diagnostic imaging primer
DESCRIPTION
Diagnostic Imaging Primer. 1 Hour (brief) introduction Sean Collins Fall 2012. Outline. Purpose of primer & thread Objectives of primer Underlying message General Principles & Plain films Computed Tomography Intro Magnetic Resonance Intro. Purpose of primer & thread. - PowerPoint PPT PresentationTRANSCRIPT
Diagnostic Imaging Primer1 Hour (brief) introduction
Sean CollinsFall 2012
Outline
• Purpose of primer & thread• Objectives of primer• Underlying message• General Principles & Plain films• Computed Tomography Intro• Magnetic Resonance Intro
Purpose of primer & thread• Primer – plant a seed of understanding of
diagnostic imaging that will grow throughout many additional DPT courses during your three years in the program
• Thread – To meet practice expectations regarding the integration of diagnostic imaging into physical therapy practice
Purpose of primer & thread• There are many threads throughout your DPT
education. Everything you learn about examination, evaluation and intervention is technically a thread through the curriculum (MMT, ROM, Endurance, Functional mobility)
• What makes Diagnostic Imaging different?– Increased use in practice is relatively new
• Response to increased availability & ease of communication – Inclusion into PT education is therefore relatively new– No single course in the curriculum “owns” the
material (neither do we have a course on MMT)
Objectives of primer• Explain the underlying process of diagnostic imaging by
x-rays, CT scan, MRI– How do these technologies create an image– What leads to “lightness” or “darkness” in the image
• Understand visually the transformation of three-dimensional anatomy into two-dimensional imaging anatomy (Carried over into Anatomy & Neuroanatomy course)
• Define basic terms and describe basic procedures of covered diagnostic imaging methods
• Explain sources of variation in diagnostic images (if presented with two images – explain how they are different and propose why)
Underlying message (1)Variation in images is obvious for:
Different anatomical sitesDifferent angles / planes of view
Variation in images is also caused by:1. Method of imaging – x-rays vs. computer modified images vs. proton signals2. Interaction of method of imaging & different tissues
You are looking at a 3d structure in 2d – even if there is a 3d reconstruction – your film or screen is only 2d
General Principles & Plain films• Radiation – energy transmitted through space of matter• Higher energy (x-ray, gamma ray) ionize atoms in matter
– Ionization can disrupt life processes• Diagnostic radiography uses short wavelength ionizing
electromagnetic radiation (therapeutic radiation uses shorter wavelengths that overlap with gamma rays)
Plain film process• Collimator controls size & shape of x-ray beam• X-ray beam passes through patient and undergoes
attenuation• Attenuation is a reduction in # of x-ray photons in the
beam due to interaction with matter and lose of energy through either scattering or photo-electric absorption
• Remnant radiation emerges from patient & contains an aerial image of patient
• Remnant radiation is captured by an image receptor• Captured image is “latent” until processed
Plain film process
Plain film / screen radiograph
1. Air (gas)2. Fat3. Water
(muscle & soft tissue)
4. Bone
Scatter of the beam will result in lower contrast
Biederman, 2006
Radiodensity impacted by thickness despite no change in actual density
Need 2 films – perpendicular to one another to gather accurate information
AP ViewViewed as if
standing in front in anatomical position
Markers:R – rightL – leftINT – int rota.EXT ext rotaWTB –
standingDECUB –
recumbantINSP, EXP
Biederman, 2006
Biederman, 2006
Contrast Enhanced• Contrast enhanced – a contrast medium is
injected or ingested– Improves visualization by increasing contrast
in areas with minimal inherence contrast– Can be radiopaque or radiolucent or dual– Angiography, mylography (myelogram)
Nuclear Imaging• Based on physiological or functional
changes (usually activity)• Radionuclide that emits gamma rays• Gamma rays are detected by gamma
camera that transforms into image• Static images, Whole body images,
Dynamic images, Positron emission tomography (PET)
Computed Tomography Intro• CT uses x-rays• Same radio densities as plain films (but
not as impacted by other tissues)• Difference:
– CT creates images based on cross-sectional slices created by up to 1000 projections from different angles
– Tighter field of view via collimators that determine slice thickness
CT Scan Types
3D CT• Can be rotated “in space”
on the computer screen – multiplanar reconstruction (MPR)
• These images are not adequately viewed in the printed format
CT Scan TypesCT Myelogram
• Myelogram is most commonly performed with CT (as opposed to conventional radiographs)
• Reminder – the injection increases radiolucency or radioopacity of structures
CT myelogram at C4-C5 – injection allows radioopacity of spinal canal
CT Scan – Selective Windowing• Windowing refers to the
range of radio densities emphasized in the image
• Bone Window (top)• Soft tissue – allows
reader to distinguish between muscles and the fat between them– 1. Glut Medius– 2. Glut Maximus– 3. Fat between
CT Scan Imaging Artifacts• Hardening: as photons in the x-ray beam pass
