Shape Distortion

Shape Distortion • Shape distortion is the misrepresentation of the object compared to the image. • Most shape distortion is the result of angulation of the tube or film. • This can be beneficial.If we can move overlying anatomy or open disc spaces, shape distortion is good.

Shape Distortion • The ideal x-ray image would be to see the area of interest free of overlying structures. • A cephalad tube angle is toward the head. • A caudal tube angle is toward the feet. • The direction and amount of angulation is determined by the angle of the area of interest.

Tube Angle: Cephalad • Tube angled toward head. • Reduce SID one inch for every five degrees of tube angulation. • Used to get angled body parts perpendicular to film.

Cephalad Tube Angle • The amount of tube angle is determined by adding the angle to 90 for erect radiographs. • Anterior structures are on top of the image. • Posterior structures are projected to the bottom of the image.

Cephalad Tube Angle • This is an A-P Cervical Spine view. • The tube angle is 15° cephalad. • The tube is set to 105° • The central ray is parallel to the lower disc spaces.

Tube Angle: Caudal • Tube angled toward the feet. • Decrease SID by one inch for every five degrees of tube angulation. • Directs Beam through angled body part.

Caudal Tube Angle • The required tube angle is subtracted from 90 for erect radiography. • Posterior structures move to the top of the film. • Anterior structures move to the bottom.

Caudal Tube Angle • The patient is positioned A-P. • The tube is angled 15° caudal or set to 75° • This is the wrong tube angle to see the cervical spine.

Tube Angulation • Erect Radiography requires adding or subtracting the required angle from 90 degrees. • Cephalad angles: Add required angle to 90 degrees. • Caudal angles: Subtract required angle from 90 degrees.

Tube Angles • The direction will be affected by the angle of the anatomy. • Example: Sacral Base Angle 30-40° • A-P angle would be 30° cephalad • P-A angle would be 30° caudal.

No Tube Angle • If we wanted to see the top two cervical vertebra, no tube angle is needed. • If the mouth was open, we would see the dens well without a tube angle. • No shape distortion.

P-A Skull Film • The P-A or A-P skull is taken without any tube angle..

Chamberlain-Townes • The Townes Projection is part of a routine skull series. • The skull is in the same relative position as the P-A view • The tube is angled to throw the anterior part of the skull away from the occipital region of the skull.

Tube Angle Observations • 1. Which view has the mandible obscuring the A-P cervical spine? • Caudal tube angle • 2. If the mouth was open, which view would best demonstrate C-1 and C-2? • No tube angle

Tube Angle Observations • 3. Which view best demonstrates the cervical disc spaces? • Cephalad Tube Angle • 4. When the tube was set to 90° was there any tube angle in relation to the patient or film? • No

Tube Angle Observations • 5. When the tube was set at 105° was this a cephalad or caudal angle? • Cephalad • 6. What do we need to do to account for tube angles with erect radiography. • Add angle to 90° for cephalad angles. • Subtract angle from 90° for caudal angles.

Beam Centering • The central ray must be centered to the area of interest . • If the beam is not centered, the shape of the area will be distorted.

Shape Distortion • The object of interest must be parallel to the film to avoid foreshortening. • If the object is angled in relation to the central ray, the object will have shape distortion.

Beam Centering • The center image has the beam properly aligned. • The right and left images have the beam not centered properly.

Observations for Beam Centering • 1. Which image most accurately demonstrated the true shape of the object? • With the beam centered to the object. • 2. When the tube is improperly centered too high or too low, what happens to the image. • The top structures are projected off center opposite to the direction of tube shift.

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Shape Distortion