Respiratory system

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Chapter 7 Respiratory system

Computed Tomography of the Respiratory System

Indications

1. In the assessment of pulmonary masses

2. To exclude metastatic disease

3. In the assessment of diffuse infiltrative lung disease (high-resolution CT)

4. Arterial assessment (pulmonary emboli, aortic dissection)

5. In the assessment of bronchiectasis and large airway stenosis. Planning for therapeutic procedures to the airways, e.g. bronchial stent insertion.¹

Technique

1. Volume scan – usually performed with intravenous contrast enhancement at a rate of 3 ml s⁻¹ with a delay of 25–40 ml s⁻¹ depending on the speed of the CT scanner. However, bolus tracking software can be used to specifically trigger the scan for the assessment of relevant vascular structures, e.g. aorta in suspected aortic dissection or pulmonary artery in suspected pulmonary embolism (see below).

2. High-resolution scan – usually performed without intravenous (i.v.) contrast:

a Patient lies supine but additional prone images ² may be required to confirm reversible dependent change

b On full inspiration, either incremental 1-mm slices at 10-mm to 20-mm intervals OR using multidetector scanner, volume scan with high-resolution reconstruction of 1-mm thick slices at 5–10-mm interval. A volume scan has the potential benefit of post-processing capabilities. Three-dimensional reformatting can help to illustrate the cranio-caudal distribution of pulmonary fibrosis, e.g. distribution of disease on coronal reformats

c Additional expiratory images can be performed in suspected air-trapping and airways disease

d Image reconstruction using ‘bone’ algorithm, i.e. high-resolution algorithm which reduces standard CT image smoothing

e Maximize spatial resolution using smallest field of view possible.

1 Boiselle P.M., Ernst A. Recent advances in central airway imaging. Chest. 2002;121(5):1651-1660.

2 Kashiwabara K., Kohshi S. Additional computed tomography scans in the prone position to distinguish early interstitial lung disease from dependent density on helical computed tomography screening patient characteristics. Respirology. 2006;11(4):482-487.

Further reading

Beigelman-Aubry C., Hill C., Guibal A., et al. Multi-detector row CT and postprocessing techniques in the assessment of diffuse lung disease. RadioGraphics.. 2005;25(6):1639-1652.

Computed Tomography-Guided Lung Biopsy

Indications

1. Investigation of a pulmonary opacity when other diagnostic techniques have failed to make a diagnosis

2. Investigation of a new chest lesion in a patient with a known malignancy

3. To obtain material for culture when other techniques have failed to identify the causative organism in a patient with persistent consolidation.

All biopsies should be planned after case discussion at a multidisciplinary meeting with a respiratory physician, radiologist and oncologist where the balance of benefit versus risk can best be assessed. Central lesions are preferably biopsied trans-bronchially either by standard bronchoscopy or transbronchial needle aspirate with endoscopic US guidance.

Contraindications

1. Vascular: bleeding diathesis, concurrent administration of anticoagulants, significant pulmonary arterial or venous hypertension. Vascular lesions such as aneurysms or arterio-venous malformations should have already been suspected at pre-biopsy discussion and should not be subjected to biopsy

2. Respiratory: contralateral pneumonectomy, presence of bullae or seriously impaired respiratory function, such that a pneumothorax could not be tolerated. The procedure should not be performed if the patient plans air travel within 6 weeks of the procedure

3. Suspected hydatid disease

4. Extremely uncooperative patient.

Equipment

1. Biopsy needle – fine needle aspirate can be performed if there is an on-site pathology service. Otherwise a cutting needle biopsy is used, of which there are several types available, e.g. Temno cutting needle or Cook Quick-Core. Coaxial needle biopsy systems can be used as these reduce the number of passes through the pleura. 18 or 20G needles are most often used

2. Full resuscitation equipment including a chest drain.

Patient preparation

1. Premedication with diazepam may be required. However, the patient must remain cooperative so that a consistent breathing pattern can be maintained during the procedure

2. The procedure may be performed on an outpatient basis with observation for 4–6 hours post procedure, but a bed should be available in case of complications

3. Clotting screen

4. Pulmonary function tests (spirometry).

Aftercare

1. Close observation post procedure for at least 1 h in a supervised ward or recovery area, preferentially with the patient lying in a position where the biopsy site is dependent, e.g. right lateral decubitus if right lateral biopsy

2. Chest X-ray in expiration at approximately 1 h post procedure. If a pneumothorax has developed then the further management will depend on the clinical condition of the patient. This may involve further close observation, aspiration or chest drain insertion, depending on the degree of patient dyspnoea and distress

3. High-risk patients such as those with pre-existing impairment of respiratory function are best admitted overnight.

Further reading

Anderson J.M., Murchison J., Patel D. CT-guided lung biopsy: factors influencing diagnostic yield and complication rate. Clin. Radiol.. 2003;58(10):791-797.

