Radiology for the Non-Radiologist Thoracic CT for the Intensivist

M. L. H. TIE Division of Medical Imaging, Queen Elizabeth Hospital, Woodville, SOUTH AUSTRALIA

ABSTRACT Objective: To discuss the recent advances in computed tomography (CT) and to present a simplified approach to CT of the chest to facilitate the understanding and diagnosis of common acute thoracic abnormalities in the critically ill patient. Data sources: Published articles and texts on thoracic disorders and CT diagnosis. Summary of review: In the acutely ill patient with complex pulmonary and cardiac disorders, a thoracic CT can be useful in determining the cardiac, pulmonary, pleural and mediastinal abnormalities present.

With an understanding of the position and appearance of normal intrathoracic structures (and artifacts) acute thoracic disorders can be easily assessed by the non-radiologist, facilitating the correct diagnosis and the appropriate management. Conclusions: Thoracic CT offers the intensive care specialist the option of evaluating the pulmonary system, pleura, mediastinum, heart, pericardium, and aorta where plain radiographs are often inadequate. New mobile CT technology offers CT to patients confined to the intensive care unit due to haemodynamic instability. (Critical Care and Resuscitation 2001; 3: 250-258)

Key words: Computed tomography, intrathoracic lesions, acute pulmonary interstitial diseases Recent advances in computed tomography (CT) have dramatically changed the practice of medicine. With more resources allocated to intensive care units, the utilisation of complex imaging techniques in this area is increasing. New developments in CT have occurred that have facilitated the ability of the clinician to diagnose many disorders, although there are important limitations. The break through in CT technology occurred with the invention of the slip ring gantry. This enabled the scanner tube to rotate constantly, which, when co-ordinated with table motion, enabled volumetric data sets to be acquired. This also enabled scans to be rapid and not require separate breath-holds for each image. Today the image can be as fine as 0.5mm with almost instantaneous reconstruction. The most recent advance in computed tomography is the “sandwiching” of several scanners into a single scanner. This is known as multislice CT. Most of the newest scanners incorporate four slices. This enables

large amounts of data acquisition per unit time, thereby increasing scan speed. Speed of data acquisition is of critical importance when patients are unable to suspend respiration (which often occurs in the critically ill patient), or to track the contrast bolus within the arterial phase when CT angiography is performed. The differences in the various scanner types are listed in table 1. High resolution thin section CT There is a common misconception that “high resolution” equates to better image quality. In CT parlance, high resolution refers to a specific processing algorithm and slice thickness which is commonly applied in chest CT. Slice thickness for high resolution equates to a 1 - 2 mm slice thickness, depending on the scanner make. The critical factor, however, is the way the acquired data is manipulated by the computer. There are several filters and edge enhancements, which can

Correspondence to: Dr. M. L. H. Tie, Division of Medical Imaging, Queen Elizabeth Hospital, Woodville, South Australia 5011

250

Critical Care and Resuscitation 2001; 3: 250-258 M. L. H. TIE

Table 2. Comparison of conventional chest CT with high resolution chest CT

modify the image to optimise spatial resolution or contrast resolution. Conventional High resolution Table 1. Characteristics of different scanner types chest CT chest CT Scanner Images/tube Breath Data type revolution holding set Slice thickness 5 - 10 mm 0.5 - 2 mm

Slice space Contiguous every 10 mm Single slice 1 Single breath 2 dimensional non helical hold per slice % of lung imaged 100 10 Single slice 1 Single breath 2 or 3 dimensional Processing Moderate edge High edge Helical hold for

multiple slices algorithm enhancement enhancement Indications General, tumour Interstitial lung Multi-slice up to Single breath 2 or 3 dimensional assessment assessment helical 16 hold for multiple slices

The classic high-resolution algorithm enhances edges and optimises visualisation of the lung interstit-ium, thus the use of high resolution CT should probably be limited to evaluation of interstitial lung disease only (e.g. interstitial fibrosis from any cause, interstitial drug reactions, lymphangitis carcinomatosis, bronchiectasis and rare entities such as alveolar proteinosis).1 The features that distinguish conventional chest CT with high resolution chest CT are listed in table 2. The other important factor is that the separation of slices is usually 1 mm at 10 mm intervals. This means that 90% of the lung is not imaged and a 5 mm nodule could easily be missed. For this reason, evaluation for masses, nodules and nodes should be undertaken with conventional chest CT. The term “thin section CT” is generically applied to any scan where the slice thickness is < 3 mm. This may be used in areas requiring small detail (e.g. in the petrous temporal bones). It does not imply a specific processing algorithm.

