Indications for imaging
To clarify anomaly seen on PA projection e.g. Interlobular effusion, Pancoast tumour (Pancoast tumor, superior pulmonary sulcus tumor, an adenocarcinoma of a lung apex causing Pancoast syndrome)
Pancoast syndrome, pain and tingling of the arm over the area of distribution of the ulnar nerve, constriction of the pupil, and paralysis of the levator palpebrae superioris muscle, due to pressure on the brachial plexus by a malignant tumor in the region of the superior pulmonary sulcus. 

Anatomy  Demonstrated
Lung apices and the medial ends of the first 4 ribs

Basic Patient Position 

There are general three principal  versions of the projection however all the general principal is to "stretch out" the lung apices and avoid superimposition of the clavicles over the apices. the differences between the projections are, 
a) Horizontal central ray patient's coronal plane angled, (lordotic)
b) Angled central ray patient erect, 
c) either of the above performed AP, PA, erect or supine.
The horizontal ray projection is best if there is a need to demonstrate a fluid level in a cystic structure, the patient erect may be safest for the patient, a PA projection will minimise radiation dose to the thyroid.

Horizontal central ray patient's coronal plane angled, (lordotic)
The patient stands AP erect approximately 30 cm from the film then leans back so the coronal plane is 30 degrees from parallel to the film, the head and dorsal skin surface in contact with the cassette, the backs of the hands are placed on the lateral aspects of the waist and the shoulders rolled forward. Exposure is made on suspended inspiration.

Apical Chest Positioning 
Meschan, I. 1955 An Atlas of Normal Radiographic Anatomy Saunders, London

Exposure Factors

Evaluation of the Image
ID and markers must be present and correct in the appropriate area of the film.
Limits of anatomy, superiorly the skin margins of the apices, inferiorly the T4, laterally the ribcage.
No rotation, The apices should be symmetrical about he midline.
Centering, T6
The medial ends of the clavicles should be projected above the lung apices.
The apex scapulae should be clear of the lungfields.
Penetration the vertebral outlines should be visible.
Density the average  density of the lungfields should be approximately 1.

Apical Chest projection (Demonstrating TB in the  Rt apex)