This is an interactive tutorial to teach you the basics to approaching a CT of the chest. Once you know this, you’ll be ready to shine! The presentation is laid out in 3 sections: We will discuss the different types of chest CT We will discuss how you should systematically approach the Chest CT every time We will discuss some common disease patterns and their differentials. Let’s begin!
There are four main types of CT’s for imaging the lungs The first is the standard CT- or the equivalent of a spiral/herlical CT Second- is the High resolution CT Third is the low dose CT Fourth is the CT angiogram We will explore all four a little more now
CT Sampling In helical CT, the patient is continuously moved through the scanner as the source and receptor rotate around the patient. The slice thickness or collimation is 3-10 cm, at centres like MSH it is roughly 5 mm. 100% of the lungs are imaged, not just thin slices, in specific spots . scans are very useful in emergency rooms where a fussy child or a delirious adult needs a critical chest image The advantage is that the entire helical CT can be done in less than a minute These images can be obtained with or without contrast The indications for Chest CT are numerous: here is a short list
In High Resolution CT, Transverse images of thin slices of lung (1-1.3 mm thick) are obtained at non-contiguous intervals, usually 1 to 2 cm apart, throughout the whole lung. The computer reconstructs the images to give high spatial resolution. This process results in images that show detail, but only 5 to 10% of the lung is sampled. This sampling is appropriate for evaluating diffuse lung disease, focal lesions may require more images.
For parenchymal disease, it is important to get as detailed a look as possible. High-resolution computed tomography (HRCT) can detect and diagnose individual diffuse lung diseases. It is useful for Diffuse Interstitial Lung Diseases like: Sarcoidosis. Hypersensitivity pneumonitis (extrinsic allergic alveolitis) Pneumoconiosis (asbestosis, silicosis) It also has greater diagnosis value in certain diseases involving airways and pulmonary airspaces Bronchiectasis Allergic Bronchopulmonary Aspergillosis Cystic Fibrosis Emphysema
We are increasingly using chest CT’s This has the potential to increase radiation burden to the general population. it is important to reduce this Conventional chest CT is usually performed using settings between 220 and 280 mAs. studies have shown that a substantial dose reduction is feasible for Chest CT because: Contrast is already inherent in the chest And absorption of radiation is lower in the lungs low-dose chest CT can be performed without seriously jeopardizing image quality. low- and ultra low-dose chest CT are used in some institutions primarily for lung cancer screening. this is not routine. Studies are underway to assess the role of low dose CT scans in early detection of lung cancer COPDers.
CT Angiography is a Ct where iodenated contrast is injected into a small peripheral vein An automatic injector controls the timing and rate of injection It is safer, more efficient and more cost effective than conventional catheter angiography, and has essentially replaced it. With the results, accurate three-dimensional "casts" of the blood vessels can be made on the computer which offers better detail than MRI or US The Indications are listed above. In PE, to accurately check for a filling defect, the scan is timed such that the contrast is in pulmonary artery during scan As for the Risks: The iodenated dye can cause severe Allergic reactions More importantly it is nephrotoxic and as such is contraindicated in pts with renal disease.
Now we’ll switch gears and focus on the basics of looking at a chest CT It is important to have a systematic way of approaching the chest. There are three windows. First is the window that highlights the soft tissues Next is the window that highlights the bony structures Last is the window that highlights the lung parenchyma When looking at Chest CT’s I would suggest switching from windows in this order or else you might get bogged down by the lung parenchyma and forget about the bones and soft tissues Remember, the key is to be systematic
Soft Tissue Windows The "soft tissue windows" Help evaluate the mediastinal structures, pleural fluid, and calcification. As you can see: Aerated lung is black; bone and calcium are white; muscle, heart, and large vessels are light grey; fat is dark grey. It is on this window that you should look for lymph nodes, both axiallary and mediastinal Make sure you look at the coronal as well as the axial views It is like a double check/verification mechanism.
