introduction to medical imaging oksana h. baltarowich, md professor radiology department of...
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INTRODUCTION to MEDICAL IMAGINGOksana H. Baltarowich, MD
Professor Radiology
Department of Radiology
Jefferson Medical College
NO DISCLOSURES
Objectives
• List the main diagnostic imaging modalities used in medical practice
• Explain the basic principles of the main types of imaging modalities
• Describe advantages of the different imaging techniques
• List disadvantages of the different imaging techniques
Medical Imaging
X-Rays: ionizing radiationRadiographyCT scan (Computed Tomography)
Gamma raysNuclear Medicine
Sound wavesUltrasound
Magnetic fields/radiofrequency wavesMRI (Magnetic Resonance Imaging)
Who Does Imaging?
• Radiologist– Consultant: Diagnostic, Subspecialties– Interventionalist
• Radiation Oncologist: Treatment planning• Cardiologist: Invasive, Non-Invasive• Vascular Surgeon: Endovascular procedures• Other specialists
– Usually for procedure guidance (Ob-Gyn, Internal medicine, Orthopedics, etc.)
Training
• Radiologist– Mandatory clinical internship– 4 years general radiology residency– Physics, radiation protection, radiobiology,
technology, diagnosis, anatomy, pathology, physiology, etc.
– 1 year fellowship in subspecialty
• Cardiologist, vascular surgeon, others– intergrated into training program
Radiography
X-rays
X-rays
• Dr. Wilhelm C. Roentgen at University of Wurzburg, 1895– Discovered and
named X-rays• Awarded first Nobel
Prize for Physics in 1901
X-Rays• Radiography
– Plain film radiography - without added contrast material
– Contrast radiography – with contrast material
– Computed Radiography (CR)– Fluoroscopy: done in real time
• Barium studies: Upper GI, BE• Angiography
• CT (Computed Tomography), aka CAT scans (Computer Assisted Tomography)
Radiographs• Electromagnetic waves (X-rays) are produced in an
X-ray tube by converting electrical energy into an electromagnetic wave• Electrons are accelerated from an electrically
negative cathode to a positive target anode• Energy is released and converted into heat and
X-rays
Radiographs• Images produced by electromagnetic waves
(X-rays)• produced by an X-ray tube• pass thru the body• are absorbed by the different tissues• reach the film and • expose the film
Radiographic cassette
Computed Radiography (CR)
• Produces digital radiographic images• Instead of film, a phosphor plate is exposed
to X-rays• Laser beam scans the plate• Light is released, intensified and converted to
electron stream• Converted by computer into digital image• Viewed on a monitor• Transferred over networks
Radiographic Densities
• Air Black• Fat Dark
gray• Water* Light
gray• Bone White
• Calcium White• Metal Very white*Water=soft tissue: organs, muscles, blood vessels, masses
Radiodensity
Radiodensity is a function of:
1. Composition (atomic number)
2. Thickness of object
3. Strength of X-ray
Radiodensity as function of thickness of object
Radiolucencyis the opposite
Radiodensity• If an object is thick and dense, less radiation
passes thru to reach the film– Radiodense– Film is underexposed and stays light
• Air gives no obstruction to X-rays – Radiolucent– Film gets overexposed and turns black
• Bone absorbs radiation, less radiation reaches the film– Film is underexposed and stays white
Abdominal Radiograph: KUB
• Air Black• Fat Dark
gray• Water Light
gray• Bone White
• Calcium White• Metal Very white
*Water=soft tissue: organs, muscles, blood vessels, masses
Radiographs
Composed of overlapping radiodensities, overlapping shadows
Tissues stacked in front of each other
Need 90 degree projection for anatomic placement
RadiographsPerpendicular projections are necessary to
localize structures in the body
Roof fell on patient
Patient Position for Chest X-Ray
Posteroanterior (PA) Chest: X-ray beam passes from posterior to anterior
RT LT
Erect Position
Free intraabdominal air: Pneumoperitoneum
Portable Films
• Patient is too ill to go to Radiology Department
• Less optimal• Portable X-ray unit • X-ray film is behind
patient • X-rays pass through
patient from anterior to posterior
Abdominal Radiographs
• Erect/upright• Supine• Prone• Decubitus• Oblique/rotated
Contrast Radiography• Injection, ingestion or placement of radiopaque material into the
body for contrast enhancement• Oral, rectal contrast: Barium, gastrograffin
Contrast RadiographyAngiography
Caution: Radiation Exposure
• Radiation workers follow safety guidelines
• Women of child-bearing age should be questioned about possibility of pregnancy before abdominal X-ray.
