FUNDAMENTALS OF IMAGING

RAD 206

Geometric factors

PRACTICE QUESTIONSTwo erect chest x-ray were done using AP and PA techniques, Estimate distance between the heart and the back surface of the patient..

Note:Direct exposure on film, No Bucky was used, SID for erect CXR is 180cm

SIZE AND DISTORTION

Umbra

Minimizing Magnification also improves Detail

Penumbra

DISTORTION

DISTORTION

SHAPE DISTORTIONDepends on

Objects ThicknessThicknessPositionPositionShapeShape

OBJECTOBJECT THICKNESSTHICKNESS

Thicker objects have more OID and

are more distorted than thinner objects

POSITION DISTORTION

OBJECT POSITIONOBJECT POSITION

POSITION DISTORTION

When we have two separate objects with different OID’s , when they are superimposed they show as one object , which produces a position distortion

• If the object plane and the image plane are Parallel, than the image is not distorted,

(hence if one of them is not parallel to the other than the image will be distorted)

• If the CR if Perpendicular the object plane than the image should not be distorted

Anatomy or objects can cause considerable distortion when imaged off central axis ( i.e. away from the center of the image)The further the distance from the center of the image, the more is the beam angle, and the more the distortion, specially for non-linear or irregular anatomy

Foreshortening : when the object is inclined to the Beam (CR) near the CR axis, its image looks shorter

Elongation:a)when the Image Receptor is inclined to the CR (object is not inclined ), or

b)When the Object is inclined lateral (away) from the CR axis,

Note: If you continue increasing inclination of the object than foreshortening will occur

SHAPE DISTORTIONForeshortening and elongation of the image

How to avoid distortion when the anatomical body part is inclined such as

The beam should be angled half the parts angle with IR, directed towards the raised side of the part 

ACTIVITY