Lecture 8: digital extraoral radiography

Imaging basics: focal trough =

"zone of sharpness" where all structures within will be reasonably defined

Imaging basics

1. Gantry rotation 2. focal trough 3. horizontal distortion 4. vertical distortion

Extraoral radiography: many techniques have been replaced by 3D imaging modalities

1. MDCT (medical CT) 2. CBCT 3. MRI (does not use ionizing radiation)

Extraoral radiography

1. broad radiographic examinations of maxillofacial region (in short amount of time in non-invasive way) 2. diagnostic query determines selection of projection 3. superimposition + unequal magnification/distortion of anatomical structures 4. many techniques have been replaced by 3D imaging modalities

"real images" (2)

1. centered in zone of sharpness 2. all "real" structures being imaged

What can "ghost" rather than form double image?

1. cervical spine 2. hyoid bone -- occurs d/t pt positioning

Image exposure: pt instructions

1. close lips 2. swallow 3. tongue placed at roof of mouth 4. hold still (remember mvmt blurriness) -- reason for positioning instructions: to avoid unnecessary airspaces that cause burnout

Gantry rotation summary

1. continuous X-ray beam 2. that circles around pt based on center of rotation 3. receptor + X-ray source move together 4. receptor + collimator always in front of pt, X-ray source behind

Lateral ceph/skull projection -- used for eval of

1. craniofacial skeletal morphology 2. developmental growth 3. occlusal relationships 4. txt progress 5. orthognathic surgery (LeFort, e.g.) -- "side image used in ortho"

Images formed (3)

1. double images 2. ghost images 3. real images (structures truly imaged by machine in sharp way)

Chin-tilted-down mistake -- on pan

1. exaggerated smile line 2. mand incisor crowns well-imaged, but their roots are not

PA ceph/skull projection -- used for eval of

1. facial asymmetry 2. orthognathic surgery -- "same machine as used for lateral ceph, but pt + indicators have turned"

Chin-elevated mistake -- on pan

1. flat smile line 2. can't see max roots 3. mand incisor roots well-defined

Chin-elevated mistake

1. flat smile line 2. max incisor crowns well imaged bc within focal trough, but their roots are not 3. mand anterior teeth within focal trough

Double images in a nutshell

1. formed by midline structures 2. that are intercepted twice 3. indicated by green area in image -- "we cross these structures twice d/t rotational nature of beam"

Head rotation mistake

1. hardest to understand 2. when pt rotated in sagittal plane, focal trough in orange does NOT change, but the pt does 3. in this example, pt rotated towards right side (note R + L in right image) 4. since rotated to right, right area of jaw will be farther from receptor (remember: receptor always in front of pt), left area will be closer to receptor --> side farther away from receptor, closer to X-ray source appears larger (vice versa for other side)

Images formed: double images -- examples of structures that result in double images when imaged

1. hyoid bone 2. hard palate 3. cervical spine 4. etc. -- some of these structures will also "ghost"

Images formed: double images

1. images formed by structures that intercept X-ray beam twice 2. still "real images", though 3. these structures that are intercepted twice are midline structures 4. can present as separate or continuous in resulting radiograph

Images formed: ghost images

1. images formed from objects that are positioned btwn center of rotation + X-ray source 2. these images appear higher on opposite side as blurry, distorted images -- "images in the back that are captured while exposing focal trough"

Extraoral radiography: broad radiographic exams in maxillofacial region

1. images lack fine detail 2. less "invasive" 3. rapid scan time 4. relatively low radiation dose

Too far backward mistake

1. in right image, imagine red lines = focal trough 2. max + mand teeth not imaged in focal trough 3. max + mand anterior teeth are magnified d/t increased distance from receptor 4. less of cervical spine seen

Too far forward mistake (4)

1. in right image, imagine red lines = focal trough 2. when pt too far forward, both max + mand anterior teeth not imaged in focal trough + teeth are too close to sensor 3. this results in their minimization + blurriness 4. cervical spine further in toward focal trough, so seen as double image

Imaging basics: focal trough specs

1. largely manufacturer-determined as it is determined by path + velocity of gantry, X-ray beam alignment, collimator width 2. pt is positioned to best fit in focal trough based on anatomical landmarks 3. center of rotation is NOT fixed but on set trajectory

Other extraoral projections

1. lateral ceph 2. posteroanterior (PA) ceph 3. Water's 4. submentovertex 5. reverse Towne's 6. lateral oblique -- we discussed lateral cephs + PA cephs; #3-6 not commonly used unless in underserved area without access to CTs

Other errors

1. left image: can see safety pin in lower right 2. middle image: pt with cochlear implant 3. right image: pin used to secure hijab

Good example of ghost image we cannot avoid

1. mandibular ramus 2. in diagram, see how X-ray beam has to pass through mandibular ramus to expose teeth 3. since structure is closer to X-ray source + farther from receptor, it is HIGHER + DISTORTED (blurry)

Chin-tilted-down mistake

1. max anterior teeth are within focal trough 2. roots of mand teeth NOT in focal trough (so can see crown but not the roots) 3. exaggerated smile line when chin tilted down

Pt positioning -- 3 guidelines to use (remember: focal trough determined by manufacturer, but there are certain anatomical landmarks we use to get pt into focal trough)

1. mid-sagittal plane 2. canthomeatal line or Frankfort horizontal plane (remember: canthomeatal line goes to corner of eye) 3. canine reference plane (have pt smile then center line on canine)

