X-Ray Fundamentals of Image Production & Image Quality

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X-Ray Fundamentals of Image Production: Part 2

Demographics and Side Marking (Image Identification)

Legal Requirements for Radiographic Images

• Radiographic images are considered legal documents.
• They contain sensitive patient information and are part of the patient's confidential file.
• Confidentiality: Access is strictly limited to those within the patient's circle of care.
• Image Linkage: Every image must contain specific information to link it to the examination and the patient.
• Patient Data Application: Patient data must be applied to each image.
  • DR Image: Added to the image during or immediately after acquisition.
  • CR Image: Added during or after the scanning process, often after the cassette is read.

Information Required on an Image

• Patient's full name (first, middle initial, last).
• Date of birth.
• Record numbers.
• Unique hospital number of the patient.
• PACS Accession Number:
  • A unique number assigned by the database for each image of a routine at a specific time (day, hour, minute).
  • Example: All images from a dislocation, followed by images from reduction attempts, must have unique accession numbers for each set/routine.
• Date of the exam (and time, if possible).
• Institution where the exam was performed.
• Correct Side Marker: Best practice is to use a lead marker placed within the exposure field.

Side Marker Protocol

• Standardization of Viewing Procedures requires specific marker placement:
  • Marker must be placed within the X-ray field on the image receptor.
  • Marker must be applied in the anatomical position.
  • Marker must be on the correct side (double-check critically).
  • Where possible, the marker should be on the lateral aspect of the body part.
  • The marker should not cover anatomy.
  • The marker must be flat to avoid distortion.
• Best Practice: Use a side marker that includes the MRT's (Medical Radiation Technologist's) initials.
• Electronic Markers: Electronically added markers after exposure may not be legally acceptable as they are not permanent.
• Resource: Refer to CAMRT Best Practices on markers and annotations for further guidelines: https://camrt-bpg.ca/patient-management/records-reporting/markers-and-annotation-rtr/

Image Examples (Page 7)

• Page $7$ displays examples of images with embedded patient and exam data (e.g., patient name DOLINER^EMILY, Acc: $248863$PDI, $2011$ Jul $07$).
• These examples are used to evaluate if the images meet the specified standards for information content.

Image Production: Part 3

Cervical and Thoracic Spine: Lateral Position

• Clinical Indications: Pathology involving the thoracic spine, such as compression fractures, subluxation, or kyphosis.
• Note for Upper Thoracic Vertebrae: If upper thoracic vertebrae are of primary interest ($ext{T1-T3}$), perform the cervicothoracic (swimmer's) lateral position in addition to routine thoracic spine projections.
• Technical Factors (Routine Thoracic Spine Lateral):
  • IR size: $14$ x $17$ inches ($35$ x $43$ cm), portrait orientation.
  • Minimum SID: $40$ inches ($100$ cm).
  • kVp range: $80-95$.
  • Grid: Required.
  • AEC (Automatic Exposure Control): Not typically used with breathing technique.
  • Orthostatic (Breathing) Technique: Requires low $ext{mA}$ and $2$ to $3$ seconds of exposure time.
  • Scatter Reduction: A lead mat placed on the table behind the patient reduces scatter to the IR.
• Shielding: Shield all radiosensitive tissues outside the region of interest.
• Patient Position: Lateral recumbent or erect position.
  • Lateral Recumbent: Head on pillow, knees flexed, hips and knees flexed with support between knees.
  • Erect Position: Arms outstretched, weight evenly distributed on both feet.
• Part Position:
  • Align the posterior half of the thorax (between midcoronal plane and posterior aspect) to the CR and midline of the table/IR.
  • Raise patient's arms to right angles to the body with elbows flexed.
  • Support waist so the entire spine is near parallel to the table. Palpate spinous processes for alignment.
  • Ensure no rotation of shoulders or pelvis exists.
• Central Ray (CR):
  • CR perpendicular to the long axis of the thoracic spine.
  • Direct CR to T7 (approximately $3$ to $4$ inches ($8$ to $10$ cm) below the jugular notch or $7$ to $8$ inches ($18$ to $20$ cm) below the vertebra prominens).
  • Center IR to CR.
• Recommended Collimation: Collimate on two sides of the anatomy (four sides if possible).
• Respiration: Use orthostatic breathing technique or suspend respiration.
  • Suspended Full Inspiration: Provides maximum uniform density of vertebrae visualized above the diaphragm.
  • Breathing Technique: Useful to blur unwanted rib and lung markings overlying thoracic vertebrae if patient can cooperate. Requires minimum $2$ or $3$ seconds exposure time with a low $ext{mA}$ setting.
• Notes:
  • Note 1: Significant secondary and scatter radiation. Close collimation and a lead mat posterior to the part are essential, especially with digital imaging.
  • Note 2: Optimal waist support ensures lower vertebrae are equidistant from the tabletop as upper vertebrae. Wide hips require more support. Broad shoulders may require a $10^ ext{o}$ to $15^ ext{o}$ cephalic CR angle if waist is not supported.
• Evaluation Criteria:
  • Anatomy Demonstrated: Thoracic vertebral bodies, intervertebral joint spaces, and intervertebral foramina. $ext{T1}$ to $ext{T3}$ may not be well visualized; a cervicothoracic lateral may be needed.
  • Position: Open intervertebral disk spaces. No rotation (superimposition of posterior vertebral bodies, less than $1/2$ inch ($1.25$ cm) space between posterior ribs).
  • Collimation: To the area of interest.
  • Exposure: Clear demonstration of bony margins and trabecular markings; no motion.

