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DMD2 > Extraoral Radiography 3 > Flashcards

Extraoral Radiography 3 Flashcards

1
Q

How is a CT scan image different to a conventional x-ray examination?

A

A CT scan obtains a tomographic slice instead of a projection image obtained with conventional x-ray examinations.

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2
Q

What is a fan beam projection?

A

A fan of data that converges onto a vertex. The apex of the fan is the X-ray tube. the projections are read at the other side of the projection.

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3
Q

What are hounsfield units?

A

The CT number which is the attenuation number of tissues.

The Hounsfield unit (HU) scale is a linear transformation of the original linear attenuation coefficient measurement into one in which the radiodensity of distilled water at standard pressure and temperature (STP) is defined as zero Hounsfield units (HU), while the radiodensity of air at STP is defined as -1000 HU.

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4
Q

What is the HU of air, lungs, fat, and water?

A

0 = water

  • 1000 = air
  • 20 to -100 = Fat
  • 300 = lungs
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5
Q

What is the difference between a voxel and a pixel?

A

A 3D volume element is a voxel

The projection of that image on a screen as a 2D image is a pixel

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6
Q

How is the value assigned to a voxel achieved?

A

Using the average HU of the tissue in that volume.

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7
Q

How is the grey scale associated with a voxel modified?

A

It depends on the window and level settings

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8
Q

What is a beam hardening artefact?

A

Beam hardening and scatter both produce dark streaks between two high attenutation objects (such as metal or bone), with
surrounding bright streaks. These can be reduced
using iterative reconstruction.

Streak artefacts in the base of the skull are caused by beam hardening.

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9
Q

What causes streaking artefact and what is it?

A

Highly attenuating material such as metal may reduce the number of detected x-rays to near 0.

The loss of data along the ray is interpreted as a streak.

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10
Q

What is the radiation dose of a typical regular CT scan, a cone beam CT, and a complete mouth survey?

A

2.0 mSv = Normal CT

Cone beam CT = 0.032 - 0.209 mSV

Complete mouth survey = 0.117 - 0.550 mSv

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11
Q

Which organs in the head is at most risk from CT scan?

A

The eyes (lens specifically)

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12
Q

What are the indications for an MSCT scan?

A

Pathology expanding into both hard and soft tissues

Planning and follow-up of treatment of large pathology (cancer)

Immediately post trauma imaging (show haemorrhage, organ ruptures, and bone fractures)

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13
Q

How does MRI work?

A

Hydrogen nucleus is a single proton with spin and charge. This is changed by MRI using a strong magnetic field to align the hydrogen spins forming parallel and antiparallel spins to the applied magnetic field.

The net magnetic momentum is then measured..

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14
Q

What is precession?

A

The protons experience a torque perpendicular to the applied magnetic field which causes the magnetization vector to precess

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15
Q

What is the Larmor frequency?

A

The frequency of the precession which is the product of gyromagnetic ratio and the strength of the magnetic field.

Gyromagnetic ratio is an inherent property of the nucleus.

This means that a change in magnetic field strength results in a change in precession frequency.

Precession frequency is different in different tissues and so allows differentiation based on water content.

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16
Q

How is the magnetization made to be at an angle to B0?

A

Resonance; protons absorb the electromagnetic waves at the precession frequency.

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17
Q

How safe are MRI scans?

A

The MRI scanner uses a very strong magnet for images meaning patients and personnel must not enter the scan room with any ferromagnetic material on their body. If they are present the magnet will attract these materials very strongly. OXYGEN BOTTLES UNDER NO CIRCUMSTANCES SHOULD BE BROUGHT TO MRI SCANNER ROOM.

Burns are a possibility due to RF fields which cause currents to form in metallic stuff, electrical equipment (in contact with the patient), and the patient himself if he forms closed loops with extremities.

There may be potential unknown complications but this is unknown at this stage.

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18
Q

How does signal reception take place in MRIs and what is relaxation time?

A

Precessing spins cause a time-varying voltage signal in a coil or antenna and this occurs at the precession frequency.

Relaxation is when the protons relax from their precession and return to facing the original direction (parallel/antiparallel) and the signal then diminishes over time. The relaxation time is used for image contrast.

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19
Q

How is localization achieved to form a good resolution image?

A

Through the use of gradient coils.

These electromagnets establish additional magnetic fields in the x-, y-, and z- directions.

The different magnetic strengths in the 3 axial directions produces different precessional frequencies and different resonance frequencies.

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20
Q

What units are used to measure magnetic field? How strong is the magnetic field in an MRI?

A

Tesla and Gauss

1 Tesla = 10k Gauss

MRI magnetic field = 1.5 tesla or 3.0 Tesla

Experimental MRI >7 Tesla

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21
Q

How is the magnet of an MRI rotated around the patient in an MRI machine?

A

Helium is used to revolve magnet around which is expensive to replace (~30k). MRI costs approximately 3 million dollars CT is ~300k.

22
Q

How are structures on the body differentiated using an MRI machine?

A

T1 and T2 are the most commonly used contrast weightings. T2 will cause all water in the image to give higher signal. This means the cerebrospinal fluid shows up.

White is not radiopaque but rather it is high signal.