Aviram G., Schwartz D.S., Meirsdorf S., et al. Transthoracic needle biopsy of lung masses: a survey of techniques. Clin. Radiol.. 2005;60(3):370-374.

Manhire A., Charig M., Clelland C., et al. BTS Guidelines: Guidelines for radiologically guided lung biopsy. Thorax. 2003;58(11):920-936.

Richardson C.M., Pointon K.S., Manhire A.R., et al. Percutaneous lung biopsies: a survey of UK practice based on 5444 biopsies. Br. J. Radiol.. 2002;75(897):731-735.

Methods of Imaging Pulmonary Embolism

1. Plain film chest radiograph. The initial chest radiograph is often normal. Numerous signs have been described in association with pulmonary embolism but, overall, the chest radiograph is neither specific nor sensitive.

2. Ventilation/perfusion (V/Q) radionuclide scanning. The technique is described later in this chapter. Interpretation of V/Q images is not clear-cut and there are a number of causes for the typical V/Q mismatch. Diagnostic criteria developed divide results into normal, low-probability, intermediate (or indeterminate) and high-risk groups. Specificity and sensitivity are such that, if the criteria place the images in the normal group, pulmonary embolism is virtually excluded and if the images fit the criteria for high risk it is very likely (85–90%). In the intermediate risk group specificity is poor and further imaging may be required.

3. Doppler ultrasound of leg veins to confirm deep vein thromboses.

4. Multidetector CT pulmonary angiography (CTPA) of the chest to diagnose pulmonary emboli down to subsegmental level. Can be complemented with a CT venography sequence performed at the same examination as CTPA (see Chapter 10).

5. Pulmonary arteriography was traditionally the ‘gold standard’ and will detect most pulmonary emboli but has now largely been replaced by multidetector CT.

Diagnostic pathways can be planned using an algorithm of clinical probability of pulmonary embolus together with the result of D-dimer assay.

Further reading

Roy P.M., Colombet I., Durieux P., et al. Systematic review and meta-analysis of strategies for the diagnosis of suspected pulmonary embolism. BMJ. 2005;331(7511):259.

Stein P.D., Woodard P.K., Weg J.G., et al. Diagnostic pathways in acute pulmonary embolism: recommendations of the PIOPED II investigators. Radiology. 2007;242(1):15-21.

Radionuclide Lung Ventilation/Perfusion (V/Q) Imaging

Indications

1. Suspected pulmonary embolism

2. Assessment of perfusion and ventilation abnormalities, e.g. in congenital cardiac or pulmonary disease

3. Quantitative assessment of right-to-left shunting (perfusion only)

4. Quantitative assessment of differential perfusion and ventilation before lung cancer, lung transplant or lung volume reduction surgery.^1,²

Contraindications (to perfusion imaging)

1. Right-to-left shunt – because of the risk of cerebral emboli

2. Severe pulmonary hypertension.

Neither of these are absolute contraindications (indeed, perfusion imaging can be used for assessment of shunts), and it may be considered acceptable to reduce the number of particles administered in these cases.

Radiopharmaceuticals

Perfusion

^99mTc labelled macroaggregated albumin particles (MAA), 100 MBq max (1 mSv ED), 10–40 μm in diameter which occlude small lung vessels (<0.5% of total capillary bed).

Ventilation

1. ^81mKr (krypton) gas, 6000 MBq max (0.2 mSv ED). Ideal generator-produced agent with a short T_1/2 of 13 s and a γ-energy of 190 keV. Simultaneous dual isotope ventilation and perfusion imaging is possible because of different γ-energy to ^99mTc. Wash-in and wash-out studies are not possible. Expensive and limited availability

2. ^99mTc-Technegas, 40 MBq max (0.6 mSv ED).³ Ultrafine Tc labelled carbon particles, 5–20 nm in size. No simultaneous ventilation and perfusion imaging. Longer residence time in lungs than aerosols, so SPECT and respiration-gated studies possible. Similar diagnostic efficacy to krypton. Expensive dispensing system

3. ^99mTc-DTPA, aerosol, 80 MBq max (0.4 mSv ED). Simultaneous ventilation and perfusion imaging not possible. Cheap and readily available alternative to krypton, but less suitable in patients with chronic obstructive airways disease or chronic asthma because clumping of aerosol particles is likely

4. ¹³³Xe (xenon) gas, 400 MBq max (0.4 mSv ED) diluted in 10 l and re-breathed for 5 min. Long T_1/2 of 5.25 days and a γ-energy of 81 keV. Ventilation must precede perfusion study because low γ-energy would be swamped by scatter from ^99mTc. Wash-in and wash-out studies are possible. Poor-quality images and rarely used.