Prime Exposure Factors: $ext{mA} imes ext{S}$ (mAs)

• Definition: $ext{mAs} = ext{mA} imes ext{s}$ (milliamperage-seconds).
  • $ext{mA} = ext{measure of x-ray photon flow}$.
  • $ext{s} = ext{length of time photons are flowing}$.
• Effect on Exposure to IR: Directly affects the amount of photons leaving the tube.
  • More $ext{mAs}$: More photons leaving the tube
    ightarrow more exposure to IR
    ightarrow darker image.
  • Under exposed: Not enough photons
    ightarrow image is too bright.
  • Over exposed: Too many photons
    ightarrow image is too dark.
• Calculation Examples:
  • $20 ext{ mA} imes 0.5 ext{ s} = 10 ext{ mAs}$.
  • $10 ext{ mA} imes 0.5 ext{ s} = 5 ext{ mAs}$.
  • $5 ext{ mA} imes 0.5 ext{ s} = 2.5 ext{ mAs}$.
• Exposure Time vs. $ext{mAs}$ Quantity: Different combinations of $ext{mA}$ and time can yield the same $ext{mAs}$ (e.g., $1 ext{ mA} imes 2.5 ext{ s} = 2.5 ext{ mAs}$ and $50 ext{ mA} imes 0.05 ext{ s} = 2.5 ext{ mAs}$).
• Key takeaway: While $ext{mAs}$ determines the number of photons, individual $ext{mA}$ and $ext{s}$ values can vary. A higher $ext{mAs}$ value will always have the largest number of x-ray photons in the beam, regardless of the $ext{mA}$ and $ext{s}$ combination.

Prime Exposure Factors: kV

• Definition: kV (kilovoltage) or kVp (kilovolt peak).
• Effect on Exposure to IR:
  • Higher kVp
    ightarrow Higher energy photons
    ightarrow More photons produced
    ightarrow More penetration through patient
    ightarrow More exposure to IR
    ightarrow Darker image.
  • Under exposed: Not enough high-energy photons
    ightarrow image is too bright.
  • Over exposed: Too many high-energy photons
    ightarrow image is too dark.

Prime Exposure Factors: SID

• Definition: SID (Source-to-Image Distance).
• Effect on Exposure of IR: When kV and $ext{mAs}$ selections remain constant, changing SID significantly affects the exposure to the IR.
• Inverse Square Law: The intensity of the x-ray beam is inversely proportional to the square of the distance from the source.
  • As SID increases, beam intensity decreases
    ightarrow less exposure to IR.
  • As SID decreases, beam intensity increases
    ightarrow more exposure to IR.
• Lab Experiment: An experiment demonstrates the impact of changing SID, for example, comparing $150$ cm SID to $50$ cm SID with constant kV and $ext{mAs}$ values, showing a significant change in Exposure Index (EXI).

Exposure Index (EI) - Only in Digital Radiography

• Purpose of EI Numbers:
  • Both CR (Computed Radiography) and DR (Digital Radiography) systems automatically adjust the visual quality (optimize) of the image. This has both positive (improved image appearance) and negative (difficulty assessing true patient exposure) consequences.
  • Digital processing systems display exposure indicator numbers to verify that the exposure factors used were in the correct range for optimum image quality with the lowest radiation dose to the patient (ALARA).
  • EI provides information on under- or over-exposure of the patient.
  • Must be monitored by radiographers to ensure exposures are of diagnostic quality and do not result in excessive patient radiation.

What EI Depends On

• The amount of radiation that strikes the receptor.
• Affected By: Technical factors ($ext{mAs}$ & kVp), total receptor area irradiated (collimation), and distance (SID).
• Display: The EI value is displayed for each exposure, always at the workstation and sometimes on PACS (Picture Archiving and Communication System).

Relationship Between EI Value & Exposure

• EXI (DR - Exposure Index): Directly related to exposure.
  • Acceptable Range: $200-600$; Target: $400$.
  • Greater than $600$ (e.g., $1000$) = IR is over exposed (patient received too much radiation).
  • Less than $200$ (e.g., $50$) = IR is under exposed (patient received too little radiation, potentially causing noise).
• S-value (CR - Sensitivity Value): Inversely related to exposure.
  • Acceptable Range: $100-400$; Target: $100$ (tabletop) & $200$ (bucky).
  • Greater than $400$ (e.g., $1000$) = IR is under exposed (patient received too little radiation).
  • Less than $100$ (e.g., $50$) = IR is over exposed (patient received too much radiation).
• Important Note: You should not repeat an image based on EXI or S-value alone. Always use the EI information to correct technical factors for subsequent images in the routine, considering visual image quality.

