Chest: Spiral CT of the Chest Part I

HRCT, Tumors and Nodules

Leo P. Lawler, MD, FRCR.

Assistant Professor


Introduction.

The chest contents may be divided into those structures whose primary function is gas exchange and those whose primary function is exchange of oxy and de-oxygenated blood to the heart. The Chest Part I (lung parenchyma) presentation is mainly concerned with the former and Chest Part II (systemic and pulmonary arteries) will concentrate on the latter.

The anatomical building block of the lung is the secondary pulmonary lobule made up of inter-lobular septations surrounding alveoli and a central bronchovascular bundle of lobular bronchi and pulmonary arteries. The central and peripheral airways and the alveoli they subtend comprise a sophisticated network allowing for delivery of atmospheric components to the blood stream and the excretion of unwanted waste-products of metabolism. They may be thought of as a scaffolding of connective tissue, blood vessels and lymphatics supporting the large surface area of epithelial lined airways and alveoli.

Many of the more common disease processes that present to the CT imager concern diseases of the intersitium (interstitial lung disease), which require high resolution CT studies (HRCT). The remaining lung tissue diseases affect the gas exchange surfaces in the form of nodules and masses and require non-contrast and contrast enhanced lung parenchyma protocols.

High resolution CT-technique.

The goal of high resolution CT is to provide static and functional gross pathological information on the lung parenchyma. To achieve this, protocols are specifically designed. Firstly narrow beam collimation and slice width are required that approximate the 3rd, 4th and 5th order branches of the structures we are interested in. The high natural contrast of the air-soft tissue interface limits the need to increase mAs to compensate for quantum mottle in these thin slices. Thus 1-2mm slice widths are used with 512x512 matrix. Next we apply high resolution kernels (~bone algorithms) for reconstruction which will produce noisier images but will give high spatial frequency information so that septations and airways are very well defined. Most patients are scanned supine. Unfortunetly dependant atelectasis (the normal collapse of dependant lung) occurs exactly in the place and with a very similar character to some of the interstitial lung diseases. In such cases additional limited lower lung imaging with the patient prone can be of value do discriminate the two conditions. For the majority of interstitial lung diseases inspiration images alone will suffice. When there is a question regarding air-trapping or lung volumes throughout the respiratory cycle inspiratory and expiratory images are indicated but are used only when the doubling of radiation dose is justified. When the clinical question is specifically for high resolution chest imaging intravenous contrast is not administered. The imaging field of view must of course include the lung parenchyma and pleura but chest wall and superficial tissues are less important for dedicated HRCT studies. Smaller field of view will enhance pixel resolution. It is important to remember that high resolution chest imaging does not suffice for routine lung and mediastinal imaging.

Single detector studies are usually performed with a limited number of pre-selected 2mm slices through arbitrary levels in the upper, mid and lower lungs (e.g. aortic arch, hilum, 2cm above diaphragm). For most cases the HRCT is providing a sample of the lung tissue and this will be adequate but less than 10% of the whole lung is sampled and in the setting of multifocal disease sensitivity can be reduced.

In multidetector row helical CT(MDCT), slice widths may be fashioned independent of beam collimation. Four 1mm detectors (4detector acquisition system) may be combined to create 1 or 1.25mm slices at 5-10mm intervals at arbitrary levels and from the same data set less noisy routine parenchyma 4-5mm slices may be obtained. Thus the patient needs only to be scanned once but all desired reconstructions must be performed before the raw data is dumped. MDCT is providing greater potential for large z-axis coverage with high resolution for wider lung sampling.

HRCT image display depends on whether hard or soft copy interpretation is performed. For hard copy interpretation target reconstructions with smaller field of view will improve pixel size. It is preferable to never print high resolution images on film greater than 12 on 1. If right and left lung images are reconstructed thus they should be displayed opposite one another on film. Windows and levels are somewhat institution dependant and the most critical factor is that a consistent protocol is used for all patients so that a level of comfort with the normal and abnormal attenuation findings can develop. Altered windows may hide emphysematous changes or over-estimate bronchiol-vascular walls and sizes. Soft copy interpretation provides greater control (and variability) for HRCT interpretation. Cine scrolling can be of value to follow branching non-planar structures.

There is not a clearly defined role for 3D post-processing as yet but minimum intensity projection (MinIP) studies have proven of some value for assessing air trapping and whole lung reconstructions have been used to better anatomically localize diseases.

The clinical role of HRCT

Detect lung disease in those at asymptomatic and at high risk

Detect lung disease in those symptomatic and with a non-specific CXR

Characterize disease in those with abnormal CXR and PFT’s

Assess the disease extent, response to therapy and the optimal area to biopsy



HRCT signs

If present some of the signs we hope the HRCT will reveal include; Septal lines, interstitial thickening

Bronchectasis, cystic change, emphysematous destruction

Honeycombing

Alveolar opacity, ground glass, mosaic pattern

Pleural effusion

Air trapping




Note: a normal HRCT does not exclude interstitial lung disease HRCT Clinical Conditions

Almost anything that affects the lung can affect the structures we image on HRCT. For dedicated HRCT we are mainly concerned with conditions that affect predominantly the airway or predominantly the interstitium and there is overlap. The list is exhaustive but here are some more common examples.