Equipment

1. Gamma-camera, preferably multi-headed for SPECT

2. Low-energy general purpose collimator

3. Gas dispensing system and breathing circuit for ventilation.

Patient preparation

1. For ventilation, familiarization with breathing equipment

2. A current chest X-ray is required to assist with interpretation.

Technique

Perfusion

1. The injection may be given in the supine, semi recumbent or sitting position (N.B. MAA particle uptake is affected by gravity)

2. The syringe is shaken to prevent particles settling

3. A slow i.v. injection is given directly into a vein (particles will stick to a plastic cannula) over about 10 s. Avoid drawing blood into the syringe as this can cause clumping

4. The patient must remain in position for 2–3 min while the particles become fixed in the lungs

5. Imaging may begin immediately, preferably in the sitting position.

Ventilation

^81mKr gas

This is performed at the same time as the perfusion study, either by dual isotope acquisition or swapping energy windows at each patient position:

1. The patient is positioned to obtain identical views to the perfusion images and asked to breathe normally through the mouthpiece.

2. The air supply attached to the generator is turned on and imaging commenced.

^99mTc-DTPA aerosol

This scan is performed before the perfusion study, which may follow immediately unless there is clumping of aerosol particles in the lungs, in which case it is delayed for 1–2 h:

1. A DTPA kit is made up with approx. 600 MBq ^99mTc per ml

2. ^99mTc-DTPA is drawn into a 5 ml syringe with 2 ml air, then injected into the nebulizer and flushed through with air

3. The patient is positioned initially with their back to the camera, sitting if possible

4. The nose-clip is placed on the patient who is asked to breathe normally through the mouthpiece. The air supply is turned on to deliver a rate of 10 L min⁻¹

5. After reaching a sufficient count rate the air supply is turned off. The patient should continue to breathe through the mouthpiece for a further 15 s

6. The nose-clip is removed and the patient is given a mouth wash, then imaging is commenced.

Images

Anterior, posterior, left and right posterior obliques.

Since perfusion and ventilation images are directly compared, it is important to have identical views for each. Foam wedges between the patient’s back and the camera assist accurate oblique positioning.

Additional techniques

1. SPECT imaging appears to significantly improve the specificity of the technique by reducing the number of intermediate probability scans.

2. Standardized interpretation of images can improve the clinical effectiveness of V/Q scans, and criteria have been published which are widely used.⁴

Aftercare

Complications

Care should be taken when injecting MAA not to induce respiratory failure in patients with severe pulmonary hypertension. In these cases, inject very slowly.

1 Wang S.C., Fischer K.C., Slone R.M., et al. Perfusion scintigraphy in the evaluation for lung volume reduction surgery: correlation with clinical outcome. Radiology. 1997;205(1):243-248.

2 Win T., Tasker A.D., Groves A.M., et al. Ventilation-perfusion scintigraphy to predict postoperative pulmonary function in lung cancer patients undergoing pneumonectomy. Am. J. Roentgenol.. 2006;187(5):1260-1265.

3 Howarth D.M., Lan L., Thomas P.A., et al. ^99mTc Technegas ventilation and perfusion lung scintigraphy for the diagnosis of pulmonary embolus. J. Nucl. Med.. 1999;40(4):579-584.

4 Freitas J.E., Sarosi M.G., Nagle C.C., et al. Modified PIOPED criteria used in clinical practice. J. Nucl. Med.. 1995;36(9):1573-1578.

Further reading

Gray H.W. The lung. In: Sharp P.F., Gemmell H.G., Murray A.D., editors. Practical Nuclear Medicine. 3rd edn. London: Springer-Verlag; 2005:179-204.

Riedel M. Diagnosing pulmonary embolism. Postgrad. Med. J.. 2004;80(944):309-319.

