The Basics of Ultrasound, Nuclear Medicine and MRI Scans Flashcards
How are ultrasound images created?
This sectional images using echoes returned from tissue interfaces
Position of echo display derived from known speed of sound
Strength of echo determined by difference in tissues acoustic impedance
What determines the brightness of display on the monitor in ultrasound?
Strength of echo
What produces an acoustic shadow?
Almost 100% of sound reflected
Where is ultrasound not very useful?
Brain
Lung
Areas behind bowel gas
What is the convention for displaying sagittal sections in ultrasound?
Patient’s head on left of image
What is the convention of displaying transverse sections in ultrasound?
Patient’s right side on left of image
What is the intensity of solid organs in ultrasound?
Mid-level - appear grey
Define anechoic
Few, if any, echoes
Display is black
Which tissues are anechoic in ultrasound?
Fluids
Define echogenic
Bright in ultrasound
What are acoustic shadows?
Bright areas causing darker echoes behind them in ultrasound
What is posterior enhancement?
Dark areas causing brighter echoes behind them in ultrasound
What is Doppler ultrasound?
Weak echoes that return from moving blood have slightly different frequency to transmitted ultrasound
Amount and direction of frequency change depends on velocity and direction of blood flow
How can a Doppler ultrasound be displayed?
Colour
Velocity waveform
What can a Doppler ultrasound do?
Detect blood flow
Determine direction of flow
Detect stenoses
Where is contrast enhanced ultrasound (CEUS) used?
Cardiac
Liver
Renal
What is the contrast used in CEUS?
Microbubbles injected intravenously
Dramatically enhance signal
Is CEUS safe?
Yes, very safe
Can be used in presence of renal failure
What are whiter images in ultrasound called?
Hyperechoic
More echogenic
What are darker images in ultrasound called?
Hypoechoic
Less echogenic
What are the advantages of ultrasound?
No radiation Real time Portable Non-invasive Relatively cheap
What are the disadvantages of ultrasound?
Operator dependent Limited field of view Blind spots Obesity - Problem mainly for abdominal and pelvic imaging
What are the clinical applications of ultrasound?
Obstetrics and gynaecology Abdominal Cardiac Vascular Musculoskeletal Small parts Interventional/intraoperative
What are the two types of nuclear medicine?
Diagnostic = imaging that makes use of radioisotopes Therapeutic = therapy using radioisotopes
How does radioisotope imaging work?
Attached to particular molecules
Introduced into body
Emit gamma rays
Gamma rays recorded by gamma camera
What is a radiopharmaceutical?
Radioisotope tagged to an agent that will target organ of interest
What is the most common gamma ray emitter used?
Technetium 99-m
Define hot spot
Focal area of increased isotope uptake in nuclear medicine
Generally displayed as darker
Define cold spot
Focal area of decreased isotope uptake in nuclear uptake
Define photopaenia
Relative decrease in isotope uptake
Define FDG avid
Greater FDG uptake and retention in PET
Appears as darker area
How are ventilation perfusion scans performed in nuclear medicine?
2 different agents given
- 1 inhaled
- 1 IV
Mismatch used as sign of pulmonary embolism
What is radioisotope biliary imaging most often used for?
Assessing gall bladder pathology
What agent is mostly used in PET studies?
18F-fluorodeoxyglucose (FDG)
Uptake reflects metabolic activity
What are FDG PET studies used for?
Tumour detection
- Powerful staging modality
Other than tumours, what else takes up FDG?
Normal tissues, especially
- Brain
- Myocardium
Inflammatory tissue
How is excellent spatial resolution created in PET studies?
Combined with CT
What are the advantages of diagnostic nuclear medicine?
Can demonstrate metabolic activity and function to much greater extent than other modalities
Sensitive in detection of many forms of malignant tumours
What are the disadvantages of diagnostic nuclear medicine?
Ionising radiation Relatively expensive Poor anatomical resolution - Overcome by combining with CT Some studies lengthy Can't guide invasive procedures
How does MRI work?
Patient placed in very strong magnetic field
H atom acts like magnet - aligns itself along direction of magnetic filed
Radiofrequency signal applied > protons excited > change alignment
Radiofrequency signal stopped > protons return to initial alignment = relaxation
Radiofrequency signal emitted - used to create image
What are the clinical applications of MRI?
Most common - CNS - Musculoskeletal Tumour detection - Liver - Pancreas Bile ducts - Bile duct stones - Other causes of bile duct obstruction Small bowel imaging - Especially in Crohn's disease
What are T1 weighted sequences?
Provide anatomical resolution
Display stationary fluid as dark
Pathology tends to be dark
What are T2 weighted sequences?
Fluid sensitive
Demonstrate pathology as bright
Stationary fluid as bright
What are brighter areas in MRI called?
High signal
Hyperintense
What are darker areas in MRI called?
Low signal
Hypointense
Define isointense
Tissue that’s same intensity as surrounding tissue
What are fluid attenuatin inversion recovery (FLAIR) sequences in MRI?
Useful for CNS imaging
Null CSF signal - looks black
Pathology bright
What are contrast enhanced scans in MRI?
Contain gadolinium Alters relaxation properties of tissues Used usually in combination with T1 sequences Contrast enhancement appears bright - Highlights pathology
What is diffusion weighted imaging (DWI) in MRI?
Shows freedom of water molecule motion
Areas of diffusion restriction = high signal
What are MR angiograms and venograms?
Images of flow within blood vessels
What is MR cholangio-pancreatography (MRCP)?
Allows imaging of biliary system and pancreatic ducts without administration of contrast
Stationary fluid shows up as very bright
What are the contraindications to MRI?
Non-MRI compatible pacemakers Intracranial aneurysm clips Cochlear implants Endovascular stents Shrapnel/metallic foreign bodies Claustrophobia - Relative contraindication - May need sedation Patient size - may not fit scanner
What are the contraindications to contrast administration in MRI?
Significant renal impairment because of risk of developing nephrogenic systemic fibrosis
What are the advantages of MRI?
No ionising radiation
High spatial resolution
Superior soft tissue contrast resolution compared to CT
Multiplanar display
Can produce angiography/venography/cholangiography without administering contrast
What are the disadvantages of MRI?
Cost
Extended duration of exam
Less widely available than CT
Claustrophobia