• Ask about LMP and check pregnancy test, if in doubt.
Caution: Pregnancy
Computed TomographyX-rays
Computed Tomography• Ionizing radiation used to obtain cross-sectional
images of the body• Table moves through large donut-shaped scanner
– Fast moving X-ray tube (thin X-ray beam rotates)– Numerous electronic detectors
• Rapid acquisition of images• Contrast agents necessary for most scans
– Oral– Intravenous iodinated
Computed Tomography• Multidetector CT scans
– Advancement from tomographic imaging (slices) to volume imaging
– Produce a volume of data that can be manipulated– Reconstruct at 1-10 mm increments– Axial, sagittal, coronal, 3D reconstructions
Computed TomographyCoronal and sagittal reconstructions
Computed Tomography
• Large field of view– Entire cross-section of body
• Improved differentiation of soft tissue densities• Excellent spatial resolution
– Small 3-4 mm lymph nodes, vessels• Automated scanners
– Less operator-dependent• Ultra-fast scanners
– Suspended respiration– Less bowel motion artifact– Trauma
ADVANTAGES
CT Density Differentiation
CT of the Female Pelvis
• Exposure to ionizing radiation• Allergic reactions to intravenous contrast
– Mild to severe (anaphylaxis) • Contrast nephropathy
– May cause renal failure– Caution in diabetics with nephropathy
• Problems of dehydration– Cautious use in multiple myeloma, sickle cell
disease • Soft tissue differentiation not as good as MRI
DISADVANTAGES
CT WindowsWindowing displays the image in differing shades of grayCorrespond to brightness and contrast
Soft tissue window Lung (air) window
CT Windows
Soft tissue window Bone window
Windowing displays the image in differing shades of grayCorrespond to brightness and contrast
CT Diagnosis• Trauma
Shattered spleen Fractured sacrum
CT Diagnosis• Pulmonary embolism• Intravenous contrast material is necessary
CT Diagnosis• Angiography• Bowel disorders
Aortic aneurysm Perforated sigmoid diverticulitis
CT 3D Volume Rendering
Aorta, plaque Aortic arch
CT Angiogram
Diagnostic Ultrasound
Sound Waves
Diagnostic Ultrasound• Audible sound 20Hz-20KHz• Ultrasound >20Hz• Medical ultrasound 1-20
MHz• Transducer sends high
frequency sound waves into the body and gathers their reflections (echoes)
• Converts echoes into electronic signals
• Displays images on a monitor
Diagnostic Ultrasound• Sound travels very well through fluid• Ultrasound is good for anything containing
fluid, as long as there is no interference for the sound beam to reach the fluid
Diagnostic Ultrasound• Interferences to sound waves: air, bone,
metal, thick tissues, deep structures
Diagnostic Ultrasound
• Multiplanar imaging in real-time• Non-invasive, safe, no radiation
– Pregnancy, Pediatrics• Relatively inexpensive• Widely available• Portable, bedside• Good contrast of tissue
layers in many organs
ADVANTAGES
Diagnostic Ultrasound
• Good contrast of tissues layers
ADVANTAGES
Pelvic Ultrasound versus CT
Ultrasound
CT
Ovarian Neoplasm
Diagnostic Ultrasound
• Relatively small field of view• Operator dependent: inconsistent reproducibility • Depends on sound penetration
– Air, bone, obesity
DISADVANTAGES
Diagnostic Ultrasound
• Trade-off between depth (beam penetration) and resolution– For deeper penetration need lower frequency
transducers, resulting in lower resolution
DISADVANTAGES
3.5 MHz 12 MHz
Doppler Ultrasound• Evaluation of blood flow
– Patency, direction, character of flow in vessels– Vascularity in a mass
Carotid artery Testicular flow
FD = (FR - FT ) = 2 • FT • v • cos
c
v
FT
FR
The Doppler EquationThe relationship of Doppler frequency shift
to velocity of a moving object
C =1540m/sec speed of sound in tissues
3D Ultrasound
1st Trimester fetus 3rd Trimester fetus
Magnetic Resonance ImagingMagnetic fields
Radiofrequency waves
Magnetic Resonance Imaging