See which structures you can identify The width of the mail pulm artery should be less than 3cm. If it is more, then the pt likely has Pulm hypertension. 2 nd mouse click Ok: here’s a hint 3 rd click And here’s the answer This is just one of the slices that you should know. But there is more anatomy that you should learn when looking at a chest CT. For more information, go to the anatomy teaching file. 4 th click Staff always like to know what this tiny structure is. Don’t be fooled by its size, it is mighty important.
Once you are done looking at the soft tissues, change to the bone window In this view, the bony structures are emphasized Scroll down and watch the vertebrae Now scroll up and watch the sternum and manubrium Then scroll down looking at the R ribs Scroll up looking at the left ribs You will see fractures, lytic lesions, sclerosis and other things if you follow this approach every time
Of course now we come to the “most important window” The lung window. In this window, air appears black, aerated lung dark grey, and other structures white. Again there is an approach to looking at this. First we look at the airways: central- trachea Look fopr patency and secretions if it is rounded then the patient has taken an adequate inspiration And then we look at the R bronchial tree Then we move to the left side of the bronchial tree Look for anatomic variants, mucus plugs, airway collapse. Last we look at the lung parencchyma. Do it by lobes. Try to find the fissure lines. They are either well demarcated black lines or simply avascular. Start with the RUL then RML then RLL. Repeat for the L lung. Often the radiologisists will be looking for nodules here. They are white tiny flecks among a sea of white tiny flecks. How can you identify them? The answer is to follow the vessles. If the white flecks move, then this is a vessel. If they do not move on three consecutive slices you can be sure they are a 3Dstructure and not just contrast in the vessels.
Ok so the first two sections covered the basics. Now let’s get a little more sophisticated and explore the Common pathology that you are bound to see. Learn them here, then call them out when you are reading a scan and you are sure to impress.
Air bronchograms allow you to see the air-filled bronchi, because the surrounding lung is of increased attenuation An air bronchogram implies that proximal bronchi are patent Cause u can see that air going thourgh it. This sign was originally described to identify lung disease and distinguish it from pleural disease Now it has become another name for air-space consolidation remember that it may be seen in association with a variety of lung parenchymal abnormalities: atelectasis, lung consolidation, and interstitial thickening to name a few.
Bronciectasis Is the abnormal dilatation of the medium-sized bronchi (>2 mm in diameter) caused by destruction of the muscular and elastic components of bronchial walls. The proximal bronchi are less affected because they have more cartilage and are more resistant to dilation. Damage to the muscular and elastic components of the bronchial wall is caused by either infectious or inflammatory cytokines. The damaged bronchi cannot clear the secretions which leads to colonization and infection with pathogenic organisms. This contributes to purulent expectoration and results in further bronchial damage Thus bronchiectasis is a c ycle of bronchial damage, bronchial dilation, impaired clearance of secretions, recurrent infection, and more bronchial damage. There are 3 Types: In cylindrical bronchiectasis, the bronchi are uniform, their walls are parallel and they don’t taper Cystic: is a severe form. The bronchi look like cysts and it usually extends to the pleural surface. Varicose: is an uncommon form. The bronchi have a beaded appearance and areas of dilation followed by relative narrowing. HRCT has replaced bronchography as the diagnostic tool of choice. HRCT is noninvasive and has a sensitivity of 96% and a specificity of 93%. Diseases associated with bronchiectasis are as follows: Infection with Typical organisms like Klebsiella , Staphylococcus aureus, Mycobacterium tuberculosis Bronchial obstruction: with endobronchial tumors, broncholithiasis, and foreign body aspiration. The others are listed above
The secondary pulmonary lobule is a fundamental unit of lung structure and reproduces the lung in miniature. Airways, pulmonary arteries, veins, lymphatics and the lung interstitium are all represented. Perilobular Pulmonary disease – is dieases that occurrs close to interlobular septa and the periphery of lobules Septal thickening reflects perilobular pulmonary disease, ie. abnormalities of the interlobular septa or peripheral alveoli. The interlobular Septa are well developed in the apices, and septal thickening is often well depicted in this region. The most common causes are listed here
