Diagnostic Imaging Primer 1 Hour (brief) introduction Sean Collins Fall 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 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 sites Different angles / planes of view Variation in images is also caused by: 1. Method of imaging – x-rays vs. computer modified images vs. proton signals 2. 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.Fat 3.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 – perpendicul ar to one another to gather accurate information

AP View Viewed as if standing in front in anatomical position Markers: R – right L – left INT – int rota. EXT ext rota WTB – standing DECUB – recumbant INSP, EXP

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 Types CT 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 fractures 2.Degenerative changes 3.First in serious trauma 4.Spinal stenosis 5.Loose bodies in joints Less time & expense than MRI Accurate measure in any plane Less 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

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 acquisition 2.High resolution with thin slices 3.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