Lecture 1

An Overview of Imaging Modalities to be Covered in this Course


      1. 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 Regimes

       Electromagnetic Waves

        Visible Light

          dermatology, gastroenterology, obstetrics, pathology, direct visible observation

        x-rays, γ-rays

          mamography, CT, MRI, nuclear medicine (PET, SPECT)

      Mechanical Waves

        Ultrasound 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 Waves

    Ultrasound Imaging

    Ultrasound 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

Normal Breat Tissue


    Malignant Breast Mass

Malignant Breast Mass


    Breast with Two Cysts

Breast with Two Cysts


    Doppler Ultrasound Imaging

    Umbilical Cord in Fetus (used to assess blood flow to placenta and fetus)

Umbilical Cord


Image Modalities II: EM Waves


    X-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

Lung Cancer



    X-ray Movie

      - rapid succession of x-ray images produced in real time

      - now available as an option on some linacs (Vanderbilt)



    Radiograph 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

Mamography (Wikipedia)


    Mamograms Showing Calcifications

Breast 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

Neck CT


    Nuclear Medicine Imaging

    Primary Differences from Previous Modalities Discussed

      - transmission vs. emission images

      - anatomical vs. functional imaging


      - 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


      - 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 : radiograph

    Side View of Brain

Side View of Brain


    Positron Emission Tomography (PET)


      - 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

Brain Scan


    Magnetic Resonance Imaging (MRI)


      - 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

MRI Image