Radiologic Technology Chapters 7 and 8 Review

Radiographic and Fluoroscopic Equipment Design

The x-ray tube is protected by a lead-lined encasement known as the housing. This component is critical for safety and directionality. Inside this housing, the cathode (negative electrode) and the anode (positive electrode) are contained within a vacuum tube, characterized by a glass or metal envelope that maintains the vacuum environment necessary for electron travel. The process of heating the filament located within the cathode assembly to emit electrons is defined as thermionic emission. To facilitate the movement of these electrons toward the anode, a high voltage, measured in kilovoltage ($kV$), is applied. During this process, electrons travel from the cathode to the anode at approximately half ($1/2$) the speed of light. However, the resulting x-rays themselves travel at the full speed of light, which is approximately $186,000\,miles/sec$.

Filtration is an essential safety feature used to eliminate x-ray beams with long wavelengths. By removing these low-energy photons, filtration effectively decreases the patient's skin dose. Total filtration in a system is the sum of two components: inherent filtration, which is provided by the tube envelope itself, and added filtration, which typically consists of aluminum or copper sheets. To further manage the beam, beam restrictors are utilized to restrict the size and shape of the primary beam. The modern standard for this is the Automatic Collimator, which employs Positive Beam Limitation ($PBL$). $PBL$ technology automatically collimates the x-ray beam to match the specific size and shape of the image receptor being used, thereby increasing image quality and decreasing patient dose by minimizing the amount of scatter radiation produced within the patient's body.

Radiographic Equipment Mechanics and Movement

The x-ray tube is mounted on a system that allows for various directional movements to accommodate different clinical scenarios. Longitudinal movement refers to the tube moving the length of the table, while transverse movement refers to movement across the width of the table. Vertical movement defines the tube's ability to move up and down toward or away from the table. When the tube is angled toward the patient's head, the movement is termed cephalic, whereas angling toward the feet is termed caudad. To synchronize the Bucky tray with the horizontal movement of the tube, a feature called autotracking is used.

The patient table also possesses specific mechanical capabilities. The Trendelenburg position involves tilting the tabletop so that the patient's head is positioned lower than their feet. Within the table system, the Bucky tray holds the image receptor or detector. Positioned between the tabletop and the image receptor is the Bucky grid. A grid consists of lead strips separated by plastic or aluminum interspacing material. The primary function of the grid is to absorb scatter and secondary radiation before it can reach the image receptor, thus preventing image degradation.

Digital Radiography and Control Systems

The control panel for radiographic equipment contains several critical interfaces: the power switch, technical factor selectors ($mA$, time, and $kV$), focal spot selectors, and icons for anatomical identification. It also includes phototimer sensors, fluoroscopic controls, the rotor, and the exposure switch. Automatic Exposure Control ($AEC$), previously known as a phototimer, is an essential tool for maintaining consistency. Digital imaging is categorized into two main types: Computed Radiography ($CR$) and Digital Radiography ($DR$).

$CR$ utilizes special cassettes containing an imaging plate made of phosphors, specifically europium-activated Barium fluorohalide. This plate is processed by loading it into a $CR$ reader where it is scanned by a laser. In contrast, $DR$ utilizes Flat Panel Detectors ($FPD$) located within the tabletop. Both systems rely on an Analog to Digital Converter ($ADC$) to translate electronic signals into digital data. For data management and communication, the industry relies on several systems: Hospital Information Systems ($HIS$), Radiology Information Systems ($RIS$), and Picture Archive & Communication Systems ($PACS$). The universal language for these systems is $DICOM$ (Digital Imaging and Communication in Medicine), which allows equipment from different vendors to communicate. The $DICOM$ header contains vital metadata, including the patient name, Medical Record number ($MR\#$), birth date ($BD$), date and time of the exam, and the facility name. These records are integrated into the Electronic Health Records ($EHR$) for digital patient management.

Fluoroscopic Imaging Principles

Fluoroscopy is distinct from static radiography because it produces dynamic or moving images. These procedures are used to view physiologic events in real-time or to guide the insertion of medical devices. The fluoroscopic x-ray tube is typically located under the tabletop, and the system includes an Image Intensifier designed to increase the brightness of the image for better visualization. For bedside or mobile requirements, portable units or $C$-arm and $O$-arm mobile fluoroscopic units are used.

Radiation safety in fluoroscopy is managed through several mechanisms. Fluoroscopic controls include a cumulative timer that triggers a buzzer at $5\,minutes$ of beam-on time to alert the staff. To reduce the radiation dose during these procedures, techniques such as pulse fluoroscopy and last image hold are employed. Dose measurement is often tracked using a Dose Area Product ($DAP$) meter.

