IB CSL
CROSS-SECTIONAL IMAGING
Dr CGC Ooi
Radiology,
Tue 27-08-02
Plain radiographs
2-dimensional
Planar image
Summation of shadows from overlapping structures
Exiting X-rays have attenuated beams - image caught on photographic film
See mainly bony skeleton (dense, absorb radiation more than ST)
AXR
- Planar image
- Able to visualise bony skeleton, outlines of organs, bowel gas
- Line from spine to iliac crest - psoas muscle
Intravenous contrast
- Opacifies the pelvicalyceal system, ureters, bladder
- Does not demonstrate detailed morphology of intra-abdominal organs
Arteriogram: opacifies arteries
- Mediastinum is obscured
- Hilar areas with arteries and veins
- On right: right and left pulmonary arteries opacified
Cadaveric cross-section
- Slice from long axis of body
Cross-section
- Definition: Drawing of a piece or slice made by cutting across e.g. tree-trunk
Cross-sectional modalities
- Ultrasound
- Computed tomography (CT)
- Magnetic Resonance Imaging (MRI)
ULTRASOUND
1960's
Principles of sonar to image body organs (bats, whales) - emit sound, hits surface, reflected back
No x-rays: main advantage
Transducer is placed on the skin
Short bursts of ultrasound waves are produced by the transducer
This penetrates the body and hits boundaries (organs)
Sound waves get reflected back when they bounce off internal structures
Transducer receives these reflected waves
Different tissues reflect sound waves differently - transformed into image by computer software (intensity and position of wave)
Transducer acts as transmitter and receiver of sound waves
In the beginning, it took a long time to build up an image - time to allow US waves to be produced and reflected back
Now the ultrasound probe can produce images in "real-time (high frequency)
Passes through fluid easily with little returning echoes
Passes through soft tissue organs
- Water / fluid: black (reverberation)
- Bone: white
- Grey: partial passage of US
- Can't pass through bone or calcified structures - all echoes returned
- Cannot penetrate gas well - a lot of returning echoes (eg. SI)
Colour Doppler US
- Images moving red blood cells within vessels
- Colour codes the vessels according direction of flow
- "Red" when blood flows towards the transducer and "blue" when blood flows away from it
- Doppler shift effect: when US hits moving cell, there is a perceived change in frequency - this is augmented to form image
- Blood vessels in the kidneys, liver, brain, masses -> eg. Kidney and liver graft: fusion, ejection, stenosis - measure BF through small arteries, resistance index, acceleration time
- Carotid arteries, limb vessels (esp. veins)
- Augmentation: squeeze bottom of foot, pushes blood through, shows that vein is clear (if clot present, squeezing doesn't cause augmentation)
- Note: venograms formerly used to look at veins (plebitis, thrombosis) - now use Doppler US + Doppler spectral tracing
Foetal imaging
- Foetal development (DM, IVDU mother, MR babies)
- Demonstrates bodily functions e.g. breathing, urination, sucking and movement
- Foetal anomalies
- Dates pregnancies: from wk 5 onwards
- Image complications of pregnancy - haemorrhages, foetal distress
Abdominal imaging
- Medulla appears slightly darker on US
- Texture, contour, size, pelvicalcyceal system
- Gallbladder with stones
- "Acoustic shadowing" behind the stones when US waves cannot pass through
US guided biopsy
User friendly
Can use guide or free-hand
Need good 3D spatial resolution
US guided drainage
Drain put into liver
US Summary
- Foetal imaging
- Reproductive organs, liver, kidneys, pancreas, gallbladder, heart
- Eg. Vaginal probe: ectopic pregnancy
- Blood vessels (colour Doppler)
- Superficial structures such as thyroid, breast, eye
- Differentiating solid from fluid structures
- Guiding biopsy or interventional procedures
US Advantages
- Non-ionising
- Quick
- Cheap
- Easy to perform
US Disadvantages
- Cannot penetrate air, fat or bone well
- Cannot image lung (unless consolidation or effusion)
- Operator dependent
- Small region of interest cf. CT scan
COMPUTED TOMOGRAPHY
1970's - Godfrey Hounsfield
Data presented in true cross-section
Narrow beam of x-rays "scans" a section of the patient by rotation of the x-ray tube 360 degrees around the patient
'tomo' = slice / section
'graphy' = describing
The x-rays exiting from the patient's body contains a profile of densities or attenuation
With conventional x-rays, these x-ray profile is registered on film
With CT it is measured by detectors
Each time the x-ray tube and detectors make a 360 degree rotation, a "slice" is acquired
The "slice" is focused to a thickness (1mm - 10mm) using lead shutters in front of the x- ray tube and detectors
Instead of plain SXR, brain can be imaged
Replicates morphology seen at cadaveric resection
Cross sectional display without overlapping shadows
Give contrast in thorax to show BV
Depicts anatomical relationship of normal and abnormal structures
Evaluates soft tissue, bones and lungs in one examination
Computer stores all recorded densities
"Hounsfield units" (HU)
- Water arbitrarily taken as "0