How to Adjust Techniques for Next Exposure

• Examples with DR (EXI Target Range 200-600):
  • Image #$1$: $40 ext{ kVp}$, $4 ext{ mAs}$, EXI $24$ (under-exposed). To increase EXI and exposure, increase kVp or $ext{mAs}$.
  • Image #$2$: $60 ext{ kVp}$, $4 ext{ mAs}$, EXI $341$ (within target, good exposure).
  • Image #$3$: $80 ext{ kVp}$, $4 ext{ mAs}$, EXI $1322$ (over-exposed). To decrease EXI and exposure, decrease kVp or $ext{mAs}$.
• Examples with CR (S-value Target for Tabletop = 100):
  • Image #$1$: $60 ext{ kVp}$, $1 ext{ mAs}$, S = $447$ (under-exposed).
  • Image #$2$: $60 ext{ kVp}$, $4 ext{ mAs}$, S = $170$ (acceptable, slightly under-exposed compared to $100$ target).
  • Image #$3$: $60 ext{ kVp}$, $20 ext{ mAs}$, S = $30$ (over-exposed).

Possible Exposure Indicator Errors

• EI values may be corrupted if:
  • Part/beam/plate are misaligned.
  • Excess scatter (deflected x-ray photons striking IR).
  • Gross overexposure or gross underexposure.
  • Read/scanned incorrectly.
  • Excessive foreign objects (e.g., metal implants).
• Crucial Rule: Do not base your reason to repeat an image solely on EI. Always correlate the EI value with the visual appearance of the image.

Exposure Index Summary

• Purpose: To verify that acceptable quality digital images have been obtained with the least possible dose to the patient (ALARA).
• Digital processing occurs in the background to always optimize how an image looks (pre-processing).
• It is important to select the correct body part and view so the unit knows how to optimize the image best.
  • DR: Typically selected at the acquisition workstation before exposure.
  • CR: Often selected after the cassette is scanned.
• Important Points:
  • Digital radiography optimizes the image, which means the visual appearance alone doesn't directly indicate patient radiation dose.
  • The EI value specifically tells you how much the patient has been exposed to radiation.
  • Always consider the EI value plus the visual qualities of the image when deciding whether to repeat an exposure.

Analyzing EI Values

• DR Room Examples (Constant $60 ext{ kV}$):
  • $1 ext{ mAs}$, EXI $241$ (Lower end of acceptable range).
  • $4 ext{ mAs}$, EXI $1019$ (Overexposed).
  • $20 ext{ mAs}$, EXI $4902$ (Grossly overexposed).
  • Trend: As $ext{mAs}$ increases, EXI value also increases, indicating higher exposure.
  • Repeats: Images at $4 ext{ mAs}$ and $20 ext{ mAs}$ are likely overexposed, but a repeat depends on visible image quality and diagnostic necessity.
• CR Room Examples (Constant $60 ext{ kVp}$):
  • $1 ext{ mAs}$, S=$447$ (Underexposed, S-value should be lower for acceptable exposure).
  • $4 ext{ mAs}$, S=$170$ (Acceptable range for S-value).
  • $20 ext{ mAs}$, S=$30$ (Overexposed, S-value should be higher for acceptable exposure).
  • Trend: As $ext{mAs}$ increases, S-value decreases, indicating higher exposure (inverse relationship).
  • Repeats: Image at $1 ext{ mAs}$ suggests underexposure and $20 ext{ mAs}$ suggests overexposure, depending on visual quality.

How Do You Know You Created a Good Image?

Differential Absorption

• Necessity: Differential absorption is essential to visualize details in an x-ray image.
• Definition: Different tissues absorb x-ray photons differently.
• Key Concepts:
  • Absorption: X-ray photons removed from the beam when their energy is taken up by matter (patient or object).
  • Scatter Radiation: Interaction where photons emanate from the patient in all directions. It degrades image quality.
  • Attenuation: The process of reduction of x-ray photons as the beam penetrates matter. It is a combination of absorption + scatter.
  • Differential Attenuation: Different tissues attenuate photons differently, which creates the image.
  • Transmission/Penetration: The passage of x-ray photons through an object without interaction.

X-Ray Image Quality

• Refers to the factors that make a radiograph visible and accurate for diagnosis.

Properties of the Radiographic Image

1. Brightness (digital term; previously Density for film).
2. Contrast.
3. Spatial Resolution.
4. Noise.
5. Distortion.
6. Artifacts.

Exposure Terminology: Brightness (Digital Terminology)

• Definition: Refers to the bright or dim areas of an image when viewed on a monitor.
• Relationship to Exposure: Increased brightness = whiter area of the image = less exposure to the IR.
• Causes:
  • Underexposed: Too few photons struck the IR
    ightarrow image appears brighter/whiter.
  • Overexposed: Too many photons struck the IR
    ightarrow image appears darker/less bright.
• Control: Primarily controlled by the computer (post-processing) within digital systems. It is influenced by kVp and $ext{mAs}$ selections but can be changed by the MRT after exposure.
• Specificity: This term is specific to digital and computed radiography only. Film radiography uses