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An Overview of Imaging Modalities to be Covered in this CourseRadiology1. The branch of medicine that deals with the use of radioactive substances in diagnosis and treatment of disease. 2. The use of ionizing radiation for medical diagnosis, especially the use of x-rays in medical radiography or fluoroscopy. 3. The branch of medicine devoted to the study of images obtained by x-ray, ultrasound, CT, or MRI, and to the treatment of cancer by radiation therapy. Waves and Energy RegimesElectromagnetic WavesVisible Light dermatology, gastroenterology, obstetrics, pathology, direct visible observation x-rays, γ-rays mamography, CT, MRI, nuclear medicine (PET, SPECT) Mechanical WavesUltrasound imaging, Doppler ultrasound imaging Fundamental Idea of Imaging- information must come from anatomical sites PET and SPECT vs. other imaging modalities - image quality vs. safety and comfort of the patient Image Modalities I: Mechanical WavesUltrasound ImagingUltrasound Transducer- produces pulses of vibration - detects echoes - pulse-echo imaging - tissue/air and tissue/bone interfaces are highly echoic (ultrasound not useful for thorax or brain) - very little energy deposition; not harmful (preferred imaging modality for obstetric patients) Normal Breast Image
Malignant Breast Mass
Breast with Two Cysts
Doppler Ultrasound ImagingUmbilical Cord in Fetus (used to assess blood flow to placenta and fetus)
Image Modalities II: EM WavesRadiographyX-ray Shadow Image- image can be recorded with film or digital detectors - useful for diagnoses of broken bones, lung cancer, and cardiovascular disorders Image Showing Detection of Lung Cancer
FluoroscopyX-ray Movie- rapid succession of x-ray images produced in real time - now available as an option on some linacs (Vanderbilt)
MamographyRadiograph of the Breast- typically used to screen women for breast cancer - dedicated mamography equipment using low energies, special filters, and special detectors can result in very high-quality images with low dose to the patient Mamogram Showing Malignant Tumor
Mamograms Showing Calcifications
Computed Tomography (CT)“Tomography”: Tomo = Slice (“Picture Slice”)- first imaging modality that required use of computer (1970s) - x-ray tube rotates 360o around the patient - x-rays detected by linear detector arrays opposite the emission tube - computer reconstructs image of a slab (5 mm thick) of tissue that is transverse to the direction of propagation of the x-rays - displayed anatomy not obscured by other structures - can produce Multi-Planar Reconstruction (MPR) CTs and MPR of Neck
Nuclear Medicine ImagingPrimary Differences from Previous Modalities Discussed- transmission vs. emission images - anatomical vs. functional imaging Characteristics- radioactive compound enters patient (ingestion, injection, inhalation) - compound is distributed according to patient physiology - projection image formed from emitted x- or γ-rays - produces nuclear medicine planar image Examples- thallium tends to concentrate in normal heart tissue, so diseased portion appears as “cold spot” in image - iodine tends to be absorbed by thyroid, so image can show if thyroid cancer has metastasized
Single Photon Emission Computed Tomography (SPECT)SPECT : nucl med planar image :: CT : radiographSide View of Brain
Positron Emission Tomography (PET)Characteristics- radioactive positron-emitting elements are incorporated into physiologically relevant compounds - oppositely directed annihilation photons are detected by an array of detectors surrounding the patient - detectors watch for “simultaneous” emission of two photons - computers reconstruct 3-D distribution of PET agent - shows physiology instead of anatomy PET vs. SPECT- PET is more expensive than SPECT - PET is more sensitive than SPECT - more physiologically relevant elements can be used in SPECT (C, O, F)
PET Image
Magnetic Resonance Imaging (MRI)Characteristics- uses B = 10,000 – 60,000 Bearth Bearth = 5 x 10–5 T = 0.5 G Note: B(small bar magnet) = 0.01 T = 100 G - most use NMR properties of proton - ΔE of spin-flip energy levels is proportional to Blocal - B-field gradient set up across patient - radio pulse sent into patient - “spin-echo imaging” MRI vs. CT- high-quality MRI takes about 10 min; high-quality CT takes 10 s, so CT better for pediatric patients, heart, lungs - electronic monitoring equipment cannot be used with MRI, so CT better for trauma patients MRI Images
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