23
Q

What is done in an emergency for a patient to take an MRI?

A

A specially equipped crash cart containing non-ferrous objects is used to render support to a patient.

Even the patient gurney or bed must be made of non-ferrous materials.

24
Q

What is done for patients with implanted pacemakers or ferromagnetic materials?

A

Special precautions are taken specific to the problem.

25
Q

How is MRI related labelling of equipment done?

A

Green square = safe

Yellow triangle = conditional

Red circle with line = not safe

26
Q

SLIDE TO LEARN:

A

LOOK AT COMPARISON OF CT TO MRI SLIDE

27
Q

SUMMARY:

A

CT scans generate 3D images with higher dose to patient whereas MRIs have no ionizing radiation.

CT scans produce good bone detail, MRIs are good for soft tissue detail

After patient is placed in a strong magnetic field, the tissue alignment is disturbed by a radio frequency pulse.

The time taken to relax back to the tissue’s equilibrium state is measured and the differences in relaxation time for different tissues is used to form the MRI image.

28
Q

What is the piezoelectric effect? How is this used to get an US image?

A

A potential difference applied to a crystal will cause distortion of the crystal.

Removing the potential difference results in crystal returning to its original shape.

By doing so it oscillates within ultrasonic frequency ranges.

These ultrasound waves are then directed into the patient’s tissues.

29
Q

What is the US transducer?

A

The device that produces and detects US waves.

30
Q

How often is a pulse produced by an US transducer?

A

Every millisecond a pulse of ultrasound is produced. This pulse lasts for 1 microsecond. The remaining 999 microsecond are used to detect the pulse (echo) returning from tissues.

31
Q

Air is not a good medium for ultrasound; How does the transducer still detect signals so well?

A

A coupling agent (gel) is used to get rid of the air.

32
Q

How are different tissues distinguished on US?

A

Different echos will be detected from different tissues enabling one to distinguish between different types of tissues.

The reflected US waves are transformed into an electrical signal which is used to compose a digital black/grey/white live image..

33
Q

What frequencies are used to detect different types of tissues via US?

A

Low frequency is used for deeper tissues.

High frequency is used for more superficial structures.

34
Q

How are deeper structures affected by their location and how is this issue compensated for?

A

Each tissue reflect US differently and as the waves are reflected they travel back through more superficial layers which render echos weaker. for this reason it is necessary to have time gain compensation

35
Q

What is the difference between echoic and an-echoic signals on an US?

A

Echoic refers to hyper-echoic signals which appear bright or hypo-echoic signals which appear dark/grey.

An-echoic signals appear as black

36
Q

What kinds of medical procudures use US?

A

US guided FNA

US guided botox injections (Salivary glands of patients)

Adjunct to sialography in diagnosis of Sjogren’s syndrome, sialadenitis, sialolithiasis, tumour of the salivary gland.

Imaging of the masticatory musculature

Imaging of lymph nodes

US colour doppler effect to investigate perfusion of tissues.

Intermaxillary suture expansion follow up.

Healing process evaluation

37
Q

What shapes of transducers are available?

A

Convex array

Flat linear array

Mini convex array

Trans-oesophageal echo probe

Small ones are used for Anal, vaginal, and oral cavity. *

38
Q

What are the advantages and disadvantages of using US?

A

Advantages:

Non-invasive
No radiation
Quick
Comfortable for patient
Ideal for soft tissues

Disadvantages:

Rather poor resolution
Air and bone cannot transmit
Operator dependent
Interpretation is often difficult

39
Q

Is US good for TMJ diagnosis?

A

Unsure at this stage conflicting results in the research.

40
Q

What happens to US signal if there is bone?

A

Image blocked by bone tissue. US is only really good for soft tissue visualization

41
Q

What common feature is seen in sialodochitis?

A

Sausage appearance of vessel (blobby appearance)

42
Q

What do small black dots on the parotid indicate?

A

Inflammation

43
Q

How effectively can US replace radiographs?

A

Studies thus far have shown very high correlation between the typical radiograph and the ultrasound image findings.

44
Q

What psychological factors can impact on the interpretation of radiographic images?

A

Distraction (All your attention is drawn to 1 thing in the image)

Pareidolia (the tendency to perceive a specific, often meaningful image in a random or ambiguous visual pattern)

Biased assumptions

Observant

Surroundings

Imagination

45
Q

What kind of things are seen as a result of pareidolia?

A

Usually people recognize faces.

46
Q

What could cause biased assumptions?

A

Patient’s story

Colleague’s remarks / notes

SoD: Clinic you are in

47
Q

How is the radiation source angled for image taking on an OPG?

A

8 - 12 degrees upwards

48
Q

Where is the image detector located relative to the anatomy on an OPG?

A

On the same side as the anatomy being observed. This is opposite to the radiation source.

49
Q

What important instruction should be given to patients before taking an OPG regarding the positioning of their tongue? Why?

A

Tell the patient clearly to keep tongue against the roof of the mouth (the bulk if possible). Not just the tip of the tongue.

This is because the soft tissue of the tongue will attenuate the image enough to make anterior teeth more visible.

50
Q

Why must lips be closed on an OPG?

A

Dark shadow covering canine to canine will appear if not done.

DMD2 (89 decks)

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