Distortion?

lucky_yooh07 · Joined: 26 Jan 2009 Posts: 12

Could someone explain distortion to me?

Calculus · Joined: 09 Feb 2009 Posts: 4

Distortion - The alteration of the original shape (or other characteristic) of an object, image, sound, waveform or other form of information or representation.

Remember that Contrast and Density are two photographic properties that comprise visibility of detail, well, in addition to Recorded Detail, Distortion is the second of two geometric properties affecting radiographic image quality.

Unlike the photographic properties of density and contrast which control the visibility of detail, the geometric properties control detail itself. Distortion is a misrepresentation of the size or shape of the structure being examined. It creates a misrepresentation of the size and/or shape of the anatomical part being examined. This misrepresentation can be classified as either size or shape distortion.

Distortion, like recorded detail, exists even when it cannot be seen due to poor visibility or, in other words, when the density and/or contrast are poor. The evaluation and adjustment of distortion require a thorough familiarity with normal radiographic anatomy. Unless the normal shape and size of a structure are actually known, comparison of the size and shape cannot be accomplished.

Distortion can be difficult to determine even when normal sizes and shapes are known. Because the objective of radiography is to provide accurate images of structures, methods of minimising distortion are important to diagnosis.

wvaio · Joined: 22 Sep 2007 Posts: 82

Distortion is not always good and not always bad. There is always distortion on every image because the image is never exactly the same as the patient. Reason being, the divergent beam. This being said, it is necessary to control the effects of distortion to create a diagnostic image.

There are two different types of distortion: size and shape

Size distortion is controlled by distances. If you increase your SID(source to image distance) you can decrease the inherent magnification. That is why we use 72" on a chest X-ray. The added SID creates an image that more closely represents the actual size of the heart and other structures. This is also why we perform a left lateral instead of a right lateral. Placing the heart closer to the IR(image receptor) reduces the OID(object to image distance) and therefore reduces the magnification.

Shape distortion is controlled by angulation and by alignment. Alignment of the cassette, the part being imaged, and the central ray are critical. Generally speaking, you want the part to be parallel to the cassette and the central ray to be perpendicular to the part and cassette. Shadows can be used to imagine the effects of distortion. If you take a flashlight and shine it toward your hand onto a table, you shadow is bigger than your hand because of divergence. The further from the table your hand is, the bigger it gets and vice-versa. If you keep your hand flat and then angle the flashlight the shadow of your hand will become elongated. This is distortion. If you move the flashlight back up to straight up and down and then angle your hand down to a 45 degree angle, your hand will appear shorter than it actually is. Finally, if you take a book and lay it at a 45 degree angle and then shine a flashlight down onto your hand, your hand will appear longer than it actually is. Light has the same fundamental effects as X-rays; both possess a divergent beam. Grab a flashlight and try these tests. Distortion will make complete sense.

Distortion can be used to enhance an image. The RAO sternum is a perfect example. If you utilize a 30" SID, the posterior riblettes will be "distorted" because of the short SID and the long OID, making them lose detail . Since the sternum is so close to the IR it will still possess its recorded detail and be less obscured by posterior riblettes. (Riblettes is more fun to say than ribs)

One more example of the benefits of distortion is the L-spine. While it is usually performed in an AP projection, using a PA projection would better visualize the joint spaces because they would mirror the divergent beam due to the lordotic curve of the lumbar. ///lll\\\ vs. \\\\lll////

Hope that helps. If you have more questions, feel free as always to ask.
Wvaio
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“Whether you think that you can, or that you can't, you are usually right.” --- Henry Ford

hjhogle · Posted: Fri Feb 13, 2009 7:41 pm Post subject:

Calculus - when quoting from text or other published sources, referencing that source is the way to go. Otherwise, it looks like you're attempting to pass off someone else's material as your own.

Which I know you didn't intend.

hjh, RT

dawnzky05 · Joined: 13 Feb 2009 Posts: 3

We should always consider the machine function and related factors causing distortion. On the other hand, patient's condition might also cause this one.

Calculus · Joined: 09 Feb 2009 Posts: 4

hjhogle:

I didn't realise I'd be required to post references on an internet chat forum.

hjhogle · Posted: Sat Feb 14, 2009 2:57 pm Post subject:

Ha! This is an anonymous forum .. you're hardly "required" to do anything at all (including actually being a professional imager).

Certainly if you have a problem with citing a source, leave it out.

hjh, RT

Calculus · Joined: 09 Feb 2009 Posts: 4

Good to hear

Wink

wvaio · Joined: 22 Sep 2007 Posts: 82

Hjh is very correct.While this is an anonymous message board, it should still be necessary to quote a source when quoting verbatim. Carlton and Adler spent their time, effort, and energy to write the text in the book and should be credited for such. Not doing so questions the integrity of the forum and is unprofessional. Please give credit where credit is due. Everyone would want the same consideration.

Wvaio