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Computer TomographyTechnique

Lecture 8:CT scan Thorax

September, 2012

Prepared by: Behzad Ommani

Master of Medical EngineeringInstructor Radiology Group

It can often identify confusing superimposed shadows seen on the plain film chest radiograph.

Indications :1. Pleural cavity2. Mediastinum3. Cardiovascular structures4. Bony cage of ribs5. Sternum

Thorax

• The data is continuous, because a whole volume is scanned.

• Small areas of pathology are unlikely to be missed; this can happen with conventional CT scanning if the slices are not contiguous.

• The whole volume can be scanned in one breathhold, so there is no misregistration of data due to respiratory movement.

Thorax spiral and Conventional

• Contrast enhancement is more uniform throughout the study so the amount used may be reduced, optimised and captured to perform CT angiography, or multiphased studies.

• It can be faster for the patient; this has excellent indications for paediatric, geriatric and trauma scanning, and for increasing workloads.

Thorax spiral and Conventional

Patient position :

• Supine, Both arms should be raised above the head. This is the planning scan and is usually an anteroposterior (AP) view.

Start position : Above the sternal notchEnd position : Dome of Diaphragm

Thorax

• Both arms should be raised above the head. This is sometimes not possible and may result in streak artefact across the images.

To avoid or reduce this the following options or a combination of them may compensate:

1. raise one arm if possible2. increase exposure3. change the algorithm to soft rather than standard to

reduce noise on the image.

Thorax

Thorax Protocol

• Slice thickness 8 mm• Table increment 8 mm• Kilovoltage 120 kV• mAs per slice 100 - 200 mAs • Algorithm standard• Scan field of view 30 - 48 cm• Display field of view 30 - 48 cm

• levels used to provide optimum visualisation of structures of the thorax are as follows:

• Mediastinum/soft tissue structures: 350 WW, 40 WL

• Lung parenchyma: 1500 WW, -550 WL

• Bone: 2500 WW, 250 WL

Thorax Protocol

• immediately prior to scanning, may enable a longer breath-hold. For a very breathless patient it may be advisable to scan from the diaphragms up: the apices move less than the diaphragms on respiration, so limiting movement artefact.

• If breath-holding is impossible, gentle breathing may be better since the movement reduction algorithm of helical scanning may remove some artefact. In practice, scans performed in this manner are usually of a reasonable diagnostic quality.

Thorax Breath

• The correct and timely administration of contrast media can increase the amount of information gained from the unenhanced thorax scan.

Oral Contrast Medium

• Experience has shown that this is not commonly used in thorax scanning since its passage through the oesophagus is often too rapid to be captured on the scan.

Thorax Contrast

• An oral, water-soluble contrast medium such as Iopamidol; 10 ml to 150 ml of water, is suggested.

• The barium sulphate type of CT oral contrast medium can be used, as its viscosity will allow it to travel more slowly through the oesophagus, but its density may cause streak artefacts and so detract from the images. Also the contraindications should be noted prior to administration.

• The patient should then be asked to drink approximately 100-150 ml of oral contrast. This is most easily done using a flexible straw, but it must be remembered that it is very difficult to drink when lying flat

Thorax Contrast

Intravenous Contrast Media

• This is used routinely in thoracic scanning in order to demonstrate the vasculature of the mediastinum and pleural cavity, and is particularly useful where there is confusion between lymph nodes and vessels.

Thorax Contrast

Multiple lesions demonstrated on a routineunenhanced thorax scan; shown histologically to be pulmonary metastases

• Diagnosis of malignant disease in the pleura, bronchi or mediastinum: to demonstrate any tumour and its extent.

• Staging of malignant disease: to indicate whether it is operable; involvement with other organs, vasculature and lymph nodes; progression of disease pre and post treatment.

• Diagnosis of pulmonary metastases.• Staging of metastatic disease• Lymphadenopathy.• Lung and airway disease• Pulmonary collapse• Trauma

Thorax Contrast Indication

• Enhanced scan parameters are similar to those of the routine unenhanced scan, following the administration of 50-100ml of an intravenous, preferably non-ionic, contrast medium, 350 mg iodine/ml.

• This is usually administered through the ante-cubital vein at a rate of 2 ml/s., either by hand or pump injection.

Contrast

• Thoracic aortic aneurysm/dissection

• Detection of pulmonary thromboemboli in segmental

• Pulmonary vessels uperior vena cava (SVC) obstruction(Pulmonary angiography is still seen as the gold standard' investigation

but, it is invasive)

• Arteriovenous malformation

• Aortic graft assessment.

CT Angiography

• Beam collimation 5-7 mm• Reconstruction interval 2-7 mm• Pitch 1.5-2, to allow a single breath hold• Standard algorithm Lung/detail or bone algorithm if

appropriate• Matrix 512• kVp 120• mA 200• Imaging Mediastinal settings, lung settings if

appropriate

CT Angiography protocol

HRCT

Indications :1. interstitial lung disease2. bronchiectasis, airway disease3. differentiation of focal lung disease4. differentiation of pulmonary nodules

• A routine thorax scan is not indicated here, since it is only the lung parenchyma in fine detail that is important.

• Incremental conventional slice by slice scanning is used to acquire thin slices of data.

Patient position :

• Supine, Both arms should be raised above the head. This is the planning scan and is usually an anteroposterior (AP) view.

Start position : Above the sternal notchEnd position : Dome of Diaphragm

HRCT Protocol

• The patient is in the supine position, although it may be of advantage to position the patient prone since ground glass' appearance which can occur with interstitial lung disease may be confused with fluid in the lung bases if the patient is supine.

HRCT Protocol

Thorax Protocol

• Slice thickness 1-3 mm• Table increment 10 mm• Kilovoltage 120 kV• mAs per slice 100 - 200 mAs • Algorithm standard• Scan field of view 30 - 48 cm• Display field of view 30 - 48 cm• Window width 1550/1700• Window level -550