Patient preparation Show
Ask the patient to remove clothes from the waist up, put on a hospital gown, remove any jewelry (necklace, earrings), and, if necessary, tie hair up on top of the head. For posteroanterior (PA) projections, instruct the patient to sit or stand upright. Patients are positioned to face the film-screen cassette in order to minimize magnification of the anteriorly positioned heart and consequent obscuration of the lungs. Make sure the patient is standing straight and is equally distributing the weight of the body on both feet. The upright position is preferred for the following reasons:
Adjust the height of the cassette so that its upper border is about 2 inches above the shoulders so that the lung apices are not cut off. Ask the patient to extend the neck, chin, and head upward and vertical. The neck and chin otherwise tend to superimpose the trachea and uppermost lung regions. The patient's arms are placed overhead or on their hips with elbows angled anteriorly. This will rotate the scapulae off the chest, thereby preventing their superimposition over the lungs. In female patients with large pendulous breasts, it is very important to minimize breast shadows. In some patients, depending on the density and size of breasts, breast shadows cannot be totally eliminated. Breast shadows render the examination suboptimal by superimposing over the lower part of the lung fields and obscuring any pathology. Ask the patient to pull the breasts upward and laterally (outwards), then remove her hands as she leans against the cassette holder to keep them in position. Place a lead shield between the x-ray tube and the patient's pelvis. Gonadal shielding from the level of the iliac crests, or slightly higher, to the mid-thigh area should be used on all patients of reproductive age. Rotation Even a small degree of rotation distorts the mediastinal borders, and the lung nearest the film will appear less translucent. The following points should be stressed to obtain a true PA view (without rotation):
Central ray For PA, the chest landmark that is used for locating the center of the lung fields is the vertebra prominans (T1). It corresponds to the apical regions of both lungs. The vertebra prominans can be palpated at the base of the neck. The hand-spread method can be used to locate the central ray. To use this method, the technologist should know his or her own hand-spread width (most hands can reach 7 inches). For the average adult, the central ray should be directed to the spinal column (mid-sagittal plane) approximately 7 inches (18 cm) for a female and 8 inches (20 cm) for a male down from the vertebra prominans. This corresponds to the level of T7 and the inferior angle of scapula, a source to image receptor (SID) distance of 72 inches (180 cm). Hypersthenic body types require only 7 inches down from the vertebra prominans, whereas athletic sthenic/hyposthenic types will require 9 inches down from the vertebra prominans. Film holder (image receptor) placement The horizontal dimension of an average chest is greater than the vertical dimension. This requires that a 14 x 17-inch film holder or image receptor (IR) be placed crosswise. However, depending on the body type, the most common practice is placing the film holder or IR lengthwise. Either lengthwise or crosswise, the goal is to adequately include the lateral lung margins. Collimation Collimation should be adequate to allow for some margin of error in central ray placement and lung expansion during deep inspiration. On each side of the posterior chest the illuminated field margins should correspond to the outer skin margins. The upper border of the illuminated field should be at the level of vertebra prominans (4 cm above the apex of lungs).This will result in a lower collimation border of 1-2 inches below the costophrenic angle, if the central ray was correctly centered. Respiration Be sure to make the exposure upon a second full inspiration by the patient. The patient should take as deep a breath as possible, and then hold it to fully aerate the lungs. Taking a second deep breath before holding it allows for a deeper inspiration, as more air is inhaled during the second breath than during the first breath. The best way to determine the degree of inspiration is to start at the top of the patient's rib cage, with rib number one, and count down to the tenth or eleventh rib posteriorly. A general rule for average adult patients is to show a minimum of 10 ribs on a good PA chest radiograph. Older patients have less inhalation capability, with a resulting low lung volume, which requires a higher central ray location. Evaluation criteria for a good PA projection
Variations An expiratory film may be helpful under some circumstances. A small pneumothorax (air in the pleural cavity) may be difficult to detect on a routine inspiratory PA film. On expiration, the volume of the thorax and lungs is reduced but the amount of air in the pleural space remains essentially unchanged. The pneumothorax then occupies a larger percentage of the area of the thorax and is more easily visible. Another indication for an expiratory film is to demonstrate air trapping. The bronchi increase in diameter with inspiration, and decrease with expiration. With a foreign body or tumor in a main bronchus, a valve action may occur, with air bypassing the obstruction on inspiration and becoming trapped on expiration With expiration, the normal lung is reduced in volume and becomes more radiopaque. The obstructed portion of the lung retains its air, thereby retaining its radiolucency and forcing the mediastinum to shift toward the contralateral side. If a patient has a unilateral respiratory wheeze, air trapping is likely, and an expiratory film may be helpful. Imaging Technique
AuntMinnie.com contributing writer October 19, 2001 Dr. Ahmad is a second-year resident affiliated with Columbia University's New York-Presbyterian Hospital. He is the owner and developer of RadQuiz.com, a compendium of radiology teaching files on the Internet. Copyright © 2001 AuntMinnie.com How far above the top of the shoulders should the IR be positioned for any decubitus position of the chest?Chest Abdomen practice questions #2- class 3. How many ribs should be visible above the diaphragm on a PA chest radiograph?The exposure should be made at full inspiration and should show rib 10 posteriorly above the diaphragm and rib 6 anteriorly. Both costophrenic angles and the lower parts of the diaphragm should be visible.
Where are the patient's hands placed for a PA projection of the chest?Chest: Patients are positioned for chest radiographs with an erect posture of the chest and a slightly extended chin. The hands are placed on the hips with the palms facing out, and the shoulders are rolled forward.
What is the degree of body rotation for the PA oblique Lao position of the chest during a cardiac series?Before midterm. |