Chest CT Scan: Indications and Anatomy

• Computed tomography (CT) scan of the chest is primarily used to evaluate and define diseases identified by routine chest X-rays.
• Even though plain chest radiographs can diagnose many lung disorders, they have limitations in providing definite diagnoses.
• CT scans offer shorter examination times, thinner sections, and higher resolution compared to radiographs.

CT Importance in Chest Imaging

• Detects diseases affecting the lungs, mediastinum, and chest wall.
• Assists in planning interventions such as biopsies or surgeries.

Chest Anatomy

• The thoracic cavity lies between the neck and abdomen.
• It stretches from the diaphragm inferiorly to the superior thoracic aperture superiorly and is bounded by the thoracic wall.
• The thoracic apparatus includes the first thoracic vertebrae, the upper border of the manubrium sterni, and the first ribs.
• Anteriorly, the thoracic cavity is bordered by the sternum, while posteriorly it is bounded by the thoracic part of the vertebral column.
• The thoracic cavity is divided into two compartments: the mediastinum in the center, flanked by a pleural cavity on either side, which houses the lungs, while the mediastinum contains the heart, associated vessels, and other vital structures.

Lungs

• The lungs consist of a spongy material called the parenchyma, which contains the fine structures of bronchial trees and pulmonary circulation.
• The exchange of oxygen and carbon dioxide occurs at the alveolar level within the parenchyma.
• The right lung has three lobes: upper, middle, and lower, separated by the horizontal fissure between the upper and middle lobes, and the oblique fissure between the middle and lower lobes.
• The left lung has two lobes: a superior and inferior lobe, divided by an oblique fissure.
• Hilum is where the bronchi, blood vessels, lymph vessels, and nerves enter and exit each lung.

Chest Wall

• The chest wall includes the ribs, sternum, clavicles, chest muscles, neurovascular bundles, skin, and subcutaneous tissues.

Diaphragm

• The diaphragm is a muscular structure below the lung bases.
• It is divided into right and left hemidiaphragms, with the right side positioned higher on a chest radiograph due to the presence of the inferior liver.
• The term cardio-phrenic angles describes the area where the heart's border makes contact with the diaphragm.

Pleura

• Each lung is surrounded by a thin-walled sac called the pleura.
• The pleura has an inner visceral layer and an outer parietal layer.
• The potential space between these layers is the pleural space, becomes significant when filled with fluid (pleural effusion), air (pneumothorax), or blood (hemothorax).

Mediastinum

• The mediastinum, located between the lungs, contains the heart, great vessels, proximal pulmonary arteries, aortic root, proximal bronchial trees, pulmonary veins, esophagus, thymus gland, and lymphatic vessels.

CT Imaging Technique

Patient Preparation

• Patients should fast for 4-6 hours before the scan if IV contrast is used.
• The procedure should be explained to reduce motion artifacts.
• Position patient supine with arms raised above the head to reduce beam hardening artifacts.

Scanning Parameters

• kVp: 120-140 helps for optimal image contrast.
• mA: Automated dose modulation is based on patient size.
• Slice Thickness: Use thin slices (1-3 mm) for high-resolution images, especially for the lungs.
• Pitch: 0.8-1.2 is optimal for spatial resolution.

Contrast Media Protocols

• Use iodinated IV contrast to enhance mediastinal structures, vascular anatomy, and masses.
• Injection rate is 3-5 mL/s.
• The typical dose is 1-2 mL/kg body weight.

Timing

• Pulmonary Arterial Phase: 15-25 seconds post-injection for evaluating pulmonary embolism
• Venous Phase: 45-70 seconds for mediastinal and chest wall evaluation.

Breath-Hold Technique

• Patients should hold their breath during scanning to avoid respiratory motion artifacts.

Reconstruction Algorithms

• High-Frequency Algorithms: Use algorithms for lung parenchyma.
• Soft Tissue Algorithms: Use for mediastinal structures and chest wall.

Window Settings

• Lung Window: WL: -600 HU, WW: 1500 HU, for optimal visualization of lung parenchyma.
• Mediastinal Window: WL: 40 HU, WW: 400 HU, for soft tissues.
• Bone Window: WL: 300 HU, WW: 2000 HU, for chest wall structures.

Clinical Indications for Chest CT Imaging:

• Lungs: Detection of infections (e.g., pneumonia, tuberculosis), evaluation of lung tumors/nodules/interstitial lung disease/ pulmonary embolism and emphysema.
• Mediastinum: Staging of lung cancer, detection of mediastinal masses (e.g., thymomas, lymphomas), and evaluation of vascular abnormalities (e.g., aortic aneurysm, dissection).
• Chest Wall: Trauma assessment, detection of chest wall tumors/infections), and evaluation of post-surgical complications.

CT Guided Lung Biopsy:

Indications

• Investigation of new pulmonary opacity after the failure of the bronchoscopy.
• Investigation of new lung/pleural/chest wall lesion in a patient with known malignancy.
• Obtaining the material for culture when other techniques have failed.

Contraindications

• Not absolute, and evaluated by multidisciplinary teams.
• Vascular: Bleeding diatheses, patients on anticoagulants or having significate pulmonary arterial.
• Respiratory: Contralateral pneumonectomy, significant emphysema, and significantly impaired respiratory function.
• Suspected hydatid disease.
• Uncooperative patient.

Equipment

• Sampling Needle: Fine needle aspiration(20 or 22G). and cutting needle biopsy (18 or 20G).
• Full resuscitation equipment, including equipment for chest drain is also required.

Patient Preparation

• Sedative premedication, avoid as the patient must remain conscious for consistent breathing.
• The procedure routinely be performed on a day-case outpatient basis, with observation and availability for 4–6 hours.
• Clotting and pulmonary tests.

Technique

• Procedure to the patient particularly with regard to breathing instructions and avoid deep breath.
• Position patient in stable position on back, front or side, determined the route from skin to lesion for emphysematous lung tissue.
• Aseptic technique Utilized.
• Inject local anesthetic into skin anaesthetic will act over a greater area of pleura and avoids the pleura.
• Sampling needle is advanced towards the lesion and samples are taken.
• Limited CT at the end of the procedure determine if a pneumo or parenchymal bleed is present.

After-Care

• Close observation post-procedure for at least 1 hour, the patient laying in the puncture site to stop the breath if need it.
• Departmental PA chest x-ray is performed to ensure any developed of the pneumothorax.
• If a Patient developed Pneumothorax there will be needed further management depend if there symptomatic or not.
• Ensure that if the patient those with pre-existing impairment are admitted.

Complications

• Pneumothorax: chest drain insertion, patient with coughing.
• Pulmonary hemorrhage is local.
• Hemoptysis.
• implantation of malignant.
• Death Rate: 0.15%.

Computed tomography (CT) scans of the chest are vital for evaluating diseases initially detected by chest X-rays. CT scans provide higher resolution and shorter examination times. They help detect diseases of the lungs, mediastinum, and chest wall, and aid in planning interventions.