Loading ...
Global Do...
News & Politics
7
0
Try Now
Log In
Pricing
1 CROSS-SECTIONAL IMAGING Department of Diagnostic Radiology, The University of Hong Kong • 2-dimensional • Planar image • Summation of shadows from overlapping structures Plain radiographs • Planar image • Able to visualize bony skeleton, outlines of organs, bowel gas AXR K L P Intravenous contrast • Opacifies the pelvicalyceal system, ureters, bladder Does not demonstrate detailed morphology of intra-abdominal organs Arteriogram: opacifies arteries Mediastinum is obscured Cadaveric cross-section 2 Cross-sectional modalities • Ultrasound • Computed tomography (CT) • Magnetic Resonance Imaging (MRI) RT diaphragm IVC ULTRASOUND 1960’s • Principles of sonar to image body organs • No x-rays ULTRASOUND • 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 onto internal structures •Transducer receives these reflected waves ULTRASOUND •Different tissues reflect sound waves differently - transformed into image by computer software ULTRASOUND • 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” ULTRASOUND 1960’s 2000 3 ULTRASOUND •Passes through fluid easily with little returning echoes •Passes through soft tissue organs Thyroid gland with simple cyst ULTRASOUND •Can’t pass through bone or calcified structures – all echoes returned •Cannot penetrate gas well - a lot of returning echoes Ca2+ tracheal rings thyroid Liver Kidney ULTRASOUND • Images moving red blood cells within vessels • Colour codes the vessels according to direction of flow • “Red” when blood flows towards the transducer and “blue” when blood flows away from it Colour doppler •Blood vessels in the kidneys, liver, brain, masses •Carotid arteries, limb vessels DOPPLER ULTRASOUND Fetal imaging ULTRASOUND 1960’s 2000 • Dates pregnacies • Fetal development • Demonstrates bodily functions e.g. breathing, urination and movement • Fetal anomalies • Image complications of pregnancy - haemorrhages, fetal distress FETAL IMAGING 4 ULTRASOUND ULTRASOUND GB Common bile duct Portal vein ULTRASOUND Transverse Ao IVC pancreas S. Bowel Pancreas ULTRASOUND Diaphragm Liver Gallbladder with stones “acoustic shadowing” behind the stones when US waves cannot pass through ULTRASOUND US guided biopsy US guided drainage ULTRASOUND 5 • Fetal imaging • Reproductive organs, liver, kidneys, pancreas, gallbladder, heart • Blood vessels (colour doppler) • Superficial structures such as thyroid, breast, eye • Differentiating solid from fluid structures • Guiding biopsy or interventional procedures ULTRASOUND ULTRASOUND Advantages • Non-ionising • Quick • Cheap • Easy to perform Disadvantages • Cannot penetrate air, fat or bone well • Operator dependent • Small region of interest C/W CT scan COMPUTED TOMOGRAPHY • 1970’s - Godfrey Hounsfield • Data presented in true cross-section COMPUTED TOMOGRAPHY Narrow beam of x-rays “scans” a section of the patient by rotation of the x-ray tube around the patient Computed tomography slice/section describing COMPUTED TOMOGRAPHY • 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 Detectors COMPUTED TOMOGRAPHY 6 COMPUTED TOMOGRAPHY • Each time the x-ray tube and detectors make a 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 PUTED TOMOGRAPHY 1st generation CT scanner • A narrow X-ray beam passes through the patient, and is detected by detector placed opposite • Both source and detector scan across the patient in linear transverse fashion • System is rotated by 10 and linear motion repeated • Similar process repeated until 1800 is completed 2nd generation CT scanner • Instead of pencil beam, a fan beam of X- rays was used • Similar linear-rotate movement 2nd generation scanners 1st and 2nd generation scanners • Linear – rotate principle. Long scan time • Since x-ray source moves around the patient, the body tissues are scanned form multiple angles and directions • The energy absorbed from these multiple projections are digitized and synthesized using a mathematical algorithm to generate an image 3rd generation CT scanner • Modification to the original system • There is a ring of detectors placed around the patient • The x-ray tube rotates around the patient in a 3600 and are absorbed by the ring of detectors • Circular motion instead of linear motion (rotation) • These older scanner gathered information in discrete slices i.e. patient is scanned 1 slice at a time • In 1990’s helical CT became possible through the use of slit rings to allow the x-ray tube to continue scanning in a circular way while the patient is fed into the CT gantry • This resulted in acquisition of a continuous volume of data without spatial or temporal gaps Helical (spiral) CT 7 Helical (spiral) CT The term spiral or helical describes the path the x-ray tube takes as it continuously scans the patient Helical (spiral) CT • Faster than old scanners • Acquisition of volumetric data • Able to scan in 1 or 2 breath-holds Multi detector CT • Instead of 1 row of detectors, there are 4, 16, 32, 64 …. rows Multi detector CT • Reduces scan time • Retrospectively reconstruct dataset into thinner or thicker sections • Increases spatial resolution Cardiac imaging, lung imaging, whole body scans, 3D reconstructions Instead of plain SXR The brain can be imaged 1975 2000 COMPUTED TOMOGRAPHY 8 2 1 4 3 COMPUTED TOMOGRAPHY Replicates morphology seen at cadaveric resection Aortic arch Trachea Right vagus nerve Costal cartilage Manubrium COMPUTED TOMOGRAPHY Cross sectional display without overlapping shadows T T T T COMPUTED TOMOGRAPHY Middle-aged woman with difficulty breathing and stridor SVC Lt BCV T vertebra COMPUTED TOMOGRAPHY COMPUTED TOMOGRAPHY •Depicts anatomical relationship of normal and abnormal structures •Evaluates soft tissue, bones and lungs in one examination COMPUTED TOMOGRAPHY 100 50 30 0 -10 -100 soft tissue water Bone, Ca2+ } fat air HU 9 • Displays a selected range of densities (window width, WW) • Around a selected density level (window level, WL) 100 80 30 0 -10 -100 soft tissue water Bone, Ca2+ } fat air COMPUTED TOMOGRAPHY 150 100 50 0 -50 -100 Mediastinal window Window Level at 50 HU Window Width of 250 HU 175 75 200 0 -200 -400 -600 -800 -1000 Lung window Window Level at -700 HU Window Width of 1000 HU -1200 COMPUTED TOMOGRAPHY Mediastinal window Lung window WL,WW: 50/250 HU WL,WW: -700/1000 HU COMPUTED TOMOGRAPHY Soft tissue window Bone window WL,WW: 50/250 HU WL,WW: 300/2000 HU COMPUTED TOMOGRAPHY 3D-CT images 10 Applications • Further evaluation of abnormal plain film findings (CXR, AXR) • Further evaluation of abnormal findings O/E • Cancer staging • Post-therapy monitoring • CT-guided biopsies or drainages COMPUTED TOMOGRAPHY • Homogeneous • Well-defined • Thin walled • Hypodense (9HU) • No enhancement FURTHER EVALUATION OF PLAIN FILM Simple renal cyst C/O: abdominal distension and weight loss O/E: enlarged liver CT- liver mass in right lobe of liver Biopsy - Hepatocellular carcinoma FURTHER EVALUATION T Stage IIIb lung cancer Inoperable CANCER STAGING Operable lung cancer CANCER STAGING T Post chemotherapy: partial response (>50%) T TREATMENT RESPONSE 11 Post-chemotherapy U Before chemotherapy TREATMENT RESPONSE CT guided biopsy CT-GUIDED PROCEDURES 2 DIMENSIONAL REFORMATION Two-dimensional reformation to show more clearly the spatial relationship between normal and abnormal structures MAGNETIC RESONANCE IMAGING • 1980’s - commercially available scanner • Uses magnetic field and radiowaves to create CX image • No x-rays • Multiplanar Research on MR by Block and Purcell, Nobel prize 1952 A particle with a spin can be likened to a small bar magnet with a north and south pole 12 The protons align themselves in the magnetic field and also precess • Magnet is the main component exerting a M field 0.5-3 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 protons more to higher energy state • RF energy is absorbed by different protons in the body • This causes a 90º change in their spin orientation •When RF is turned off, there is realignment of the spin orientation to that of the M field •This change in orientation causes a RF signal to be emitted •Converted into image by digital computer RF pulse off, protons return to resting state transmitting RF pulse A typical examination consists of a series of 2-6 sequences (each 2-15 minutes) MAGNETIC RESONANCE IMAGING 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 1 - coronal plane 2 - sagittal plane 3 - axial plane “Multiplanar” Great advantage over CT MAGNETIC RESONANCE IMAGING 13 MR coils MULTIPLANAR IMAGING Axial neck Sagittal spine Axial brain Coronal brain MULTIPLANAR IMAGING MAGNETIC RESONANCE IMAGING • 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) Able to characterize lesions better than CT or US T1-weighted T2-weighted MAGNETIC RESONANCE IMAGING MAGNETIC RESONANCE IMAGING Axial plane 14 Coronal Sagittal MAGNETIC RESONANCE IMAGING SOFT TISSUE PATHOLOGY MR angiogram Arteriogram MAGNETIC RESONANCE IMAGING Young lady with recurrent haemoptysis P. Art P. Vein Magnetic Resonance Angiogram Arteriovenous malformation seen as a nidus with a large feeding artery and draining vein MAGNETIC RESONANCE IMAGING • 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 15 Cannot image lung and bone cortex MAGNETIC RESONANCE IMAGING MAGNETIC RESONANCE IMAGING Advantages Disadvantages • Non-ionising • Contrast resolution • Multiplanar • Non-invasive • Relatively long scan time • CI: ferro-magnetic implants, cochlear implants, pacing wire, • Life-support systems incompatible with MF • Expensive ?Longterm safety for fetus 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 • US and MR are non-ionising - children CONCLUSION • 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 • 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 Drawing of a piece of slice made by cutting across e.g. tree-trunk Cross-section Definition