• Patient placed inside a large cylinder-shaped magnet
• Radio waves 10,000 - 30,000 stronger the earth’s magnetic field are sent thru body
• Nuclei of body’s (hydrogen) atoms shift position
• As they move back they send out radio waves
• Scanner detects the signals• Creates image based on
location & strength of signals
Magnetic Resonance Imaging
• Large field of view– Cross-section of entire body– Volume imaging (from tomographic imaging,
slices)• No ionizing radiation as with CT • Not as operator dependent as ultrasound• Much fewer contrast allergies and less risk of
contrast nephropathy than with iodinated agent used in CT
• Excellent contrast resolution among tissue layers, esp. fat, hemorrhage
ADVANTAGES
Magnetic Resonance ImagingLarge field of view
Excellent Contrast Resolution
Ultrasound vs CT vs MRIField of View, Excellent Contrast Resolution
Ultrasound
MRI CT
Magnetic Resonance Imaging• Exquisite neuroanatomical detail• Musculoskeletal disorders• Cardiovascular
Magnetic Resonance Imaging
• Motion related artifacts– Bowel peristalsis– Respiratory motion
• Cost $$$• Image production is time-consuming with
complicated protocols & scan time• Intravenous contrast is often required to
improve tissue differentiation• Claustrophobia• Obesity
DISADVANTAGES
Magnetic Resonance Imaging
• Contraindication in patients with ferromagnetic metallic objects, implants, foreign bodies:– Metallic fragments in eye– Cochlear implants– Cardiac pacemakers– Brain aneurysm clips– Certain heart valves– Neurological stimulators
• Orthopedic devices are not harmful to patient, but create artifacts
• Intrauterine devices are safe
METALLIC OBJECTS
Nuclear ImagingGamma waves
Nuclear Medicine• Uses small amounts of radioactive material
for diagnosis and treatment • Molecular Imaging: images reflect biological
processes that take place at the cellular and subcellular levels
• Evaluates physiological function rather than anatomic structure
• Uses radiopharmaceuticals: agents that have trace amounts of radioactive atoms attached
• Radiation emitted from the patient is imaged by a gamma camera, SPECT or PET scanner
Nuclear Medicine Applications
• Oncology: tumor localization, staging, metastases
• Cardiology: myocardial perfusion scans• Gastrointestinal: acute cholecystitis, biliary
tract, GI bleeding• Pulmonary: ventilation, perfusion• Infectious disease: localize infections (subtle)• Therapy, e.g. I-131 for thyrotoxicosis, thyroid
cancer
Common Radionuclides
• Intravenous administration– Technetium-99m– Iodine-123 and 131– Thallium-210– Gallium-67– Fluorine-18 Fluorodeoxyglucose– Indium-111 labeled leukocytes
• Inhaled gaseous/aerosol radionuclides– Xenon-133– Krypton-81m– Technicium-99m gas
Gamma Camera
Bone Scan: Metastases
PET Scan: Cancer Detection
• Positron Emission Tomography
• 18F-fluorodeoxyglucose (FDG)
• Primarily used for diagnosis, staging & monitoring of cancers: lung, breast, cervical, colorectal, esophagus, head & neck, lymphoma, melanoma
Lymphoma pre and post chemotherapy
PET-CT ScannerCombined PET scanner and CT scanner
PET-CT• Imaging by “fusion” of anatomy and physiology• Superimposition of the anatomic images of a CT scan
and the co-registration of physiologic uptake of a radionuclide agent (molecular imaging)
PET-CT ScansFusion imaging: Lingular mass
CT PET PET-CT
Summary
• Medical imaging is essential for medical practice
• Interpretation of various imaging modalities requires training and experience
• Radiologist specializes in various imaging modalities– Consultant– Interventionalist
Sample Question
Of the following imaging modalities, which test has the least harmful effects for a fetus?
a. CT
b. Ultrasound
c. MRI
d. Nuclear medicine
Thank you for your attention!
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