- Eye can only appreciate only 20 shades of greyscale
- ST = 30-50 HU
- Bone > 100 HU
- Fat = 10 to -80
- Air = 100's
- Displays a selected range of densities (window width, WW)
Around a selected density level (window level, WL)
- Mediastinal window
® WL, WW: 50/250 HU
Lung window ® WL,WW: -700/1000 HU ® BV white; Bronchus black
Soft tissue window ® WL,WW: 50/250 HU ® Similar window to mediastinal window
Bone window ® WL,WW: -700/1000 HU
Windows are set by the CT scanner
3D-CT images
Applications
- Further evaluation of abnormal plain film findings (CXR, AXR)
- Further evaluation of abnormal findings O/E
- Cancer staging (also use PET scan)
- Post-therapy monitoring
- CT-guided biopsies or drainage
Further Evaluation of Plain File
Simple renal cyst
- Homogeneous
- Well-defined
- Thin walled
- Hypodense (9HU) - near water
- No enhancement after contrast
- Therefore not tumour, hydronephrosis
- Bowel is not displaced much, therefore must be retroperitoneal (kidney)
Cancer Staging
® Inoperable
Operable lung cancer ® No mediastinal LN's
Treatment Response
- Post chemotherapy:
- Partial response (>50%)
CT-Guided Procedures
CT guided biopsy
2D Reformation / Reconstruction
- Two-dimensional reformation to show more clearly the spatial relationship between normal and abnormal structures
- Upper pole cyst of kidney
- Line through sagittal section (side-side)
- Note: coronal: back to front
MAGNETIC RESONANCE IMAGING
Research on NMR by Block and Purcell, Nobel prize 1952
1980's - commercially available scanner
Uses magnetic field and radiowaves to create CX image
No x-rays
Multiplanar
Similar to CT scanner, but tunnel much deeper
Based on study of hydrogen protons / atoms
A particle with a spin can be likened to a small bar magnet with a north and south pole (a moving charge behaves like a magnet - eg. Proton in the body)
If you apply an external field, the protons align themselves - rotate and precess (wobble)
Magnet is the main component exerting a M field 3-5 Tesla (30,000x > gravity) -
Magnetic field excites protons in the body to align around the direction of M field and to precess at a pre-requisite rate
When a patient enters cylindrical magnet a RF signal is turned on and off
RF energy is absorbed by different protons in the body
This causes a 90º change in their spin orientation (protons move to a higher energy state)
Different protons have different reactions to this
When RF is turned off, there is realignment of the spin orientation to that of the main M field (protons try to move back to original position and start wobbling again - this causes release of energy (RF signal emitted)
Converted into image by digital computer
Multiplanar due to coils of wire within the bore of the magnet - create gradients of magnetic field in 3 main directions:
- Z gradient (axis) - axial
- X gradient (axis) - sagittal
- Y gradient (axis) - coronal
A typical examination consists of a series of 2-6 sequences (each 2-15 minutes)
Multiplanar
1 - coronal plane
2 - sagittal plane
3 - axial plane
Great advantage over CT
MR coils
Many different coils
Head, ankle, abdomen etc
Multiplanar Imaging
Axial neck: left to right
Sagittal spine: T2-weighted, fluid and CSF is bright
Axial brain: T1-weighted, water is black, fat is bright
- MR gives excellent contrast discrimination
- 2 main extremes of contrast in MR imaging is fat and water
- "Signal Intensities" of tissue
- T1-weighted image: fat appears bright and water dark
- T2-weighted image: water is bright and fat not as bright (intermediate signal)
- Different types of water: oedema, inflammation, IC water, EC water etc.
- Gadolinium given for contrast: changes magnetic field
MRI Summary
- Excellent contrast resolution makes it suitable to image soft tissue tumours particularly in the CNS (spine) and musculoskeletal system
- Increasingly used for liver and abdomen
- Pelvis - reproductive organs (non-ionising)
- MR angiography - non-invasive
- Cannot image lung and bone cortex
® lung consists of air \ no protons
Advantages
- Non-ionising
- Contrast resolution
- Multiplanar
- Non-invasive
Disadvantages
- Relatively long scan time
- CI: ferro-magnetic implants, cochlear implants, pacing wire
- Ferro-magnetic: eg. Hip replacement before titanium - heats up and moves
- Aneurysm clip will unclip
- Life-support systems incompatible with MF
- Expensive
- ?Long-term safety for foetus (not in 1st trimester; 2-3rd trimester OK)
- No allowed near machine: watches, credit cards, oxygen tanks (squashed someone in USA last year)
CONCLUSION
Cross-sectional imaging is often required for further evaluation of lesions found on plain films or on examination
Choice of US, CT and MR (complimentary)
US and MR are non-ionising - children (MR: sedate child)
US limited largely by air and bone but is most available and most suitable for children
CT is quick, and can image most body parts (but with radiation)
MR, with best contrast resolution and multiplanar, is most suited for CNS and MS lesions. In other situations, it should be used as problem solving tool (expensive: technician time, film, machine maintenance)