Figure 1. Coronal maximum intensity projection reconstructions of the chest. In this case the tumour in the right hilum and its metastases are clearly visible in relation to the superior vena cava.

Three dimensional imaging Advances in computing power in the last few years has also enabled vast amounts of data to be processed to provide 3-dimensional (3-D) rendered images in almost real time. The use of 3-D imaging is helpful in assessing structures that are in the same plane as the scan. These include discoid atelectasis in the lungs, folds of bowel and vessels. The 3-D images are also often helpful in surgical procedures allowing the surgeon to plan the extent and limits of the operation. Coronal maximum intensity projection reconstructions (MIP) also allow cross sectional imaging in any plane to aid surgical planning (Figure 1).

Other CT technology Mobile CT The recent introduction of the Philips Tomoscan M scanner into Australia heralded a new era in the practice of CT in the intensive care unit. This is a “CT scanner on wheels” which enables CTs to be performed at the bedside in the intensive care unit. The main limitation to such a device has been financial due to staffing requirements. It is cumbersome to operate and requires

251

M. L. H. TIE Critical Care and Resuscitation 2001; 3: 250-258

at least two operators to manage (Figure 2). However, in a high volume department with sufficient staffing resources it should prove to be an important tool for the intensive care specialist and may also be useful in the operating theatre for surgical guidance.

Figure 2. The mobile CT scanner being used in the Queen Elizabeth Hospital intensive care unit. Notice that at least two technologists are required to operate the machine. NORMAL THORACIC ANATOMY Pulmonary anatomy Fissures and lobes The recognition of fissures on the CT scan is relatively easy. Fissures are pleural planes that overlie the distal-most air spaces. Thus fissures, if scanned in perpendicular planes, will have the appearance of a thin line surrounded by areas of relative paucity of vessels or airways (Figure 3). However, the pleura is often not thickened and the thin line is not seen. In these cases the location of the fissures are implied by the plane of “avascular” lung that is observed. The horizontal fissure, in particular, usually manifests only as an avascular slice on CT, due to the horizontal scan plane. Localising the fissures facilitates localisation of lesions into the upper, lower or middle lobes. Bronchi and pulmonary vessels The bronchi and pulmonary arteries are branched like a tree with a dichotomous branching. Bronchi are air filled and thus black on CT. Pulmonary arteries have a density of soft tissues when seen on plain chest CT, and have a density of diluted contrast when contrast is administered. They are usually traceable to the main pulmonary arteries on sequential scans. Pulmonary veins, on the other hand, have fern like or fishbone

branching, and can be traced to the left atrium, thus differentiating them from arteries. The aorta and other major intrathoracic arteries and veins are shown in figures 4 and 5.

Figure 3. A normal chest CT demonstrating hypovascular planes (arrowheads) corresponding to the location of fissures.

Figure 4. A chest CT demonstrating intrathoracic vascular structures. The two most anterior structures represent the left brachiocepalic vein crossing the midline to join the right. The three more posterior vessels represent the branches of the aortic arch. From right to left they are the right brachicephalic artery, the left common carotid artery and the left subclavian artery. Heart The anatomy of the major intrathoracic vessels are shown in figures 6 and 7. The normal anatomy of the heart is outlined in figure 8. Note the ventricular septum, left ventricular thickness, pulmonary artery size and pericardial thickness.

252

Critical Care and Resuscitation 2001; 3: 250-258 M. L. H. TIE

PULMONARY PATHOLOGY Atelectasis and consolidations Atelectasis and consolidations are probably the commonest manifestations of lung pathology in the intensive care unit. Their appearance on CT are nonspecific (Figures 9, 10 and 11). Similar to plain chest radiography, atelectasis results in volume loss and shift of structures; consolidation, on the other hand, does not result in volume loss. In reality, however both disorders often coexist. Both result in increased density of lung parenchyma, and both can produce air broncho-grams, though this is commoner in consolidation. The role of CT is primarily to detect lesions undetectable on chest X-ray (e.g. anterior pneumothoraces, loculated pleural effusions, malpositioned appliances and inciden-tal lesions).

Figure 5. A chest CT demonstrating the aortic arch which passes from right to left and anterior to inferior, giving a diagonal course on axial CT. S = superior vena cava, A = aorta, T = trachea.