Ground Glass Opacity Ground-glass opacity is a frequent but nonspecific finding The underlying abnormality is diverse; any condition that decreases the air content of the lung parenchyma without totally obliterating the alveoli can produce ground- glass opacity. The histology will show: minimal thickening of the alveolar walls and septal interstitium an alveolar lumen partially filled with fluid, macrophages, or neutrophils Ground Glass opacity is different from true consolidation: In true consolidation, the area appears white since the lung opacity obscures the vessels In Ground Glass opacity: The degree of increased lung opacity is not sufficient to obscure pulmonary vessels. Ground-glass opacity is potentially reversible with appropriate therapy, if the underlying disorder is treated early, because none of the changes in lung structure are permanent. Some active but potentially reversible processes that produce ground-glass opacity include: pulmonary edema; alveolar proteinosis;
Emphysema is the permanent enlargement of distal air spaces to the terminal bronchioles, and destruction of the walls without obvious fibrosis. DDx: Alpha Antitrypsin: often in young patients at the lung bases Immundeficiency syndromes: PCP/Human immunodeficiency virus (HIV) Connective tissue disorders: Marfan /Ehlers-Danlos HRCT is more sensitive and highly specific for diagnosing emphysema than conventional radiographs. However, this information does not alter therapy and thus, a CT scan is not part of the routine care of patients with COPD
There are 3 types of emphysema: Centriacinar/lobular: This begins in the respiratory bronchioles and spreads peripherally. It is associated with long-standing cigarette smoking. It is found predominantly in the upper half of the lungs. 2. Panacinar: Here the entire alveolus is destroyed. This type is predominant in the lower half of the lungs. It is associated with homozygous alpha1-antitrypsin (AAT) deficiency. 3. distal acinar/paraseptal: This involves the distal airway, alveolar ducts, and alveolar sacs and is found localized around the lung septae or pleura. The apical bullae may spontaneously rupture causing pneumothorax.
Lung cancer is the leading cause of cancer death Early lung cancer, [primary tumor is less than 3 cm] may lead to 5-year survival rates of 70-80%. prompt diagnosis and management is v impt Generally, a pulmonary nodule must reach 1 cm in diameter before it can be identified on a chest radiograph. But Ct is good for detecting nodules as small as 3-4 mm. Clinical history will help assessing to assess whether nodules are benign vs. malignant: History of smoking History of malignancy Travel: Travel to areas with endemic mycosis (eg, histoplasmosis, coccidioidomycosis, blastomycosis) or tuberculosis (TB) Occupational: asbestos, radon, nickel, chromium, vinyl chloride, and polycyclic hydrocarbons History of TB or mycosis Benign lesions: less than 15 HU has a greater than a 95% predictive value for benignity fat within halo sign: smaller than 0.5 cm, stable for 2 years Malignant: multiple irregular or spiculated margins. More contrast enhancement- Caution because active infectious lesions may also enhance, If you can’t spot the nodule… perhaps u sholdn’t go into radiology
The Ddx of a chest nodule on CT is listed in the table more than 95% are neoplasms (most likely primary), benign lesions (most likely hamartoma) or infectious granulomas Causes: Neoplastic (malignant or benign) Inflammatory (infectious) Granuloma - TB, fungus Inflammatory (noninfectious)
CT angiography is highly sensitive for PE And can spot endoluminal thrombi in central as well as second to fourth division pulmonary arteries). Pulmonary thromboemboli appear as well defined hypodensities in the pulmonary artery. CT can lend itself to additional findings: pulmonary infarct, which is a well defined wedge shaped density With the apex pointing to the pulmonary artery and the base along the pleural surface. Spiral CT can determe age of the thrombus. Usualy if it is pushed to one side, or srescentic, it is old and chronic… The CT is useful to r/o other pathologies, which may present similarly clinically to PE’s: pneumonia, pnuemothorax or aortic dissection. The sensitivity of CT Angio now closely matches the more invasive gold standard of pulmonary angiography. Limitations of Spiral CT Pulmonary Angio The main limitation of Spiral CT Pulmonary Angio is in the evaluation of arteries below the 4th segmental level. But the clinical significance of such subsegmental pulmonary arterial thrombi has not been established. Interpretation pitfalls of Spiral CT Angio Anatomical landmarks and variants especially intersegmental nodes frequently causing filling defects. Also vascular tumor invasion may look like this. In fact, the radiologist looking at this scan couldn’t tell if the filling defects on the L were PE or tumor invasion. Technical psuedo filling defects may also mimic PE: due to inappropriate selection of injection parameters, flow rate, concentration, scan delay or breath hold. Remember, clinical correlation is required!
