18 - Introduction to Medical Imaging Flashcards
What are the three planes used in medical imaging? How do they each divide the body?
Sagittal - divides into left and right
Coronal - divides into anterior and posterior
Transverse - divides into superior and inferior
What are the 5 principal densities on an x-ray image?
Air (black) Fat Soft Tissue Bone Metal (white)
How do X-rays give an image?
- Beam of high energy electrons pass through the body onto a receiver
- Some are absorbed or scattered (attenuation) - some tissues more than others
- More dense tissue = more attenuation = lighter appearance
What advantages do electrical cassettes have over x-ray film when taking an x-ray?
Cassettes create an electrical signal on exposure to x-rays and send it to a Picture Archiving and Communication System (PACS)
- Can zoom
- Can alter the densities
- Measure distances and angles
What are AP and PA x-rays? Which is more routine?
AP - x-ray beam passes through the patient anterior to posterior to reach the detector
PA - x-ray beam passes through the patient posterior to anterior to reach the detector
PA is used more routinely, AP is used for patients that are sitting or bed-bound
Why can the heart appear abnormal in AP X-rays?
In an AP x-ray the heart is slightly further from the receiver. The x-ray beam diverges with distance so the heart appears larger in AP x-rays. May give the impression of cardiomegaly.
What acronym is used to read a chest X-ray?
A - Airways B - Breathing C - Circulation D - Disability E - Everything else
What acronym is used to read an abdominal X-ray?
A - Air
B - Bowel (small and large)
D - Densities (bones)
O - Organs
What are some advantages and disadvantages of X-rays as an imaging technique?
Advantages:
- Quick
- Portable
- Cheap
Disadvantages:
- Radiation (low level)
- One plane so only 2D
- Can’t visualise all areas (e.g. not the brain as it lies within the skull)
- Poor soft tissue imaging
What are some common uses of X-rays?
- Chest - infection, pneumothorax, trauma, oedema, effusion
- Bowel - dilatation, perforation
- Orthopaedic - fracture, trauma
- Post-procedure - nasogastric tube, pacemaker, PICC (peripherally inserted central catheter)
- Dentistry
What is fluoroscopy? What contrast media are commonly used?
Uses a constant stream of x-rays to create a frame-by-frame movie of anatomy in motion. Enhanced by contrast.
- Barium
- Iodine
- Gadolinium
What are the common uses of fluoroscopy?
- Angiography
- Contrast GI studies
- Therapeutic joint injections
- Arthrograms
- Screening in theatre
What are the advantages and disadvantages of fluoroscopy?
Advantages:
- Dynamic
- Cheap
- Useful in interventional procedures (e.g. stent insertion after an MI)
Disadvantages:
- Radiation - clinician exposure must be minimised
What is CT scanning?
Rotating gantry with x-ray tube on one side and detectors on the other, same principle of attenuation as with x-rays. Images are put together by a computer.
How many Hounsfield Units are measured for each of these densities?
- Air
- Water
- Cortical bone
Air = -1000 HU
Water = 0 HU
Cortical bone = 2000 HU
In a CT scan, how is the patient positioned and what is the view seen on the scan?
Patient usually positioned supine.
Imaging done in transverse plane and seen looking from the feet upwards. Left of image = right side and vice versa.
What are the advantages and disadvantages of CT scanning?
Advantages:
- Quick
- Good spatial resolution
- Can scan most areas
Disadvantages:
- Radiation
- Requires breath holding (not all patients may manage)
- Overuse when fishing for a diagnosis
What are some common uses of CT scanning?
Diagnosis - cancer, stroke, bone injury, blood flow
Guide further treatment - radiotherapy, biopsy
Monitor conditions - cancer treatment
What is MRI and how does it work?
Magnetic resonance imaging
- strong magnetic field aligns hydrogen atoms. Some point towards the head and others towards the feet. Some unmatched ions remain
- When RF pulse is applied, the unmatched ions spin in the opposite direction
- Pulse is turned off, atoms return to normal position and emit energy
- Computer processing of the signal generates an image
What are the two different MRI weightings and how do they differ?
T1 and T2
T1 - fat is white, water is black
T2 - water is white, fat is black
What are some advantages and disadvantages of MRI?
Advantages:
- No radiation
- Good contrast resolution
Disadvantages:
- Expensive
- Time consuming
- Claustrophobic and some patients may not fit
- Need to lie still
- Loud
- Metalwork (although lots of surgical metalwork is now MRI safe)
What is scintigraphy?
- Nuclear imaging technique using radiopharmaceuticals that emit gamma rays
- Highly sensitive
- Gives functional and anatomical information
- 2D (PET scan is 3D version)
What is a PET scan and how is it used?
Positron Emission Tomography
Radionuclides that decay by positron emission are bound to glucose. PET scan detects the high energy gamma rays given off.
Hot spots are areas of high glucose metabolism - heavily used in oncology as cancers are highly metabolically active
How does ultrasound form an image?
- High frequency sound waves emitted from transducer
- Sound wave is reflected back when the density (impedance) of a tissue differs
- Probe detects the reflected waves
Distance determined by time taken to return
Density determined by impedance (proportion of wave that is reflected back)
Why can ultrasound not see past bone, air or stones?
Much greater difference in tissue density so sound is completely reflected and doesn’t reach the underlying tissues (acoustic shadowing)
What do the following terms mean:
- Hyperechoic
- Hypoechoic
- Anechoic
- Isoechoic
- Hyperechoic - more reflective (white)
- Hypoechoic - less reflective (dark grey)
- Anechoic - not reflective (black)
- Isoechoic - Same as surrounding tissue
What is a doppler ultrasound and when is it commonly used?
Moving objects influence the sound waves and be used to image flow. Blood flow can affect the ultrasound signal:
- Coming towards = increased freq.
- Moving away = decreased freq.
Used in neck doppler to detect carotid artery stenosis.
What are some advantages and disadvantages of ultrasound?
Advantages:
- Lack of radiation
- Low cost
- Portable
- Can be inserted into cavities
- Dynamic (blood flow measurment)
Disadvantage:
- Operator dependent
- No bone or gas penetration (obscures deep structures)
- Doesn’t deal with certain body types well
