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!