Airway predominant

Pulmonary hemorrhage (primary or secondary)

Alveolitis

Hypersensitivity

Infection



Alveolar proteinosis

BOOP

Emphysema



Intersitial predominant

Edema-acute/chronic, cardiogenic/noncardiogenic

Pneumonconiosis

Asbestosis

Silicosis



Granulomatous

Sarcoid



Collagen vascular/connective tissue

SLE

Rheumatoid



Idiopathic

Uusual interstitial pneumonia

Respiratory bronchiolitis ILD

Non-specific ILD

Desquamative interstitial pneumonia



Lymphangitic spread of tumor





Lung Nodule technique

Lung nodules represent soft tissue lung parenchyma opacities less than or equal to 3cm. When found the main clinical question is whether it represents cancer or not. Contrast is not required for detection as they usually are well contrasted against the low HU lung parenchyma on lung windows though it can be of value to demonstrate the presence and quality of enhancement. For single detector imaging 2-3mm beam width and slice collimation are used and separate smaller slice widths are scanned for 1mm characterization. With MDCT 1mm detectors may be combined to fashion 3-5mm slices and smaller 1mm reconstructions may be made from the same data when required. 3D post processing is not routinely required but can help to localize a nodule and differentiate it from a vessel. Some authors have suggested volume measurements may provide a better means to follow nodules over time. When nodules are over 6mm combination PET/CT may be useful to differentiate benign and malignant tissue. Once malignancy is suspected CT guided fine needle aspiration and biopsy is utilized for tissue sampling. When it is elected to follow a nodule by CT to ensure stability we like to see no interval growth over a two year period and it is important that scan techniques are consistent to remove interscan error.

Lung nodule CT features

When using CT alone to characterize a nodule we are dependent on its morphologic features.

Edges (smooth, lobulated, spiculated)

Enhancement

Composition-fat, calcification

Vascularity (AVM?)

Temporal stability

Multplicity



Common clinical conditions presenting as lung nodules

Benign

Granuloma (usually secondary to histoplasmosis)

Hamartoma

Adenoma

AVM



Malignant

Primary bronchogenic carcinoma

Small cell

Non small cell



Metastases





Lung tumor CT

169520 people were diagnosed with lung cancer in 2001 and 154000 died of it in the same period. In the chest we are mainly concerned with primary lung masses (over 3cm and usually bronchogenic carcinoma) or metastatic disease. 3mm slices from 3mm beam widths of single detector scanners or from 2.5mm detectors of MDCT are commonly employed. Occasional thinner slices are required to better delineate the margins of tumor and involvement of adjacent structures. Contrast is a requirement and can be administered at 3-4cc per second through an antecubital vein. We routinely give 120cc of non-ionic contrast containing 350g of iodine per ml. Imaging is performed after 40second scan delay. Attempts have been made to correlate the dynamics of tumor enhancement with malignancy. Less than 15HU has been found to correlate with benignity but false negatives exist with necrotic tumors. Currently contrast is of more importance for the mediastinal structures to define any lymph node enlargement or involvement of vital structures. It is important that bone windows are reviewed on soft copy or printed on hard copy due to the high incidence of bone metastases with lung cancer. 3D post-processing is of value for staging and planning surgery or biopsy. It is also useful in setting up conformal radiation therapy and evaluating palliative airway stents.

The clinical information that is necessary from a lung tumor CT is the following;

Primary tumor

Site, size, character(cavitation, calcification), number

Is it amenable to biopsy

Is it large enough for PET



Lymph nodes

Enlargement

Accessability for mediastinoscopy or bronchoscopic biopsy



Involvement of other structures

Chest wall/spine-direct invasion or metastases

Pleura-soft tissue metastases or effusion

Systemic arteries/veins/esophagus



Prediction of future complications

Assessment for airway stenting or follow-up



A note on lung cancer screening CT scanning for lung cancer screening is a source of great interest at present. At present there is not a formal public lung cancer screening protocol or recommendation and self referral is not recommended by the ACR or American Lung Association. Screening is not to diagnose disease but to identify within the population those individuals who may benefit from further tests to diagnose cancer. Informally many clinicians will refer patients for lung CT to exclude cancer because of some history such as smoking. Lung cancer remains the single biggest cancer killer of men and women in the developed world and the statistics have not significantly changed despite advances in imaging and therapy. The problem with CT as a screening tool is that it may be too sensitive and not specific enough thus creating a large pool of patients with indeterminate nodules that must be followed over many years. Screening populations by definition have large numbers of patients who do not have cancer and there is potential harm from radiation and procedures to this population to prove that nodules are indeed benign. The other problem is that it is still unclear whether detecting lung cancers earlier would actually change the ultimate mortality outcome. Lead time bias may mean cancers are found and treated earlier but people still succumb to them at the same age and thus just live with them longer. These and other questions are under review by ongoing studies (ELCAP, ACRIN, MAYO).