Chest Radiology

Conventional radiographs make up approximately 30% of radiology volume. Should represent the the initial imaging study for all patients.

Components of a radiology chest unit:
 * High kilovoltage-potential source
 * Grid: reduces scatter
 * Phototimer: to control exposure length

Important Terminology

 * Penetration: The intervertebral disk spaces should barely be seen.
 * Rotation: Should be able to make a T, a horizontal line should run through the clavicular heads, and a perpendicular vertical line should run down the spinous processes.
 * Inspiration: The right diaphragm apex should be below the 10th posterior rib.
 * Motion: Should be sharp contrast along the cardiac margin, diaphragm, and pulmonary vessels.

AP
Radiation source is above the patient. Typically a portable unit, gotten when the patient is too sick to stand up. These units are limited in maximal kilovoltage which necessitates longer exposure time which results in greater propensity for motion artifact.

Drawbacks of an AP Film


 * Greater motion artifact


 * Magnification of intrathoracic structures (15%-20% increase in cardiac diameter)


 * Supine positioning leads to diaphragm elevation which causes compression of lower lobes and decreased lung volumes, equalization of pulmonary flow between upper and lower lung zones, and an increase in systemic venous return which causes widening in the upper mediastinum.


 * Supine positioning may also mask pleural effusions which will layer out.


 * Supine positioning may also hide a pneumothorax because the non-dependent portion is no longer the lung apices.

CT and HRCT
Major advantage of CT is better contrast resolution and cross-sectional display format. The better contrast resoultion allows the radiologist to determine composition of anatomical structures: calcium, soft tissue, and fat. The cross-sectional format prevents superimposition of structures as is typical for normal chest radiography.

Normal CT occurs in a stop, acquire, go mode while HRCT proceeds in a helical mode where the gantry does not stop during image acquisition.

HRCT uses thin scans to evaluate for bronchial or parenchymal lung disease. Expiration HRCT scans are useful for detecting air trapping.

MR
Most studies will be done by T1W or T2W. If a mass is being evaluted there may be benefit from fat suppression like STIR or gadolinium-enhancement. If a blood vessel is being evaluated then GRE volumetric acquisitions are useful. Cardiac evaluation is best done with cardiac gating.

Most important advantage of MR is ability to differentiate fat and tumor and the ability to determine tissue type based on T1 and T2 relaxation times, as well as lack of need for iodine contrast. MR has the ability to do a non-reconstructed direct scan in the sagitall and coronoal plane which are beneficial if the target area is in the axial plane like superior sulcus tumors, subcarinal and aortopulmonary window lesions, and hilar masses.

Examples of MR vs. CT superiority:


 * diagnosis of chest wall or mediastinal invasion because of increased contrast between muscle, fat, and tumor.


 * distinguish tumor from fibrosis (for followup of Hodgkin disease patients)


 * Identify fluid filled cysts, hematoma, and hemorrhage


 * evaluation of aortic disease if patient is hemodynamically stable

Examples of MR vs. CT inferiority:


 * detecting calcium


 * limited spatial resolution


 * poor imaging of pulmonary parenchyma


 * takes more time


 * more expensive

PET
Based on detecting metabolic activity of neoplastic and inflammatory lesions. If used complementary to CT can be helpful for determining lesion significance.

Ultrasound
Used for detecting, characterizing, and sampling pleural, peripheral parenchymal, and mediastinal lesions.

Techniques
Lateral decubitus: patient lies in the decubitus position for detection of pleural effusions (minimum 5mL of fluid), or a small pneumothorax (minimum 15 mL of air).

Expiratory radiograph: Obtain at residual volume (end of maximal forced expiration) to detect air trapping and to detect small pneumothorax.

Apical lordotic view: Improves visualization of the apices normally obscured by the clavicles and 1st costochondral junctions.

Chest fluoroscopy: to assess chest dynamics in patients with possible diaphragmatic paralysis.

PACS Workstation
Window level

Window width

Example settings
 * Mediastinal structure: WW=400 and WL=40
 * Lungs: WW=1500 WL=700