Thanks and good luck!
Approach to ct chest 578
CT CHEST Giselle Revah University of Toronto Class of 0T7 Dr. N. Jaffer Staff Radiologist MSH and University of Toronto
THE BASICS INTRO 1. The different options for CT imaging of the lungCT TYPE 2. An approach to looking at chest CT’sANATOMY DISEASE 3. A few disease patterns that willPATTERNS help you impress
CT TYPES 1. Standard INTRO 2. High Resolution 3. Low DoseCT TYPE 4. CT AngioANATOMY DISEASEPATTERNS
1. STANDARD CT • Slice thickness: 3-10 mm INTRO • scans a large volume, very quickly • Covers the full lungCT TYPE • +/- contrast Indications • CXR abnormalityANATOMY • Pleural and mediastinal abnormalities • Lung cancer staging • F/U metastases DISEASE • Empyema vs abscessPATTERNS
2. HIGH RESOLUTION (HRCT) • narrow x-ray beam collimation: 1-1.3mm INTRO vs. conventional 3-10mm • cross sections are further apart: 10 mm • high definition images of lung parenchyma: vessels, airspaces, airwayCT TYPE and interstitium • No contrast STANDARD CT HRCTANATOMY DISEASEPATTERNS
2. HIGH RESOLUTION (HRCT) Indications INTRO • Hemoptysis • Diffusely abnormal CXR • Normal CXR with abnormal PFT’sCT TYPE • Baseline for pts with diffuse lung disease • Solitary pulmonary nodules • Reversible (active) vs. non-reversibleANATOMY (fibrotic) lung disease • Lung biopsy guide • F/U known lung disease DISEASE • Assess Rx responsePATTERNS
3. LOW DOSE • Premise: lower dose radiation will not INTRO reduce the diagnostic functionality of the scan (eg. 250 mAs 50 mAs) • Detail is decreasedCT TYPE Uses • ScreeningANATOMY – ongoing trials • F/U – infections DISEASE – post lung transplantPATTERNS – metastases
4. ANGIOGRAPHY (CTA) • contrast injected into peripheral vein INTRO • injection timing/rate controlled automatically • dye is where you want it during scan • replaced conventional catheter angiogramCT TYPE Indications • Pulmonary embolism • Aortic aneurysmsANATOMY • Aortic dissection Risks DISEASE • Iodinated contrast: – Allergic/ nephrotoxicPATTERNS
APPROACHING THE ANATOMY INTRO Three Windows 1. Soft TissueCT TYPE 2. BoneANATOMY 3. Lung DISEASEPATTERNS
1. SOFT TISSUE WINDOW Look at these structures • Thyroid INTRO • Chest wall • Pleura HeartCT TYPE • Chambers • CA calcifications • PericardiumANATOMY Vessels • Aorta • PA • Smaller vasculature DISEASEPATTERNS Nodes • mediastinal •
Ascending aorta Main pulmonary artery SVC INTRO L pulmonary arteryCT TYPE R pulmonary artery Descending aortaANATOMY Esophagus DISEASE Azygous veinPATTERNS What is this duct?