Imaging basics: focal trough diagrams

1. on left: black curves = changing center of rotation 2. focal trough outlined in red on right; well sharpest images in red... what is highlighted in blue is focal trough? 3. what are imaged within focal trough = "real images"

Pre-scan preparation

1. optimize exposure options to pt 2. removal of any metal object that can obscure the image(s) 3. lead apron without thyroid collar 4. position pt within machine -- note: options to image only part of jaw, but she doesn't see reason for not exposing the entire thing

Common indications for pans: evaluation of

1. osseous components of TMJs 2. impacted teeth 3. eruption of abnormalities 4. pathology 5. trauma

Imaging basics: horizontal distortion

1. remember: X-ray beam is always divergent 2. middle image: object is farther away from receptor resulting in magnified image 3. right image: object too close to receptor resulting in minimized image --> not just from receptor perspective: closer object is to X-ray source, the more magnified the image

Image of dual machine that exposes pans + cephs

1. right side of device for pans 2. left side for cephs (both PA + lateral)

Imaging basics: vertical distortion occurs d/t

1. slight caudocranial inclination of beam (inherent property of beam coming from source) 2. this inclination is ~10 degrees 3. so what is lingually placed appears superior to what is buccally placed (see diagram) 4. what is in middle should have more or less accurate representation

Visible palatoglossal airspace

1. space btwn palate + tongue 2. seen when pt doesn't put tongue to roof of mouth 3. if severe (radiolucent enough), can burnout apices of maxillary teeth

Positioning errors

1. too flat 2. too angled

Water's, submentovertex, reverse Towne's, lateral oblique commonly used for

1. trauma + pathology cases -- where you cannot see details on diff images 2. Water's view specifically used for sinuses 3. submentovertex + reverse Towne's used for condyles + mand 4. lateral oblique used for one side of mand

"Gantry moves in same direction, it's the pt position that changes" - she also said "receptor + beam..." - thicker one = X-ray source - thinner = receptor

1. upper image is for lateral ceph 2. lower image is for PA ceph

Don't take a panoramic when pt still has

RPD in place -- not diagnostic bc we can't see roots of teeth nor bone

Head-tilted mistake on pan -- note:

asymmetry/rotation may be d/t pt anatomy -- they may have shifted midline, e.g., so their landmarks are "off"

Nice thing about lateral ceph

can see soft tissue profile as well -- class III on left; middle + right images demonstrate points used in measuring

Too far forward mistake -- on pan

can tell pt way too far forward bc: 1. entire maxillofacial region is minimized 2. lots of extra cervical spine on image 3. max + mand anterior regions not imaged well + blurry

Center of rotation + focal trough decided by

center of rotation in conjxn with path + velocity of gantry, how X-ray beam is aligned, what collimator looks like

Artifact caused by unexposed area

doesn't happen often, but need to hold button down for entirety of exposure -- if you let go of button, will have unexposed area (white line)

Diagram that tells what images is best for what

don't need to know these, but just know that there are diff types of extraoral images that can be used in place of 3D images -- but these methods are obsolete more or less

Double images are __, ghost images are __

double images are real images, ghost images are "not really"

"what is lingually placed appears superior to what is buccally placed" -- what does this mean?

if you're closer to X-ray source, you'll appear higher than what is farther from X-ray source

Imaging basics: Gantry rotation

looking at right image: 1. "panoramic imaging = rotating arm gantry" 2. gantry has receptor + X-ray source attached to arc 3. thick side houses X-ray source, thin side has receptor left image: 1. source moves behind pt 2. collimator moves as X-ray is being taken 3. seems like receptor is moving in opposite direction

Troubleshooting necessary for

malocclusions -- have pt bite more forward in class III to reduce distortion, e.g.

Focal trough centered around

maxillomandibulofacial region

Hearing aids artifacts

not as common as cochlear implants -- may see nasal canula for O2, shunts from brain to abdomen, e.g.

Too far backward mistake -- on pan

note how large max + mand anterior teeth appear

Overexposed image

overall dark image

Underexposed image

overall light image

Overlap btwn double + ghost images bc both are images

posterior to center of rotation -- but pt positioning determines whether ghost or double image forms

So PA + lateral cephs used as

preoperative films for both orthodontic + orthognathic surgery txts -- but also have CBCT...

Pt with earrings in during pan exposure

pt could not remove earrings -- in this instance, PAs of obscured edentulous areas indicated (bc need to know if root tips are there, e.g.)

Extraoral radiography: diagnostic query determines selection of projection

pt positioning crucial -- superimposition, unequal magnification of structures --> note: this also applies to intraoral images

Head-tilted mistake on pan

see left side is slightly higher than right -- could also be d/t pt anatomy

Size (height, weight...) of individual influences

standardization of kVp + mA -- these settings are lowered in child, higher for larger individual, e.g.

Diagram of lateral ceph, SMV, Water's, PA ceph, reverse Towne

tells how pt should be positioned relative to central beam + receptor

Note regarding panoramic images

there are inherent types of distortion that are seen

Visible palatoglossal airspace on pan

this is an important airspace bc it projects differently on every pt

"what is lingually placed appears superior to what is buccally placed" -- bc of this,

we cannot accurately determine location of inferior alveolar canal -- since we have access to 3D imaging, it is unadvised to use a pan to place implants

"the side that the pt is rotated towards is the side that

will be magnified" -- this is bc this side is farther from receptor