Computed Tomography in the Diagnosis of Pulmonary Emboli

Technique

The technique will depend on individual CT scanner technology. Single-slice scanners require a longer scanning time than multidetector scanners. Individual CT manufacturers will advise specific scanning protocols tailored to the scanner technology and speed. The volume of i.v. contrast and the scan delay may need to be increased for larger build patients.

Computed tomography pulmonary angiography – enhanced scan of pulmonary arterial system ¹

1. Volume of contrast medium – 110–150 ml or when dual phase injector available, 80–100 ml of contrast medium followed by saline chase of 30–50 ml

a Preset delay of approximately 15 s for single slice; 22 s for 16 slice scanner; 26 s for 64 slice scanner

b Bolus tracking using software supplied with most multidetector scanners. A ROI (region of interest) is positioned over the pulmonary artery at the level of the carina. After commencing the injection a ‘tracker scan’ monitors the hounsfield level at the ROI and the scan is then triggered when the density at the ROI reaches a preset value

3. Rate of injection – 4 ml s⁻¹

4. Detector width/reconstruction (mm) – 0.625/1.25

5. Scan direction and extent – caudocranial direction helps reduce respiratory motion artifact at the lung bases. Less important with faster multislice scanners. Scan from lowest hemidiaphragm to lung apex

6. Image review and post-processing ² – images should be reviewed at three settings:

a Mediastinal window (window width/window level 400/40 HU)

b Pulmonary embolism-specific window (700/100 HU)

c Lung window (1500/−600 HU).

Multiplanar reformatted images through the longitudinal axis of a vessel can be helpful to overcome difficulties encountered on axial sections of obliquely oriented arteries, aiding confidence in diagnosis or exclusion of thrombus.

1 Wittram C. How I do it: CT pulmonary angiography. Am. J. Roentgenol.. 2007;188(5):1255-1261.

2 Wittram C., Maher M.M., Yoo A.J., et al. CT angiography of pulmonary embolism: diagnostic criteria and causes of misdiagnosis. Radiographics. 2004;24(5):1219-1238.

Further reading

Remy-Jardin M., Pistolesi M., Goodman L.R., et al. Management of suspected acute pulmonary embolism in the era of CT angiography: a statement from the Fleischner Society. Radiology. 2007;245(2):315-329.

Schoepf U.J., Costello P. CT angiography for diagnosis of pulmonary embolism: state of the art. Radiology. 2004;230(2):329-337.

Pulmonary Arteriography

Indications

1. Demonstration of pulmonary emboli and other peripheral abnormalities, e.g. arteriovenous malformations, when less invasive investigations have been non-diagnostic.

2. Planning and performing therapeutic procedures such as catheter-directed thrombolysis or catheter embolectomy (in patients who cannot receive systemic thrombolysis due to bleeding risk and where surgical embolectomy is contraindicated).

Contraindications ¹

These include the general contraindications to any form of arteriography such as contrast allergy, renal failure and bleeding diatheses. There are more specific relative contraindications to diagnostic pulmonary arteriography:

1. Elevated right ventricular end-diastolic pressure (>20 mmHg) and/or elevated pulmonary artery pressure (>70 mmHg) – where the mortality rate is increased to 2–3%, instead of a standard documented mortality rate of 0.2–0.5%. Selective arteriography is advised if arteriography is still essential to patient management.

2. Left bundle branch block – right heart catheterization may induce complete heart block. A temporary pacemaker may need to be placed prior to the procedure.

Contrast medium

Low osmolar contrast material (LOCM) 370; 0.75 ml kg⁻¹ at 20–25 ml s⁻¹ (max. 40 ml).

Equipment

1. Fluoroscopy unit on a C- or U-arm, with digital subtraction facilities

2. Pump injector

3. Catheter: pigtail (coiled end, end hole and 12 side holes; see Fig. 8.3).

Technique

The Seldinger technique is used via the right femoral vein. The catheter is introduced via an introducer sheath. The catheter tip is sited, under fluoroscopic control, to lie 1–3 cm above the pulmonary valve. Pulmonary pressures should routinely be measured during the procedure. Lower injection rates are advised if the pressures are raised.

Additional techniques

1. If the entire chest cannot be accommodated on one field of view, the examination can be repeated by examining each lung. The catheter is advanced to lie in each main pulmonary artery in turn. For the right lung the patient, or the tube, is turned 10 ° LPO and for the left lung 10 ° RPO.

2. 40 ° caudal-cranial view: this view is optimal for the visualization of the bifurcation of the right and left pulmonary arteries, the pulmonary valve, annulus and trunk. With this manoeuvre the pulmonary trunk is no longer foreshortened and is not superimposed over its bifurcation.