Figure 6. A chest CT at the level of the pulmonary artery. The lambdoid pulmonary artery is seen. Note that its calibre should never exceed that of the aorta. AA = ascending aorta, PA = pulmonary artery, VC = vena cava, DA = descending aorta.

Figure 8. A chest CT demonstrating a “4 chamber” view of the heart. RV = right ventricle, RA = right atrium, LA= left atrium, LV = left ventricle, S = ventricular septum, << = aortic outflow tract, A = descending aorta.

Figure 7. A chest CT at the level of the aortic valve, the 4 chambers and pulmonary veins entering the left atrium are demonstrated. LA = left atrium, RA = right atrium, RV = right ventricle, LV = left ventricle, DA = descending aorta.

Figure 9. A chest CT lung window revealing bilateral lower lobe atelectasis.

253

M. L. H. TIE Critical Care and Resuscitation 2001; 3: 250-258

Figure 10. Identical level as figure 9 but in mediastinal windows revealing bilateral lower lobe atelectasis. Note the dark branching air bronchograms and pleural effusion surrounding the lower lobes.

Figure 11. A chest CT revealing complete left lower lobe atelectasis. Note the paucity of vessel/bronchial markings in the left lung field. A = aorta, LL = collapsed left lower lobe Pulmonary embolism Spiral CT pulmonary angiography (CTPA) is rapidly gaining acceptance as a valid test for detecting pulmonary emboli (Figures 12 and 13). In most series, the sensitivity and specificities are comparable or better than VQ scanning.2 The other advantage of computed tomography is its ability to detect other unsuspected pathology.3 Some authors even advocate replacing the VQ scan with spiral CTPA, in the diagnosis of pulmonary embolism.4 There are few firm answers, but most authors will agree that CTPA is nearly 100% specific and sensitive for pulmonary emboli in the main pulmonary arteries and close to 90% for pulmonary emboli in the segment-al pulmonary arteries. Furthermore, the sensitivities of

CTPA for subsegmental and smaller pulmonary arteries are at least as good as VQ scans. For patients with severe renal impairment or contrast allergy, VQ scanning is clearly the test of choice. The utility of CTPA specifically in the intensive care unit patient however remains to be tested.

Figure 12. A chest CT revealing bilateral segmental pulmonary emboli. These manifest as filling defects within pulmonary arteries which may be surrounded by a rim of contrast (white arrowheads). Note the right heart chamber enlargement as a manifestation of acute right heart failure.

Figure 13. A chest CT revealing central pulmonary emboli with a vermiform filling defect straddling the pulmonary artery bifurcation (i.e. classic “saddle embolus”). Pulmonary hypertension Acute elevation of pulmonary arterial pressure results in acute engorgement of the pulmonary artery. Thus, in cases of acute pulmonary embolus, dilatation of the pulmonary trunk (which is normally of similar calibre to the aorta) suggests a haemodynamically significant embolus. In chronic pulmonary hypertension, prominent

254

Critical Care and Resuscitation 2001; 3: 250-258 M. L. H. TIE

central pulmonary vessels are present but there is early “pruning” of the pulmonary artery branches, resulting in paucity of lung markings peripherally. The lung parenchma should also be assessed to determine if there are changes of chronic obstructive pulmonary disease, fibrosis or emphysema which may point to a diagnosis of cor pulmonale (Figures 14 and 15).

Figure 14. A chest CT of a patient with idiopathic pulmonary hypertension. Note right atrial and right ventricular dilatation and prominent central pulmonary vessels.

Figure 15. A chest CT from the patient in figure 14, with dilatation of central pulmonary arteries. Note that the pulmonary trunk (PA) is larger than the ascending aorta (A). Mediastinal adenopathy or masses Adenopathy in the intensive care unit patient presents a diagnostic dilemma as to whether the nodes are reactive (i.e to inflammation) or due to primary pathology (e.g. lymphoma). In most cases, the clinical history, comparison images or concurrent chest CT findings will contribute to making the correct diagnosis

(e.g. presence of axillary adenopathy, ipsilateral lung infiltrates, etc). Acute respiratory distress syndrome ARDS is a clinical diagnosis. The CT findings of ARDS are extensive ground glass opacity and consolidation, which may be symmetric or asymmetric,5 in addition to the manifestations of the cause of ARDS for example due to pneumococcal pneumonia. There is much variability in the prevalence of pleural effusions, air bronchograms, pneumatocoeles and Kerley B lines. There is some interest in the use of CT in predicting the prognosis of ARDS,6,7 although the value of this is unproven. However, chest CT is quite non-specific in diagnosis of the causes of ARDS in most cases.8