2. BONE WINDOW Manubrium/ INTRO SternumCT TYPEANATOMY DISEASE R Ribs L RibsPATTERNS Vertebrae
3. LUNG WINDOW AIRWAYS Bronchial Tree INTRO CentralCT TYPE PARENCHYMA RULANATOMY LUL Fissure DISEASEPATTERNS Fissure RLL LLL
COMMON PATHOLOGIC FEATURES INTRO 1. Air Bronchograms 2. BronchiectasisCT TYPE 3. Septal Thickening 4. Ground Glass OpacityANATOMY 5. Emphysema 6. Nodules 7. Filling Defect DISEASEPATTERNS
1. AIR BRONCHOGRAMS Description INTRO • Bronchi become visible due to increased attenuation of surrounding lung • Implies proximal bronchi patency • Excludes pleural or mediastinal lesionCT TYPE DDx • Non-obstructive atelectasisANATOMY • Pneumonia • Pulmonary edema • Hemorrhage DISEASE • Bronchioloalveolar carcinomaPATTERNS • Lymphoma
2. BRONCHIECTASIS Dilatation of medium-sized bronchi (>2 mm) impaired clearance recurrent infection bronchial INTRO damage What type Types is this? 1. Cylindrical 2. CysticCT TYPE 3. Varicose HRCT is diagnostic tool of choiceANATOMY DDx • Infection • Immunodeficiency states • Bronchial obstruction • alpha 1-Antitrypsin deficiency • Cystic fibrosis • RA and Sjögren DISEASE • Primary ciliary • Pulmonary fibrosisPATTERNS dyskinesia
3. SPETAL THICKENING • abnormalities of interlobular septa or INTRO peripheral alveoli • thickening and outlining of the secondaryCT TYPE pulmonary lobules is best seen on HRCT • often well depicted in the apicesANATOMY Most Common Causes • pulmonary edema DISEASE • pulmonary hemorrhagePATTERNS • lymphangitic cancer spread
4. GROUND GLASS OPACITIES • common nonspecific finding INTRO • decreased air content without totally obliterating the alveoli • increased lung opacity not sufficient toCT TYPE obscure pulmonary vessels Early DDx • Alveolitis or interstitial pneumonitisANATOMY – Hypersensitivity pneumonitis – IPF – Sarcoidosis • Pulmonary edema DISEASE • Resolving pneumonia/ hemorrhagePATTERNS Dense
5. EMPHYSEMA • permanent enlargement of air spaces distal to the terminal INTRO bronchioles • destruction of the walls without obvious fibrosisCT TYPE DDx • smoking • alpha 1-Antitrypsin deficiency • IV drugsANATOMY • Immundeficiency • Vasculitis • Connective tissue disorders Young pt with bullous emphysema at the lung DISEASE bases.PATTERNS What’s the diagnosis?
5. EMPHYSEMA 3 Types 1. Centriacinar/lobular INTRO • respiratory bronchioles periphery • upper half of lungs • smokingCT TYPE 2. Panacinar • destroys entire alveolus uniformly • lower half of lungs • homozygous alpha1-antitrypsinANATOMY deficiency 3. Distal acinar/paraseptal • distal airway, alveolar ducts, and alveolar sacs • What 2 types are found DISEASE around the lung septae or pleura here? • apical bullae may spontaneouslyPATTERNS pneumothorax
6. NODULES CT can detect nodules 3-4 mm INTRO Benign • Small, unchanged over 2 years • Less than 15-20 HU • fat within (hamartoma)CT TYPE • halo sign: ground-glass surrounding nodule indicates infection Malignant • Enhancement of greater than 20 HUANATOMY • Caution: active granulomas/ infectious lesions • Spiculated • Multiple DISEASE Can you spot thePATTERNS nodule?
7. FILLING DEFECTS • Pulmonary Embolism is a well defined hypodensity in the pulmonary artery INTRO • • CTA sensitive for PE (90%) can’t evaluate arteries below 4th segmental level DDx: • Anatomical landmarks and variants eg intersegmentalCT TYPE nodes • Vascular tumor invasion • Technical psuedo filling defects (eg flow artifact)ANATOMY DISEASEPATTERNS
REFERENCES 1. Engeler CE, Tashjian JH, Trenkner SW, and Walsh JW. Ground Glass Opacity of the Lung Parenchyma: A Guide to INTRO analysis with High Resolution CT. AJR 1993; 160: 249-251. 2. Collins, J. CT signs and patterns of lung disease. Radiol Clin North Am. 2001 Nov;39(6):1115-35.CT TYPE 3. Lee JKT, Sagel SS, Stanley RJ, Heiken JP. Computed Body Tomography with MRI correlation. 3rd ed. Raven Press NYC, 1998.ANATOMY DISEASEPATTERNS