Chest Computed Tomography
The cross-sectional orientation and contrast sensitivity of CT are well suited for detecting, describing, and distinguishing among diseases of the thorax.
Common indications for a CT scan of the chest are broken into two main categories: those patients with abnormal CXR findings requiring further evaluation, and those patients with a normal CXR but with suspicion for occult disease.
Common abnormal radiographic findings prompting a follow-up CT include staging of bronchogenic carcinoma; evaluation of a nodule, mass, or opacity; and characterization of infiltrative lung disease, mediastinal, pleural, or chest wall abnormalities.
Common radiographically occult diseases include the evaluation of potential metastases, suspected aortic dissection, hemoptysis, bronchiectasis, infiltrative lung disease, endocrine abnormalities, or source of infection.
Similar to the interpretation of CXR, it is important not to bias your interpretation of the CT scan based solely on the patient’s known clinical history. While an understanding of the clinical scenario is important to focus on specific areas of the study, careful attention must be paid to the entirety of the examination to avoid missing pertinent findings.
Comparison to prior CT studies is also essential to both characterize the time progression of lesions, and to determine whether subtle findings truly represent pathology.
Ordering a CT
Not all chest CT examinations are performed in the same way. For efficient throughput, many CT departments use a variety of protocols to scan the thorax. These protocols are created to convey to the technologists information on radiation technique, reconstruction techniques, and methods of contrast use and enhancement. The protocols are usually based on clinical scenarios, such as aortic dissection, pulmonary embolism, or interstitial lung disease, and providing a meaningful indication for the examination will help ensure that the proper protocol is used.
There are several important aspects to be considered when preparing the patient for a CT scan, including the area to be scanned, the use of contrast, and the patient’s ability to tolerate the contrast.
The region of the body to be scanned should be documented, and will typically consist of a combination of the chest, abdomen, and pelvis. This decision will be made by the referring physician based on clinical context and may be adjusted by the radiologist as needed.
Increasingly, insurance restrictions do not allow for the changing of the region to be scanned by anyone but the referring clinician. Hence, careful thought at the time of writing the order can save added work later.
A chest CT tends to scan from the thoracic inlet through the adrenal gland.
An abdomen CT tends to cover the dome of the diaphragm through the iliac crests.
A pelvic CT scans from the iliac crest through the pubic symphysis.
The appropriate use of contrast is necessary to understand when ordering a CT study. A scan can be ordered with contrast, without contrast, and with and without contrast and is dependent on the indication for the study.
Contrast can be administered intravenously (most common) or orally (rare for thoracic conditions). As a general rule, IV contrast is indicated for patients with suspected hilar, mediastinal, or pleural abnormalities and in patients with potential vascular abnormalities such as a pulmonary embolus. It can help distinguish lymph nodes from hilar vessels, underscore the vascular component of arteriovenous malformations, and identify the enhancing rim characteristic of empyemas.
A noncontrast scan is generally indicated for assessing lung disease, ruling out pulmonary metastases, and for assessing nodules.
A chest CT scan with and without contrast is typically only indicated for evaluation and differentiation of an aortic dissection or intramural hematoma, initial evaluation of pulmonary arteriovenous malformations, or characterization of a known mediastinal mass.
There are four important considerations to understand if a patient can receive contrast: renal function, allergy, vascular access, and volume status.
Since contrast agents are excreted by the kidneys and may cause changes in renal hemodynamics or tubular toxicity, it is important to assess renal function prior to ordering a contrast-enhanced study, as contrast may result in irreparable damage to a borderline set of kidneys.
Many centers use a serum creatinine level because of its ease in acquisition and it can be converted via a simple equation to creatinine clearance, which is an estimation of the glomerular filtration rate (GFR). Normal creatinine clearance ranges from 100 to 160 mL/min with physiologic variation by age. Generally, IV contrast should be avoided in patients with a clearance of <30 mL/min.
Of note, patients on dialysis can receive contrast media since the contrast will be filtered in their next dialysis session.
It is also important to assess for a history of a reaction to contrast media when preparing a patient for CT. Patients should be specifically asked about iodinated contrast material, as many do not consider contrast a type of medication.
Shellfish allergy alone is not a contraindication to the use of IV contrast.
The severity of any reaction to contrast agents should also be characterized.
Generally, a patient with a history of itching or hives following prior contrast administration can receive premedication, whereas a patient with a history of prior serious contrast reactions such as laryngeal edema or anaphylaxis should not receive contrast despite premedication. The reactions can be somewhat idiosyncratic and tend to get more severe over time.
Premedication typically consists of a combination of corticosteroid and antihistamine. These medications may produce side effects of increased intraocular pressure or urinary retention, so a history of glaucoma or prostate enlargement should also be obtained.
When ordering a study with contrast, the vascular access of the patient is an important and potentially limiting consideration. Although convention may vary by institution, typically central access or peripheral antecubital access with a 20-gauge line or larger is required. Specific questions about access requirements are best addressed through consultation with the radiology department.
It is also important to remember that contrast is a bolus of fluid volume. Because of the osmolality, the contrast dose is equivalent to over 1 L of normal saline and may cause
problems for patients with pulmonary edema or cardiac issues. As a general guideline, if the patient could not tolerate a 1-L bolus of saline, they should not receive IV contrast.
CT Scans and Protocols
In the modern era, all thoracic CTs are performed on helical, multidetector CT machines. The term “spiral CT,” which gained popularity as a synonym for the PE protocol CT, is not helpful, as most scans are performed helically or spirally.
High-resolution CT (HRCT) is a scanning protocol often used to diagnose diffuse lung diseases, bronchiectasis, emphysema, and focal lung lesions.
HRCT does not require contrast and obtains detailed images that are comparable to gross tissue inspection.
HRCT uses thin slices to improve resolution and view the fine details of the pulmonary parenchyma.
In many centers, HRCT is performed in both inspiration and expiration.
Low-dose CT reduces the total radiation dose and is accomplished by lowering the tube current or kilovolt peak during the scan, which still results in readable images in the majority of patients.
This type of scan is typically indicated for lung cancer screening, in children, or if multiple follow-up examinations are required.
Low-dose CT may be limited by parameters such as patient size.
Various protocols also exist for the evaluation of pulmonary embolus, aortic dissection, and thoracic aorta pathology. The appropriate use of these protocols is best clarified through consultation with a radiologist.
Preparing the Patient
Patients can be hesitant about CT scans, which usually stem from lack of knowledge about the radiation dose and specifics of the procedure. It is helpful to relate the radiation exposure to that of natural background radiation, where one conventional chest CT is approximately equal in exposure to 3 years of natural background radiation.
It is also helpful to explain to patients that the scan can be interrupted or terminated at any time if problems arise and that they will be able to communicate with the radiographer in the control room through an intercom.
Claustrophobic patients may find it helpful to close their eyes during the examination.
Patients should also be aware of the need for controlled breathing throughout the study, as this reduces image noise due to diaphragmatic movement.
All clothing with zippers and all metallic objects should be removed to prevent confusion when interpreting the image.
Patients should be made NPO 4 hours prior to their scheduled scan. IV contrast material can occasionally be proemetic. Four hours allow the stomach to be cleared of contents so the risk of aspiration can be reduced.