Image Acquisition & Technical Evaluation Part 1

Body Habitus and Anatomical Classifications

• Definition of Body Habitus: Refers to the general size and shape of a patient. There are four primary classifications:     

• * Sthenic: Represents the strong, active, average patient type.         * Heart: Moderately transverse.         * Lungs: Moderate in length.         * Diaphragm: Moderately high.         

• * Colon: Spread evenly with a slight dip in the transverse colon.         * Gallbladder: Located in the right upper abdomen.    

•  * Asthenic: Characterized as small and frail.         * Heart: Nearly vertical and positioned in the midline.         * Lung: Long, with apices situated above the clavicles; may be broader at the base.         * Diaphragm: Low.         * Colon: Low and folds on itself.         * Gallbladder: Lower and closer to the midline.     

• * Hyposthenic: Describes a thin but healthy person (slender).         * Status: Positioned between the sthenic and asthenic types.         * Anatomy: Similar to the asthenic type, but the stomach, intestines, and gallbladder are situated fairly high in the abdomen.    

•  * Hypersthenic: Characterized by a big frame, often overweight; also known as the Bariatric patient.         * Heart: The axis is almost transverse.         * Lungs: Short, with apices lying near the clavicles.         * Diaphragm: High.         * Stomach: High, transverse, and located in the middle.         * Colon: Positioned around the periphery of the abdomen.         * Gallbladder: High.

Factors Influencing Radiographic Technique

• Muscle Tone: Affects the use of technique (muscular vs asthenic)

• Pathology Influences: Three specific items influence radiographic technique selection:     

  • 1. Type of pathology.    

  •  2. Size of the pathology.    

  •  3. Composition of the pathology.

  •  * Pneumonia.     * Emphysema.

• Special Considerations and Cases:     

• * Trauma cases.     

• * Pediatrics (Children).    

• * Geriatrics.    

• * Casts.     

• * Contrast agents.    

•  * Body Mass Index (BMI).     

• * Specific anatomical regions such as the Colon and Rectum.

Image Quality and Characteristics

• Brightness: The amount of light transmitted by the monitor combined with light reflected off the monitor that affects image appearance on a display. It is controlled by monitor functions and post-processing functions.

• Contrast: The ability of a digital system to display changes in grayscale values.     

• * High Contrast Resolution: Demonstrates more shades of gray, allowing for the differentiation between small differences in densities.    

•  * Enhancement: Contrast is enhanced by dynamic range and post-processing.

• Subject Contrast: Refers to the various body tissue densities and thicknesses, also known as differential absorption.

• Spatial Resolution: The ability to render small objects on the image (image detail).     * Metric: Described by the quantity "spatial frequency" and measured in $lp/mm$.     

• * Digital Radiography (DR): Determined principally by pixel size.     

• * Computed Radiography (CR): Determined by phosphor layer thickness and pixel size in PSP systems. A thinner phosphor layer results in higher resolution.

• Distortion: Misrepresentation of object size and shape on an image.     

• * Reduction: Reduced by positioning the part parallel to the Image Receptor (IR) and perpendicular to the Central Ray (CR).     

• * Elongation: The object or part of interest appears bigger or longer than normal.     

• * Foreshortening: The anatomic part appears smaller or shorter than normal, usually due to poor alignment of the body part; fixed by careful positioning.

Exposure Factors (Technical Factors)

• mA (Milliampere): Determines the number of electrons boiled off the filament via thermionic emission.     * Function: The controlling factor of x-ray intensity (quantity).     

• * Limitation: A change in mA does not change the kinetic energy of electrons flowing from cathode to anode; it only changes the number of electrons.

• Exposure Time: Usually kept as short as possible to reduce patient motion blur.

• kV (Kilovoltage): Controls x-ray beam energy.     

• * Beam Quality: Refers to beam penetrability.     

• * Mechanism: An increase in $\text{kVp}$ equals an increase in the kinetic energy of electrons from cathode to anode.     

• * Effects: Affects both quality and quantity; influences the scale of contrast.     

