Week 8: Ct Thorax

Question 1

CT Thorax Indications

Answer 1

• pulmonary embolism
• pulmonary nodules/mass
• infection
• trauma
• bronchiectasis
• inhalation injury
• interstitial disease
• emphysema
• coronary artery disease

Question 2

CT Thorax Routine Patient Position

Answer 2

• supine (most often but some protocols require prone)
• arms up
• short scan time to minimize breathing and cardiac/vessel motion artifacts
• scouts AP and Lat (site specific)
• single breath hold, inspiration mostly
• routine chest is scanned from apices to below costophrenic angles
• CTA for pulmonary embolism can be scanned inferior to superior

Question 4

Non Contrast CT Thorax

Answer 4

• screening
• detection or exclusion of pulmonary nodules or primary lung disease (emphysema, fibrosis, etc)

Question 5

Contrast Enhanced CT Thorax

Answer 5

• differentiate vascular from. non-vascular
• primary lymph nodes
• cardiovascular structures
• lesions
• esophageal studies

Question 6

Windowing CT Thorax

Answer 6

• soft tissue: ww:350, wl: 50
• lung: ww:1500, wl:-700

Question 7

CT Airways/Bronchography

Answer 7

• commonly used to look for narrowing of airways
• thin slices (1.25 mm or less)
• single breath hold, fast acquisition, both inspiration and expiration
• no IV or oral contrast unless tumour in airway suspected
• virtual bronchoscopy offers internal rendering of bronchial tree, walls and lumen

Question 8

High Resolution CT (HRCT)

Answer 8

• evaluates lung parenchyma or diffuse lung disease
• thin slices, ≤ 1.5 mm (thicker used on larger patients to decrease mottle, 2.0-2.5 mm used)
• fast single breath hold, expiration and prone options as well
• edge enhancing algorithm used to optimize spacial resolution
• can be incremental or volumetric
• no contrast
• may be three scan series: supine inspiration, supine expiration and then prone inspiration
• reformats at 1.25 intervals

Question 9

Why are prone images taken?

Answer 9

to help differentiate actual pathology from effects of gravity that may mimic pathology

Question 10

Incremental HRCT

Answer 10

scans only 10% of lung, reducing dose but disease may be missed

Question 11

Volumetric HRCT

Answer 11

• helical
• complete lung and airways assessment
• MIP and MinIP reformats
• increased radiation dose but mA can be reduced to compensate

Question 12

What is used for early diagnosis of interstitial lung disease (ILD)?

Answer 12

• ILD is main cause of death in systemic sclerosis
• gold standard is HRCT but regular screening faces increased radiation dose
• 9 slice low dose HRCT is a great alternative for the detection of ILD in patients with systemic sclerosis

Question 13

CTA in the Diagnosis of Pulmonary Embolism

Answer 13

• scanned inferior to superior/caudal to cranial to minimize respiratory artifact and streaking artefacts from CM
• 30% death rate if PE left untreated
• CTA must be careful considered for young or pregnant women due to the high radiation dose (tech must identify and bring attention to rad)
• dose, rate and timing of CM is critical to CTA exams, spine flush recommend
• MDCT is the mainstay if PE diagnosis
• CT pulmonary angiography studies better than traditional angiography

Question 14

Disadvantages of MDCT in PE Diagnosis

Answer 14

• cases with sub optimal vessel opacification or variations
• breathing artifacts
• overt patient motion
• require CM (patients with contraindications can’t get)
• radiation dose high (risk vs benefit)
• learning curve, requires knowledgeable and experienced techs

Question 15

Advantages of MDCT in PE Diagnosis

Answer 15

• thinner slices = improves spatial resolution
• MPRs, reconstructions further improve spacial resolution
• rapid imaging for patients who are SOB
• fast, allowing for pan contrast fill of arteries
• establish alternative diagnosis if no PE present
• cost effective

Question 16

Why scan inferior to superior in CTA PE?

Answer 16

• minimize respiratory artifact
• reduce streaking artists from CM in subclavian and SVC

Question 17

Alternative Diagnostic Study Options to CTA PE

Answer 17

• nuclear medicine - ventilation perfusion (VQ) scan: high percentage (73%) of studies results are indeterminate
• traditional angio study: expensive, invasive, complications, inexperienced rads)
• lab test ELISA: abnormal ELISA doesn’t confirm PE as many other causes may result in elevated d-dimer assays

Question 18

PE CTA Protocol

Answer 18

• may be performed hemidiaphragms to apices
• ECG gating for PE’s is controvesial
• saline flush used to eliminate beam hardening artifacts around SVC, right main and right upper arteries
• CTV a second scan is done with a 180 second delay (venous phase) from crests to knees to detect DVT
• start: 2 cm below tibial plateau,
• end: iliac crest (if pt has IVC filter, as seen on scout image, end 2 cm above the IVC filter)

Question 19

Routine Chest Protocol

Answer 19

• inspiration
• start: lung apices
• end: just below costophrenic angles
• helical
• recons 2.5mm thickness/1.25 intervals
• IV CM: 80 mL at 3 mL/s, delay 35 seconds

Question 20

CTA Thoracic Arch

Answer 20

• 2cm above the arch to 2cm below the celiac trunk
• may be gated