through structures such as the skull the beam becomes “harder” because they are absorbed more readily. Leads to dark bands in the image between radiopaque areas
• Metals: lead to streaking that can present as bright lines in the image extending radially from the metal
• Motion: movements can lead to shading or streaking. Faster scan times reduce the prevalence of motion artifacts
CT Scan Pros & Cons
Best at:1. Subtle or complex fractures2. Degenerative changes3. First in serious trauma4. Spinal stenosis5. Loose bodies in joints
Less time & expense than MRIAccurate measure in any planeLess claustrophobia
Limited in use for soft tissues due to reliance on radio density
Relatively high radiation exposure
Magnetic Resonance Intro• Based on energy emitted from hydrogen nuclei
(protons) following their stimulation by radiofrequency (RF) waves
• Energy emitted varies according to tissue characteristics
• Therefore, MRI can distinguish between different tissues
• No “radio density” now – Signal Intensity “SI”– Greater SI is brighter; less SI is dark
Magnetic Resonance Phenomenon
• MR is process by which nuclei, aligned in a magnetic field, absorb and release energy
• While many molecules display MR, for all practical purposes MRI is based on signals from hydrogen in water molecules
• Since hydrogen consists of 1 proton – the hydrogen nucleus is referred to as simply the proton in the context of MRI
MR Phenomenon• First protons are aligned by a strong magnetic field – either in
the direction of the field, or the opposite direction• There are slight differences between those in direction and
opposite which results in longitudinal magnetization• A pulse of RF waves is applied at right angles to longitudinal
magnetization• The pulse alters the alignment to a transverse plane, and the
energy absorbed in the process brings them to a higher energy state: transverse magnetization
• As the protons realign energy is released – this induces a current that gives rise to the data for creating the MRI
1. Aligned in magnetic field (longitudinal)2. RF wave
3. Altered alignment (transverse, E increased)4. Gradually return to alignment (E release)
T1 & T2 Phenomenon• T1 & T2 are different processes related to the return of the
alignment to the main magnetic field• T1 – time it takes for protons to gain longitudinal magnetization
(T1 Recovery)• T2 –protons lose their transverse magnetization (T2 Decay)Two sides of same coin – but different processes
MRI uses this to create different images that feature different tissues based on the protons response to the RF wave
TR = time to repetition (time to repeat RF wave)TE = time to echo (time at which the signal is captured)
T1 Recovery• Protons lose energy to
surrounding molecules• Time of return differs for
different tissues• Faster recovery (shorter
times – short T1) results in stronger signals from the protons of that tissue
T2 Decay• Transverse
magnetization decays because of a loss of phase coherence, owing to interaction between protons
• Slower decay – stronger the signal recorded at end of the process
T1 & T2 Weighted Imaging
T1 Weighted• Short TR and TE• Signal caught early
when difference in relax characteristics for fat has higher SI
• Good anatomical detail
T2 Weighted• Long TR and TE• Tissues that are slow to
give up energy are imaged – such as water – therefore water has high SI
• Particularly valuable for detecting inflammation
Biederman, 2006
Biederman, 2006
Image Information• Scout image• Weighting and/or TR and
TE• Slice thickness (4-8 mm)• FOV (field of view)• Date, Time, facility, body
part, plane
Protocols• Combination of sequences• No standard protocols• Combination depends on the body part
and the suspected pathology• Two main categories of sequences
– Spin echo (SE) such as T1 and T2 images– Gradient echo (GRE)
SE Sequences• Usually referred to as T1 – or T2 weighted with
specific parameters stated• Fast SE – as it sounds – faster• Proton density (PD)
– Long TR and short TE the contrast is primarily due to PD, tissues with higher PD have higher SI
– SI is similar to T1, but has greater anatomical detail• Inversion recovery (STIR – short tau inversion)
– Inversion pulse cancels out the signal from fat to further reduce its SI in T2 images
Biederman, 2006
For better example of differences see Figure 5-4 in McKinnis text
Biederman, 2006
GRE Sequences
RF wave is applied and only partly flips the magnetization field (0-90 degrees) and includes a variable flip angle
Allows reformatting to any plane – not limited to orthogonal plan – so used for complex anatomy
Overall: 1. Fast image acquisition2. High resolution with thin slices3. High contrast between fluid and cartilage
Use of Contrasts
• Intravenous gadolinium-containing contrast agents
• Gadnolium is a paramagnetic metal ion used for regular MRI, MR angiography (MRA) and MR arthrography
Imaging Characteristics of Tissues
MRI Advantages / Disadvantages
Advantages• Greater contrast for soft
tissue• Image organs surrounded
by dense bone• No ionizing radition• Less false positives
Disadvantages• Expensive• Not always available• Long imaging times• Longer operator time• Larger slices than CT• More problems with motion
artifact• Less resolution for bone• Concern about metal implants