Complications

The rate of minor complications is 1–3%. These include respiratory distress, major dysrhythmias, contrast reactions and renal failure. The current documented mortality rate is 0.2–0.5%.

For further details regarding general complications related to catheter techniques see page 216.

1 Gotway M.B., Reddy G.P., Dawn S.K. Pulmonary thromboembolic disease. In: Webb W.R., Higgins C.B., editors. Thoracic Imaging: Pulmonary and Cardiovascular Radiology. Philadelphia: Lippincott Williams & Wilkins; 2005:609-629.

Further reading

Kucher N. Catheter embolectomy for acute pulmonary embolism. Chest. 2007;132(2):657-663.

Uflacker R. Interventional therapy for pulmonary embolism. J. Vasc. Interv. Radiol.. 2001;12(2):147-164.

Bronchial Stenting

Indications

1. Compression or stricture secondary to malignant disease

2. Tracheal or bronchial stricture – either congenital or acquired, e.g. post lung transplant

3. Fistulae/dehiscence with the oesophagus or pleural cavity

4. Tracheobronchomalacia (relative indication).

Stents

1. Silicone polymers, e.g. Dumon (Novatech)

2. Metallic, e.g. Palmaz (Johnston & Johnston), Gianturco (Cook)

3. Covered – metallic mesh with polyurethane coating, e.g. covered Wallstent (Boston Scientific)

4. Hybrid – e.g. polyester/silicone or nitinol/silicone

5. Retrievable – e.g. either covered or uncovered nitinol stents, which allow retrieval or repositioning.

For malignant disease, polymer or covered stents should be used to prevent tumour in-growth.

Complications

3. Obstruction, e.g. due to exuberant granulation tissue or tumour in-growth

4. Mechanical failure, e.g. stent fracture

5. Perforation.

Further reading

Dineen K.M., Jantz M.A., Silvestri G.A. Tracheobronchial stents. Review. J. Bronchol.. 2002;9(2):127-137.

Lee K.S., Lunn W., Feller-Kopman D., et al. Multislice CT evaluation of airway stents. J. Thorac. Imaging. 2005;20(2):81-88.

Magnetic Resonance Imaging of the Respiratory System

MRI only has a limited role in assessing the respiratory system as the susceptibility artifact from the large volume of air makes imaging the lungs difficult. The mediastinum and chest wall are well assessed by MRI due to the ability to image in the coronal and sagittal plane. MRI is particularly useful in the assessment of superior sulcus and neurogenic tumours as it can give information on the nature and extent of any intraspinal extension. Fast sequences and cardiac gating may be used to minimize motion artifacts. However, CT is quicker and more widely available. Multidetector CT technology can now give excellent multiplanar and three-dimensional reconstruction.

In the future MR pulmonary angiography may have a role to play in the diagnosis of pulmonary emboli, but it will probably be limited to a small cohort of patients in whom there is a relative contraindication to radiographic contrast or ionizing radiation.¹ At present, multi-detector CT scanning is the preferred method of investigation owing to its availability and ease of performance.

1 Fink C., Ley S., Schoenberg S.O., et al. Magnetic resonance imaging of acute pulmonary embolism. Eur. Radiol.. 2007;17(10):2546-2553.

Further reading

van Beek E.J., Wild J.M., Fink C., et al. MRI for the diagnosis of pulmonary embolism. J. Magn. Reson. Imaging. 2003;18(6):627-640.

PET and PET-CT of the Respiratory System

In recent years, FDG-PET has become an essential lung-cancer imaging tool. A PET scan is currently recommended for the investigation of lung cancer where the lesion is greater than 1 cm in size. It also plays an important role in loco-regional and distant staging of non-small cell lung cancer (NSCLC). Patient preparation and the technique are fully described in Chapter 11. The scan can be performed either on an isolated PET scanner or a hybrid PET-CT scanner.

Recent studies suggest that PET is more sensitive than CT in measuring the biological effects of anti-cancer therapy. In the future PET-CT may have an additional role in predicting the response to anti-cancer treatment when patients have a scan performed to assess the early response after induction therapy.

Further reading

Devaraj A., Cook G.J., Hansell D.M. PET/CT in non-small cell lung cancer staging – promises and problems. Clin. Radiol.. 2007;62(2):97-108.

Vansteenkiste J.F., Stroobants S.S. PET scan in lung cancer: current recommendations and innovation. J. Thorac. Oncol.. 2006;1(1):71-73.

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