0. Basics of XRAY, Nuclear Medicine, US, CT Flashcards
What type of radiation is XRAY?
Electromagnetic, IONIZING radiation
How are X rays generated?
Through a vacuum tube that uses high voltage to accelerate electrons released by a cathode to a high velocity - those electrons collide with a metal target, the anode, creating the XRAY
Examples of anode metals
- Molybdenum (used in mammography)
- Wolfram
Types of X-Ray images
- static snapshot (photography)
- dynamic / functional imaging (“movie”)
- continuous XRAY radiation
What does the attenuation of XRAY depend on?
It depends on atomic number, density and thickness - dense structures appear brighter.
(it’s the opposite in fluoroscopy)
How is XRAY resolution?
- Excellent spatial resolution
- Poor soft tissue resolution
How can we visualize pneumoperitoneum in XRAY?
By layin the patients in lateral decubitus position for their XRAY, we can detect air-fluid levels and free peritoneal air
What is dense breast on mammography?
Dense fibroglandular tissue with less adipose tissue - makes it harder to see cancer & microcalcification
What is the fat-pad sign on XRAY?
Luscent crescent of fat that indicates join effusion after trauma (aka fracture)
What is the significance of posterior fat pad on XRAY?
Supracondylar fracture or intra-articular fracture
What is the GI contrast agent for XRAY?
Barium, but if contraindicated it can be switched out for iodine-based contrast agentsW
When and why can barium be contraindicated?
- It is contraindicated in cases of GI postoperative bleeding, perforation…
- Because it causes severe peritonitis or mediastinitis if it gets in contact with the peritoneum or pleura
What is a small bowel follow through exam?
- First a non contrast XRAY is taken
- Then patient drinks iodine-based contrast agent
- Images are taken after every 30-60 minutes to follow advancement of contrast agent
Advantages of XRAY
- Accessible
- Quick
- Can use contrast agents
- Specific signs in certain diseases
Disadvantages of XRAY
- Ionising radiation
- 2D
- Summation image
- Not good for soft tissues
- Can’t detect many diseases
Can we use XRAY to detect kidney stones?
No, it’s not sensitive enough, the gold standard is low dose enhanced abdominal CT
What is nuclear medicine ?
medical activities using unsealed radioactive isotopes in diagnosis, treatment, and research of diseases
Types of radiations used in diagnostics and therapy
Diagnostic : 99mTc (gamma - SPECT), 18F (positron - PET)
Therapy : alpha or beta emitting isotopes
Half life of Technetium-99m
6 hours
What does SPECT detect?
Gamma emitting radioisotopes
Which has the best resolution and sensitivity between PET and SPECT?
PET
Advantages of PET/SPECT
- Sensitive
- More specific than CT
- Gives metabolic information
Disadvantages of PET/SPECT
- Lower resolution
- Lack of precise localization
- Longer acquisition time
What radioisotope is used in bone scintigraphy and why?
Tc99m-diphosphonate - adsorbs to bones in proportion to osteoblast activity
Advantages and disadvantages of bone scintigraphy
- Very sensitive : can detect lesions before XRAY
- Not specific : fractures, inflammation, primary tumors…
What radioisotope is used in thyroid scintigraphy and why?
Tc99m-pertechnetate - taken up by thyroid cells through Na-I symporter (so same as iodine)
What are some tumors that cannot take up FDG?
Prostate, hepatocellular, some neuroendocrine tumors…
What 3 components does the creation of sonographic images depend on?
- production of high frequency sound waves
- reception of reflected waves
- conversion of that echo into the actual image
Frequency of probes in US
- Curvilinear : 3-5MHz
- Linear : 7-10MHz
How can you get better resolution and penetrability in US?
Higher frequency results in better resolution, while lower frequency results in better penetrability
What is the most used type of ultrasound in diagnostics?
B-mode
What is an echodense tissue?
A tissue that reflects all echo. It will be bright with a posterior acoustic shadowing
What is M-mode used for?
To visualize moving structures, like heart valves in echocardiography
What is power doppler used for?
More sensitive than color doppler, so it’ sused for flow with low velocity
What is the ultrasound contrast agent?
Microbubbles : bubbles with a shell made up from albumins, galactose, lipids and polymers
What is US used for in kidneys?
To detect hydronephrosis. The ureters are difficult to visualize on US, so we would use CT instead
Why is a full bladder necessary for US?
To provide an acoustic window, to avoid the air filled bowels when assessing prostate or pelvic structures
What is ureteric jet?
Detecting normal periodic flux of urine from ureters into the bladder - to exclude renal tract obstruction
What is US used to detect in the thyroid gland?
- Vascularization
- Nodules
Normal lymph node on US
- Fatty hyperechoic hilum
- Hypoechoic cortex
What is the modality used to screen stenosis?
Doppler ultrasound
What affects the risk of rupture of an aneurysm?
Its size (4-5cm have 25% risk of rupture)
What is the screening of choice to check for aortic aneurysm?
US - blood will be anechoic, thrombus will be echogenic
HOw do we check for deep vein thrombosis on US?
Veins containing thrombus will not completely collapse after compression, and the thrombus is echogenic
What does FAST US stand for?
Focused Assessment With Sonography in Trauma
What are the 5 main points of focus of FAST?
- Heart
- IVC
- Morrison’s pouch (x3)
- Aorta
- Lungs
Classical sign of pneumothorax on M mode FAST US?
Normal lung sign would be “seashore” but in PTX, there is just beach : only static
Benefit of helical / spiral CT
Multiple regions can be scanned during a single breath-hold, in just a couple of seconds
What is bright vs dark in CT?
Dark : hypodense - low density regions like air, fat
Light : hyperdense - dense structures like metal, bone
GI contrast agents in CT
- Iodine based, like gastrografin (bowel leaks, fistulae)
- Water or macrogol -hypodense, useful in suspected bowel ischemia)
Phases of contrast enhancement
- No contrast phase
- Arterial phase / angiography
- Late arterial / renal cortical phase
- Hepatic / late portal phase
- Nephrogenic / venous phase
- Excretory / delayed phase
Do we always measure the GFR before contrast CT?
No, not in emergency situations - kidney function is measured after the examination
Advantages of CT
- 3D image
- Good spatial resolution
- Good for soft tissues
- Fast
- Can visualize calcifications and lung tissue
Disadvantages of CT
- High ionizing dose
- Less good than MRI for soft tissue
- Contraindications (pregnancy, kidney failure)