Figure 16. A chest CT of a patient with ARDS. Note the extensive areas of consolidation which are predominantly found in the posterior portions of both lungs. Empyema, loculated effusions and pneumothoracies The diagnosis and characterisation of pleural effusion or pneumothorax can be difficult in the intensive care unit patient who, a) cannot be positioned for satisfactory lateral chest X-ray or lateral decubitus films or, b) has markedly distorted normal anatomy due either to surgery of congenital deformity (e.g. scoliosis see Figure 17). The diagnosis of loculated collections may have a major impact on the management of these patients as multiple chest drains or video assisted thoracoscopy may be required (Figure 18).9 In the intensive care unit setting, the diagnosis of empyema with ultrasound has been advocated.10 CARDIAC PATHOLOGY Patients with severe cardiac dysfunction are often admitted to the intensive care unit. The rapid CT scanners currently available are increasingly used to demonstrate cardiac pathology. The hallmarks of cardiac failure seen on CT are the same as those seen with chest

255

M. L. H. TIE Critical Care and Resuscitation 2001; 3: 250-258

X-ray (e.g. cardiac chamber dilatation, pulmonary vascular engorgement, pleural effusion and interstitial pulmonary oedema).

Figure 19. A 29-year-old post partum female with acute shortness of breath. Note the bilateral effusion, pulmonary vascular engorgement and ventricular and left atrial dilatation, all consistent with left ventricular failure.

Figure 17. A scout image from a chest CT scan showing complex appearance on frontal x-rays.

Figure 20. Ventricular septal deviation due to cardiomyopathy. (the same patient as in Figure 19). Note the dilated left ventricle, pulmonary vascular congestion, bilateral pleural effusions and ventricular septum deviation to the right. Cardiac masses Primary cardiac neoplasms are rare and difficult to diagnose, but can often be characterised by their echocardiographic, CT and MRI characteristics.11,12 Atrial myxomas are the commonest primary cardiac neoplasm accounting for over 50% of cardiac neo-plasms.13 They have a predilection for the left atrial septum, specifically from the fossa ovalis.

Figure 18. An axial CT scan shows satisfactory position of chest drain in the large left pneumothorax, with left lung collapse with mediastinal shift to the left and areas of saccular bronchiectasis bilaterally. Cardiomyopathy

A chest CT can demonstrate the presence of enlarged cardiac chambers and dilated vessels (Figures 19 and 20). Thinning of the cardiac wall and ventricular

Pericardial effusion The normal pericardium on CT imaging is of “pencil line” thickness. A pericardial effusion is easily recognised from a chest CT (Figures 21 and 22). A large

aneurysms can also be detected. The CT diagnosis of cardiac pathology can be made using the same principles utilised for evaluating plain chest X-rays.

256

Critical Care and Resuscitation 2001; 3: 250-258 M. L. H. TIE

Aortic dissection effusion may cause pericardial tamponade. Late stages of tamponade can result in equalisation of the heart chamber sizes (i.e. the ventricles and atria become similar in size). A large effusion may also lead to compressive cardiac effects.14 Although azygos vein reflux has been reported as a useful sign of pericardial tamponade,15 azygos vein reflux and reflux of contrast into the hepatic veins can occur in any disorder associated with an elevated right heart pressure. Nevertheless, chest CT may be the initial diagnostic modality demonstrating tamponade in cases of chest trauma.16

Chest CT or MRI are the tests of choice for excluding an aortic dissection in the hypertensive patient who has tearing chest pain and in the absence of ECG abnormality.18 In Australia, due to the poor access to MRI, chest CT is the usual modality of choice (Figures 23 and 24). The main role of imaging is to exclude dissection and, if diagnosed, to determine if the dissection involves the aortic arch (which determines if the patient is to be managed surgically or not). On chest CT the intimal flap manifests as a thin membrane of tissue separating the true from the false lumen. The false or true lumen may be thrombosed and thus reliance on the presence of contrast on either side of the membrane may yield false negative results.19 Aortic rupture will be discussed in the later article on abdominal CT.

Figure 21. A chest CT demonstrating a small pericardial effusion (arrowheads).

Figure 23. A chest CT demonstrating an aortic dissection (Stanford type A) involving the aortic arch (arrowheads).

Figure 22. A chest CT demonstrating a massive pericardial effusion (P) due to lymphoma.