• * Digital Considerations: Post-processing is the main contrast enhancement factor, while $\text{kVp}$ is the main factor for patient radiation dose.     

• * Trade-off: Advantage of a higher penetrating beam vs. the disadvantage of increased scatter on the image.

• The 15% Rule: Used to maintain the same exposure:     

• * To maintain exposure while increasing $\text{kVp}$: Increase $\text{kVp}$ by $15\%$ and cut $\text{mAs}$ in half. * To maintain exposure while decreasing $\text{kVp}$: Decrease $\text{kVp}$ by $15\%$ and double the $\text{mAs}$.

• Exposure Indicator (EI) / Deviation Index (DI):    

• * EI: A number indicating how much radiation reaches the image detector; shows if "dose creep" is occurring.     

• * DI: Indicates how far the exposure was from the ideal exposure for that specific exam; serves as a warning for dose creep.

• Dose Creep: The gradual increase in radiation exposure over time because digital images can still look "diagnostic" even when significantly overexposed.

• SID (Source-To-Image Distance): The distance from the tube to the IR.     

• * Energy Impact: Has no effect on radiation energy.     

• * Rule: Governed by the Inverse Square Law.

Scattered Radiation and Grids

• Scatter Characteristics: Emitted from the patient in all directions. It is a source of exposure to technologists and radiologists.     

• * Disadvantage: Reduces radiographic contrast and provides no useful information.     

• * Factors Affecting Scatter: $\text{kVp}$, field size/beam restriction, and patient/part thickness.

• Grids: Positioned between the patient and the IR to improve contrast by "cleaning up" scatter radiation.     

• * Requirement: Recommended for body parts measuring greater than $10\,cm$.     

• * Efficiency: Can absorb $80\%$ to $90\%$ of scatter. Designed to transmit only x-rays traveling in a straight line from source to IR.

• Grid Characteristics:     

• 1. Grid Ratio: The height of the grid strip ($h$) divided by the interspace width ($D$). Formula: $Ratio = h/D$. Ratios range from $5:1$ to $16:1$. High-ratio grids are more effective at scatter reduction but require more precise positioning.    

•  2. Grid Frequency: The number of grid strips or lines per centimeter ($cm$). High frequency grids show less distinct lines but have thinner interspace strips and higher ratios, leading to higher patient dose.     

• 3. Grid Material: Lead is used for strips; interspace material is typically aluminum or plastic fiber.

• Grid Cutoff: The undesirable absorption of primary or useful x-rays by the grid. Often caused by inadequate SID or misalignment.

Grid Types and Error Classifications

• Parallel Grid (Linear Grid): The simplest type; cleans up scatter in one direction. Primarily used with short SID or large-area IRs.

• Crossed Grid (Cross Hatch Grid): Consists of two linear grids placed perpendicular to each other. More efficient than linear grids and better for higher $\text{kVp}$, but prone to grid cutoff.

• Focused Grid: Designed with lead strips angled to match the divergent x-ray beam to minimize cutoff. Must be used at specific SIDs; high-ratio versions have less positioning latitude.

• Moving Grid (Potter-Bucky Diaphragm): A mechanism that moves the grid during exposure to blur out grid lines. The "Reciprocating Grid" is the most common motor-driven type.

• Stationary Grid: Used for mobile and cross-table radiography; usually low-ratio.

• Virtual Grids: A digital reconstruction technique that identifies incident x-ray energy/frequency to produce a radiograph with fewer scatter interactions. It reduces patient dose, improves contrast, and eliminates artifacts.

• Grid Errors and Results:     

• * Off-Level: Tube improperly positioned; results in grid cutoff across the image (underexposed/light image).     

• * Off-Centered (Lateral Decentering): Central Ray not positioned under the center of the grid; results in grid cutoff across the image.     

• * Off-Focus: Incorrect SID used (critical for high-ratio grids); results in grid cutoff toward the edges of the image.     

• * Upside Down: Severe grid cutoff toward the edges of the image.

Filtration and Equipment Factors

• Filtration: Functions to absorb low-energy photons ("soft" x-rays) to reduce patient dose. High-energy x-rays that pass through are "hard."     

