CT- Thoracic Imaging Procedures
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what makes CT imaging of the chest sometimes challenging?
motion of the heart and vascular structures
why are scans done for screening, detection, or exclusion of pulmonary nodules or primary lung diseases such as emphysema or fibrosis typically done without contrast?
because the thorax has the highest intrinsic (due to its nature) natural contrast of any body part. for ex the pulmonary vessels and the ribs have significantly different attenuation values compared with the adjacent aerated lung.
CT imaging of the airways requires thin sections of how many mm?
1.25mm or less
airway imaging is routinely performed on inspiration or expiration?
high resolution CT (HRCT) is used to evaluate what part?
lung parenchyma in patients with known or suspected diffuse lung diseases such as fibrosis and emphysema
high resolution ct (HRCT) uses thin sections of how much?
1.5mm or less
what is volumetric HRCT (high res CT)?
- uses helical mode to acquire images of the entire lung, rather than representative slices done with axial scanning where lung nodules could be missed between slices.
- Because these helical protocols cover the entire lung, they result in a more complete assessment of the lung.
- in addition, they allow post-processing techniques such as MIP and MinIP
- The only downside is radiation is increased with volumetric HRCT
around how much of the lung is scanned during an HRCT axial scan?
- volumetric HRCT scans the whole lung
why are there additional series of prone images for a HRCT?
- theres an increase in attenuation and vessel size from anterior to posterior lung regions because of the gravity on blood flow and gas volume.
- also, there can be atelectasis in the most dependent lung (the side touching the CT table) that can mimic or hide lung disease
why are HRCT protocols routinely obtained at full inspiration? why is a series of HRCT on expiration included?
- full inspiration causes the lung to expand and the contrast between low-attenuation aerated air space and high-attenuation lung structure is maximized.
- expiratory images are useful for depicting bronchiolitis and air trapping
HRCT protocols include which three series of scans?
- inspiratory supine
- expiratory supine
- inspiratory prone
in volumetric HRCT protocols, which ones are done in helical and axial modes?
only the inspiratory supine series is done in a helical mode. The additional images are done in axial fashion to reduce the radiation exposure
most pulmonary emboli are caused from thrombi originating where?
in the lower extremities known as deep vein thrombosis (DVT)
systemic circulation is defined as
blood flow from the left ventricle of the heart through the blood vessels to all parts of the body and back to the right atrium
define pulmonary circulation
blood flow from the right atrium to the right ventricle to the pulmonary artery to the lung and capillaries.
what is the role of D-dimer for ruling out PE?
- lab test done to see if there is a PE.
- If D-dimer is within limits, then the likelihood of a PE is minimal
what is CT venography (CTV)?
- this may be performed to assess for venous thrombosis within the pelvis and lower extremities
- accomplished by obtaining a second scan series in a delayed venous phase (180 secs after IV contrast injection) from the iliac crest through the knees
why are CTA protocol scans occur in a caudal-to-cranial direction?
- to minimize respiratory artifact
- respiratory motion is greatest at the lung bases therefore, in cases where patients cannot hold their breath in well it is best to start the scan there.
what are some factors to consider that may affect the quality of a CTA for PE study when injecting contrast?
- dose, rate, timing of IV contrast
- age, cardiac output, lung disease, position of catheter
why is the use of a saline flush after the injection of contrast recommended?
eliminates beam-hardening artifacts within the SVC that may obscure small emboli in adjacent vessels, particularly in the right main and right upper lobe pulmonary arteries
when is a coronary artery bypass graft surgery (CABG) recommended?
when there is a disease of the left main coronary artery or in three or more vessels.
why is it important for a CT tech to know the basics of a CABG procedure?
because it is a common indication for a cardiac CT for evaluating patients for possible surgery and for assessing graft patency after surgery
to reduce motion artifact on cardiac CTA images a patient's heart rate can be temporarily lowered by administration of what?
B-blockers (beta blockers)
what do beta blockers do to the heart when given to the pt before a cardiac CT scan?
lowers the heart rate to less than 65 to 70 beats per minute and to make the rhythm more regular to minimize heart motion
the decision to give beta blockers and the dosage for a cardiac CT scan should be made by who?
physician, nurse practitioner or a PA
most institutions also give patients nitroglycerin sublingually before coronary CT scans, what does nitroglycerin do?
- it dilates vessels to improve visualization
- helps prevent coronary spasm that can mimic stenosis on the CT image
- its given as a spray or table with dosage of 0.3 mg
what is ECG gating?
- minimizing cardiac motion by acquiring images during the point of the cardiac cycle with the least cardiac motion
- usually during end-diastole but may also be at end-systole, in other words, just before or just after the left ventricle is fully contracted
what are prospective ECG gating and retrospective gating?
two techniques that attempt to minimize cardiac motion in the study by selecting or acquiring images during cardiac segments with relatively slow cardiac motion
what does Prospective ECG gating accomplish during a cardiac CT scan?
also known as sequential or cine-mode scanning seeks to identiify the areas of lowest cardiac motion and acquire images only in those portions of the cardiac cycle, which minimizes radiation exposure
what does Restrospective ECG gating accomplish during a cardiac CT scan?
it acquires images throughout the cardiac cycle while the patient's ECG is recorded. Images are later reconstructed to create image sets at any desired phase of the cardiac cycle.
How does the Prospective ECG gating acquire images during a cardiac CT scan?
- acquiring images is based off of a portion in a patient's ECG. It acquires an image after the "R" section.
- For ex: "R", then delay, then scan
- the table then moves to the next position and waits for the portion of the ECG before taking the next picture. This is done until the entire area of interest is covered
- this method is also known as step-and-shoot, different from helical CT
- the primary advantage is the dramatic reduction in radiation dose to the patient
- primary disadvantage is that this method is very sensitive to cardiac motion artifacts and misregistration.
How does Restrospective gating acquire images during a cardiac CT scan?
- uses helical scanning throughout the cardiac cycle. using the ECG tracings that are acquired with the scan acquisition, images are reconstructed at the portion of the cardiac cycle estimated to have the least motion
- the primary disadvantage is the high dose of radiation given to the patient.
- to combat the high radiation given to patients during retrospective ECG gating, manufacturers developed methods that automatically decrease the tube current during the systolic phase (phase that has the most cardiac motiong) of the ECG tracing. This technique is called ECG-pulsed tube current modulation.
what is the greatest challenge to obtaining diagnostic images during prospective ECG gating and retrospective ECG gating during cardiac CT scans?
- heart rate variation
- can result in misregistration of the targeted anatomy
what makes the ECG pulsed tube current modulation during a restrospective ECG gating in a cardiac CT scan impossible?
high heart rate variability and the presence of arrhythmia
what is the contrast administration protocol for cardiac studies?
- 3-6ml/sec injection rate
- just like a CTA chest for PE
- 20g or larger catheter on anticubital vein is
- iodine concentration between 300 and 400 mg/ml
- smart prep, bolus tracking software
- ROI on ascending aorta for coronary arteries studies
- ROI on left atrium for pulmonary vein studies
what is a calcium score?
the amount of calcification on cardiac CT
how is imaging of coronary calcifications achieved?
- low dose technique without contrast
- scan time as short as possible
- either a prospective or retrospective cardiac gating is used
- patient is supine and scan extends from the midlevel of the left pulmonary artery down to the diaphragm
what is the main concern of coronary calcium scoring ct scans?
because it is applied as a screening examination to asymptomatic patients, the radiation dose is the main concern
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