CT is an invaluable tool in diagnosis of unsuspected pathology in the intensive care unit patient. In addition to assessment of pulmonary pathology, thoracic CT offers evaluation of the heart, pericardium, pleura mediastinum and chest wall. The advent of mobile CT has the potential to result in a dramatic change in the daily management of critically ill patients.

Pericardial masses Pericardial masses and metastases are rare, usually arising from direct invasion by lung cancer, lymphoma or metastasis, especially from melanoma.17

257

M. L. H. TIE Critical Care and Resuscitation 2001; 3: 250-258

258

5. Desai SR, Wells AU, Rubens MB, Evans TW, Hansell DM. Acute respiratory distress syndrome: CT abnormalities at long-term follow-up. Radiology 1999;210:29-35.

6. Puybasset L, Cluzel P, Chao N, Slutsky AS, Coriat P, Rouby JJ. A computed tomography scan assessment of regional lung volume in acute lung injury. The CT Scan ARDS Study Group.Am J Respir Crit Care Med 1998;158:1644-1655.

7. Maurer J, Stutzner J, Gutberlet M, Pappert D, Felix R.Value of abdominal computed tomography (CT) concerning the prognosis in patients with acute respiratory distress syndrome (ARDS) Intensive Care Med 1999;25:330.

8. Goodman LR, Fumagalli R, Tagliabue P, Tagliabue M, Ferrario M, Gattinoni L, Pesenti A. Adult respiratory distress syndrome due to pulmonary and extrapulmonary causes: CT, clinical, and functional correlations. Radiology 1999;213:545-552.

9. Lee RB. Radiologic evaluation and intervention for empyema thoracis. Chest Surg Clin N Am 1996;6:439-460.

10. Kearney SE, Davies CW, Davies RJ, Gleeson FV. Computed tomography and ultrasound in parapneumonic effusions and empyema. Clin Radiol 2000;55:542-547.

11. Araoz PA, Eklund HE, Welch TJ, Breen TJ. CT and MR imaging of primary cardiac malignancies. Radiographics 1999; 29:1421-1434.

12. Araoz PA, Mulvagh SL, Tazelaar HD, et al. CT and MR Imaging of Benign Primary cardiac neoplasms with echocardiographic correlation. Radiographics 2000; 20:1303-1319.

13. Burke A, Vimani R. Tumours of the heart and great vessels. In: Atlas of tumour pathology:fasc 16 ser 3. Washington DC: Armed forces institute of pathology, 1996;1-98.

Figure 24. A chest CT demonstrating an aortic dissection involving the ascending and descending aorta. Note the thin intimal membrane separating the true and false lumens (black arrowheads). 14. Hernandez-Luyando L, Calvo J, Gonzalez de las Heras

E, de la Puente H, Lopez C. Tension pericardial collections: sign of 'flattened heart' in CT. Eur J Radiol 1996;23:250-252.

Received: 26 September 2001 Accepted: 1 November 2001 15. Harries SR, Fox BM, Roobottom CA. Azygos reflux: a

CT sign of cardiac tamponade. Clin Radiol 1998;53:702-704.

REFERENCES 1. Mueller N. High resolution computed tomography of the

lungs. Webb WR, Muller NL, Naidich DP. 3rd Ed, Lippincot. Williams & Wilkins 2000.

16. Goldstein L, Mirvis SE, Kostrubiak IS, Turney SZ. CT diagnosis of acute pericardial tamponade after blunt chest trauma. AJR Am J Roentgenol 1989;152:739-741.

2. Remy-Jardin M, Remy J. Spiral CT angiography of the pulmonary circulation. Radiology 1999;212:615-636.

17. Breen JF. Imaging of the pericardium. J Thorac Imaging 2001;16:47-54.

3. Kim KI, Muller NL, Mayo JR. Clinically suspected pulmonary embolism: utility of spiral CT. Radiology 1999;210:693-697.

18. Hartnell GG.Imaging of aortic aneurysms and dissection: CT and MRI. J Thorac Imaging 2001;16:35-46.

4. Cross JJ, Kemp PM, Walsh CG, Flower CD, Dixon AK. A randomized trial of spiral CT and ventilation perfusion scintigraphy for the diagnosis of pulmonary embolism. Clin Radiol 1998;53:177-182.

19. Vasile N, Mathieu D, Keita K, Lellouche D, Bloch G, Cachera JP. Computed tomography of thoracic aortic dissection: accuracy and pitfalls. J Comput Assist Tomogr 1986;10:211-215.