Diagnostic Imaging Primer3 Hour introduction to curricular thread

Sean CollinsFall 2010

Outline (Topic)

• Purpose of primer & thread • Objectives of primer • Required readings • Underlying message • General Principles & Plain films • Computed Tomography Intro• Magnetic Resonance Intro • Diagnostic Ultrasound 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 logic of diagnostic imaging by x-

rays, CT scan, MRI and Diagnostic ultrasound– 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)

Required Readings

McKinnis LM. Fundamentals of Musculoskeletal Imaging, 3rd Edition, 2010, FA Davis

Reviewed in Primer –Chapters 1, 4, 5, 6

For Med/Surg OrthopedicsChapters 2 & 3

Underlying messageVariation 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 signals vs. sound wave reflections2. 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(x-rays)

• 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

Density impacted by thickness

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 or

ERECT – standing

DECUB – recumbant

INSP, EXP

Biederman, 2006

Biederman, 2006

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)

• 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

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)

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• 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 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

Diagnostic Ultrasound Intro • Cross – sectional approach based on sound

wave reflection from tissue interfaces• Pre-dates both CT scan and MRI for pelvic &

abdominal soft tissue imaging; and in the past 30 years has been increasingly used in imaging the musculoskeletal system

• Unique position – – Not widely adopted by radiologists; provides real time

image as part of clinical exam; non radiologist health professionals have welcomed as part of practice

Equipment• Pulsar – base Freq of 2-5

MHz; 1k – 5k bundles / minute at this base Freq; between bundles – silence (1% of time waves are sent)

• Transducer – other 99% of the time the transducer acts as a receiver

• Scan converter & monitor: computer that takes signal and produces digital image (256 shades of grey)

Interaction with Tissue• Absorption (left): friction converts mechanical energy to heat• Reflection: image based on reflected sound. Image depends

on how much is reflected back. Ideally is perpendicular to the structure being imaged

• Refraction (middle): change in direction• Scattering (right): uneven surface results in loss of reflection

US ImageBased on amplitude, timing and transverse location of

reflected waves• No set characteristic signal intensity – but generally

– High reflection: hyperechoic (bright)– Little reflection: hypoechoic (dark)– No reflection: anechoic (dark)– Interfaces that are dissimiliar in density (bright)

• Timing- distance determined by time• Transverse location determined by position on

transducer

Imaging Characteristics of Different Tissues

• US does not penetrate bone – bright• Tendon relative to muscle - bright• Ligaments relative to muscle – bright• T & L have different fiber patterns; L are more compact• T & L are “anistrophic” – slight changes in angle of US

beam may change from bright to dark• Muscles tend to be dark relative to tendons and fascia

– Fascia bands can be discerned in longitudinal US; and as dots in transverse US

Imaging Characteristics of Different Tissues

• Bursa – dark• Hyaline cartilage – dark• Fibrocartilage – bright• Nerve – darker than tendons, but brigher than muscle• Cysts are dark with enhancement of structures

posterior to cyst

1, supraspinatus t.2, deltoid ms3, subacromial bursa4, articular cartilage5, humeral head

Abnormal Findings• Muscle strains – disruption of fibrous bands• Tendon pathology – thickening of the tendon and

disruption of pattern; inflammation will appear as dark layers

• Bursitis – widening of dark space• Ligament strains – disruption of pattern• Cartilage damage – variable thickness• Nerve compression – flattening of nerve at point of

compression

Advantages / DisadvantagesAdvantages

• Higher resolution than MRI• Low cost and portable• No known hazards• Ready comparison with

opposite side• Change positions to put into

symptom provoking position• Use resisted contractions or

passive muscle stretching• Apply traction, compression,

ligament tension

Disadvantages• Limited in joint surfaces and

intra-articular structures• Only shows cortical outline of

bone• Does not cross air-tissue

interaces (cannot scan across lung fields)

Display of different responses to lumbar stabilization1. Rectus abdominus2. External oblique3. Internal oblique4. Transverse abdominus5. Anterior and posterior rectus sheaths

Finally……

1. Questions will appear on your first pathology exam from this material

2. Use the readings to supplement these slides3. Dr. Gerber will continue with a class on

diagnostic testing later today (Sept 8)4. Reading images is critically related to

understanding these basic concepts AND a strong understanding of anatomy