theless resistant it is to the passage of x-rays. Therefore it allows more x-rays to pass throughand reach the film. If the x-rays reach the film, that area will appear black on the film andthe structure is said to appear relatively radiolucent.

C. Radiographic opacities

An objects radiopacity is affected by 2 variables:1. its physical density or atomic number2. its thickness

1. 5 Basic radiographic opacities

Metal Mineral (or Bone) Soft Tissue Fat

Air (or Gas)


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There are 5 basic opacities that can be distinguished on radiographic films.Only 4 of these 5 are biological opacities. The most dense material is metal andtypically appears white on films, since most (if not all) x-rays are unable to passthrough (i.e., they are all absorbed). Obviously, metal appears quite radiopaque.Bone is not quite as dense, but only allows a few x-rays to pass through whencompared to other body tissues. Thus bone appears a little more radiolucent thanmetal, but is the most radiopaque tissue in the body. Fluids (i.e., water) allow evenmore x-rays to pass through and appear gray on films. Fat allows even more x-rays topass through, but still absorbs more than air, which absorbs nothing at all and appearstotally black. Important: Non-fat soft tissues (e.g., muscle, liver, spleen, bowel wall,etc.) and fluid (e.g., blood, urine, transudate, exudate, etc.) are exactly the sameopacity on radiographs (given an equal thickness radiographed). They all are softtissue opacity. This means that one cannot tell the difference between a solid softtissue structure (e.g., a solid mass) and a fluid-filled structure (e.g., a cyst) onradiographs!! To distinguish between the two, one would need to perform another test(i.e., ultrasound, MRI, or perhaps CT).

2. Object thickness

The second variable that affects an objects radiopacity is its thickness. The

thicker an object is, the more x-rays it absorbs and thus the whiter or more radiopaqueit appears on film. For example, which do you think would appear more radiopaque(white) on a film: a water balloon the size of a grapefruit or a sheet of aluminumfoil laid flat? Survey says..the water balloon! How can that be? I thought we saidthat metal is more radiopaque than water! True, but..even though the water balloonis less dense than the foil, it is so much thicker that it absorbs more x-rays than thethin little sheet of foil. An even though we said that most (if not all) x-rays areabsorbed by metal, thickness can have a great impact. X-rays CAN pass through aTHIN piece of metal (e.g., the sheet of aluminum foil). This can easily be seen on abiological specimen if you examine a routine abdominal radiograph of a dog. On aleft-right lateral abdominal radiograph, the liver (which is nothing more than a bag ofwater) will appear much more radiopaque than the spinous process of a lumbarvertebra (which is mineral opacity)!

D. Superimposition and summation

Related to the discussion on radiopacity is the concept of superimposition andsummation. With the exception of isolated bones and organs (i.e., necropsy specimens) mostradiographs are made of living animals. Where structures are superimposed in the path of anx-ray beam (which will basically be the case in all of our subjects) their relative radiopacitiesare altered. Their radiopacities are summed. A film of a dogs antebrachium, for example,has images of the skin, hair, musculature, etc., as well as the radius and ulna. Obviously,where the bones appear, soft tissue is superimposed on it. In the areas where the two bonesoverlap, the effect of this superimposition is dramatically demonstrated because those areasappear especially white (radiopaque) in comparison to the whiteness of either the radius orulna alone.


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V. Radiographic Views and Orientation

A. Necessity for 2 views

One cannot read depth into a radiograph, like you can with a photographic image!Remember that a radiograph is a 2-dimensional image of a 3-dimensional object and sincewe cannot see the surface (like a photographic image) we cannot determine depth. If youever find yourself thinking this looks closer to me than that or this looks farther away STOP! Youre trying to read depth.

Yet we are trying to effectively evaluate a 3-dimensional animal! Therefore, we mustalways take at least 2 views of whatever our region of interest is at a 90-degree angle to oneanother. Additionally, we may take more that 2 views (e.g., oblique views) to fully evaluatethe area (e.g., equine carpus).

B. Proper naming of radiographic views

There are 2 basic rules in naming radiographic views:

1. Use official nomenclature (directional terms and body parts).

2.

Indicate the path that the x-ray beam travels through the subject from itspoint of entry to its point of exit.

For example, a craniocaudal (abbreviated Cr-Cd) view of a dogs antebrachiumwould indicate that the dogs antebrachium has radiographed and that the x-ray beam passedthrough the limb in a cranial-to-caudal direction. In other words the caudal surface of theantebrachium was against the film and the x-rays passed into the cranial surface and exitedthe caudal surface. A dorsoventral (abbreviated D-V) view of a dogs abdomen indicates thatthe abdomen was radiographed with the x-ray beam passing from dorsal to ventral throughthe animal. A left-right lateral (L-R lateral) view of a dogs thorax indicates that the x-raybeam passed from left to right through the dogs thorax.

Not all views are made parallel to the 3 major body planes. For example, a

dorsolateral palmaromedial oblique view (DL-PaMO) of a dogs carpus is made with thebeam entering the dorsolateral aspect of the carpus and emerging from the palmaromedialaspect.


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C. 3 Factors of orientation

When you first begin to interpret radiographs, the first thing you must do is getoriented. Is it a thoracic limb or a pelvic limb? Is it a right limb or a left limb? What is theradiographic view? Is the film positioned correctly on the viewer? There are 3 factors thatmust be taken into account to get properly oriented:

1. What is the body part?2. What is the view that was taken?3. Is the film positioned on the viewer correctly?

Once you are provided with any 2 pieces of this information, you can derive the thirdby deductive reasoning.

A radiograph has 2 sides and 4 edges, thus there are 8 different ways it can be placedon the viewer. It is now widely accepted that one should view films in standard orientation(i.e., head-to-the-left and head-to-the-top placement) regardless of how the film was taken.This practice dramatically improves the eye/brains ability to recognize patterns, andtherefore increases the likelihood that lesions will be identified.


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VI. Radiographic Aberrations

A. Magnification

All radiographic images are larger to some degree that the actual subject. This occursbecause the x-rays originate from a small focal point in the x-ray tube (i.e., they come from apoint source) and diverge from each other as they travel from the tube. Objects very close tothe film are magnified very little. Objects further away (from the film) are magnified to aprogressively greater degree. In a left-right lateral view of the thorax, the left ribs (fartherfrom the film) will be slightly larger (in both length and diameter) than the right ribs. Thisdifferential enlargement can sometimes be useful in mislabeled or unlabeled films.

As a general rule, we always want our subject (or area of interest) to be as close to thefilm as possible. The farther the subject is from the film, the more magnification anddistortion we get. As we get our subject farther from the film, not only do we getmagnification that can interfere with normal interpretation, but we also lose detail due todistortion. Just think of holding your hand in front of a flashlight and producing a shadow ofyour hand on the wall. The closer your hand is to the wall, the shadow produced is muchmore accurate in size and much sharper. The farther your hand is from the wall, the shadowproduced is magnified and blurry.


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B. Foreshortening

Foreshortening refers to the physical principle that reduces any dimensions of anobject viewed that are not perpendicular to the x-ray beam. For example, a pencil viewedend-on or obliquely appears shorter than its true length. The same phenomenon occursradiographically. Therefore, as a general rule, we want to keep our subject as parallel to thefilm as possible.

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