• * Requirement: Total filtration must be at least $2.5\,mm$ Aluminum ($Al$) equivalent.     

• * Total Filtration: Calculated as Inherent + Added filtration.

• Compensating Filters: Aluminum or plastic tools shaped to provide uniform density when imaging irregular anatomy.

• Beam Restriction: Improves image contrast and reduces patient dose.

• Anode Heel Effect: Higher radiation intensity on the cathode side and lower on the anode side.     

• * Relationship: A smaller anode angle results in a greater heel effect.

• Generator Type: Most modern equipment uses high-frequency generators, which improve image quality and decrease patient dose via constant voltage potential.

Computer and Digital Systems Terminology

• Bit: A single unit of data; the smallest increment of data on a computer.

• Byte: Made up of $8$ bits.

• Pixel (Picture Element): The smallest element in a digital image. More pixels equate to better image resolution.

• Pixel Size: Directly related to spatial resolution. Smaller pixels result in greater detail.

• Pixel Pitch: The distance from the center of one pixel to the center of the next.

• Bit Depth: The number of bits stored per pixel, defining available shades of gray. Formula: $2^n$ (where $n$ is bit depth). For example, a bit depth of $8$ provides $2^8 = 256$ shades of gray.

• Matrix: A square arrangement of numbers in columns and rows corresponding to pixels. Typical ranges are $512 \times 512$ to $1024 \times 1024$. As matrix size increases, pixel size decreases.

• Post-processing: The ability to manipulate the image after exposure.     

• * Window Level: Controls image brightness; a higher level makes the image darker.     

• * Window Width: Controls contrast; a wider width results in lower contrast.

• Resolution vs. Visibility:     

• * Resolution: Detail level; high resolution is sharp/fine, low is blurry.    

• * Visibility: Perception of structures; good visibility allows anatomy to stand out clearly.

• Automatic Rescaling: The computer's effort to "fix" exposure errors to maintain uniform density and contrast.     

• * Too little exposure: Causes quantum mottle.    

• * Too much exposure: Causes loss of contrast.

Automatic Exposure Control (AEC) and Technique Selection

• AEC: Regulates the amount of ionizing radiation delivered to the IR.     

• * Types: Phototimers and Ionization Chambers.    

•  * Positioning: Exact centering is critical; improper cell selection leads to poor results.

• Technique Charts:     

• * Variable: $\text{mAs}$ is fixed, and $\text{kV}$ is increased as part thickness increases.     

• * Fixed: Specifies $\text{kV}$ for each part, and $\text{mAs}$ compensates for thickness.     

• * AEC Charts: Automatically adjust exposure for thickness/density; requires the View of Interest (VOI) to be centered.

• Anatomically Programmed Radiography (APR)/technique: Uses a microprocessor with internal technique charts. The technologist selects the anatomic part and size on a console, and the unit chooses the $\text{mAs}$ and $\text{kV}$ algorithm.

• Extended Dynamic Range (EDR): Digital systems can compensate for approximately $80\%$ underexposure and $500\%$ overexposure.

Questions & Discussion

• Question: What interactions make up differential absorption?

• Answer: Photoelectric interactions, Compton interactions, and transmitted interactions.

• Question: What type of exam might you want "distortion"?

• Answer: Skull (Towne’s view/AP axial), Elbow (radial head), Foot (AP with angle), and Calcaneus (axial projection).

• Question: What is another name for "technical factors"?

• Answer: Exposure Factors.

• Question: What is the law that states you can have any combination of mA & exposure time to produce identical receptor exposure?

• Answer: Reciprocity Law.

• Question: What 3 factors affect scatter radiation?

• Answer: $\text{kVp}$, field size/beam restriction, and thickness of patient/part.

• Question: What type of radiographic procedure can take advantage of the heel effect?

• Answer: T-spine, Femur, and Santa’s belly (KUB).

• Question: Name a type of pathology that would require a decrease in technique.

• Answer: Emphysema, Osteoporosis, and Degenerative arthritis.