Physics of Radiographic Imaging and Attenuation

During exposure, several types of radiation are present. Primary radiation is the beam exiting the tube toward the patient. Radiation that exits the tube housing from parts other than the intended port is called leakage radiation, which the lead housing is designed to prevent. When x-rays interact with the atoms of the patient, they create secondary or scatter radiation, which causes a fogging or graying effect on the image. $X$-rays that successfully pass through the patient to reach the image receptor are known as remnant or exit radiation. Furthermore, backscatter radiation can hit the back of the image receptor and bounce back $180^{\circ}$ toward the receptor.

Attenuation refers to the process where x-rays are partially absorbed as they pass through matter. Materials such as bone or barium demonstrate increased attenuation and appear white on an image; these are termed radiopaque. Materials like air allow x-rays to pass through easily, demonstrating decreased attenuation and appearing black; these are termed radiolucent. The atomic numbers of materials significantly impact attenuation: Bone has an atomic number of $12$, Iodine is $53$, and Barium is $56$.

Exposure Factors and Image Quality

The four prime factors of radiographic exposure are milliamperage ($mA$), exposure time, kilovoltage ($kV$), and Source to Image Receptor Distance ($SID$). Milliamperage ($mA$) represents the amount of electrons passing from the cathode to the anode, while exposure time is the duration of this flow. The product of these two factors is $mAs$ ($mAs = mA \times \text{time}$). For example, a setting of $300\,mA$ at $0.25\,seconds$ results in $75\,mAs$. Furthermore, $400\,milliseconds$ is equivalent to $0.400\,seconds$.

$AEC$ (Automatic Exposure Control) automatically sets the exposure time based on patient thickness to provide the shortest possible exposure time and minimize motion. Medical conditions can require $AEC$ adjustments; for instance, pneumonia, which increases fluid in the lungs and decreases receptor exposure (appearing as white areas), might require a compensation of $+1$ or $+2$. Conversely, emphysema or $COPD$ increases air in the lungs, which increases receptor exposure and creates a blackened appearance. $mAs$ primarily controls the quantity of x-rays and the overall receptor exposure. $kV$ represents the speed or energy of the electrons and controls the penetrating ability of the beam. As $kV$ increases, the penetrating ability, quantity, and quality of the beam all increase. Optimal $kV$ settings vary by anatomy: Mammo at $28\,kV$, Hand at $50\,kV$, Knee at $70\,kV$, Skull at $80\,kV$, and Barium studies at $110\,kV$.

Quantitative Measurement and Image Processing

The standard $SID$ for most exams is $40\,inches$, though chest and lateral cervical spine exams use $72\,inches$. Radiographic quality is categorized into photographic properties (receptor exposure) and geometric properties (spatial resolution and distortion). Image quality is evaluated based on receptor exposure, scale of contrast, spatial resolution, distortion, and artifacts. The Exposure Index ($EI$) or Digital Exposure Index ($DEI$) measures the exposure to the receptor. In $DEI$ systems, yellow indicates underexposure, green indicates proper exposure, and red indicates overexposure. For $CR$, the specific $S$ number range for an abdomen is $100-400$ and for a general chest is $200-600$.

Mathematical relationships are critical in maintaining quality. The inverse square law states that the intensity of the beam is inversely proportional to the square of the distance ($I_1 / I_2 = (d_2 / d_1)^2$). If $SID$ is decreased by $1/2$, intensity increases $4$ times; if $SID$ is doubled ($2\times$), intensity decreases to $1/4$. The receptor exposure maintenance formula is a direct proportion used to adjust $mAs$ when distance changes. Processing involves the Look-Up Table ($LUT$), which adjusts brightness and contrast, and automatic rescaling to set the image to an acceptable viewing level. Window level controls image brightness, while window width controls contrast or grayscale. Underexposure (low $kV$ or $mAs$) results in quantum noise (mottle), appearing grainy, while overexposure results in increased scatter and a long scale of contrast.

Contrast, Resolution, and Distortion

Contrast is the visible difference between any two areas on a radiograph. Short scale contrast consists mainly of blacks and whites (low $kV$, more beam restriction, or grid use), whereas long scale contrast features many shades of gray (high $kV$). The chest region typically requires short scale contrast, while the abdomen requires long scale contrast. Spatial resolution is defined as the sharpness of structural edges. Unsharpness can be caused by motion (voluntary like pediatric/geriatric or involuntary like heart/peristalsis), material, or geometry. Resolution is optimized when pixel size is decreased and matrix size is increased.

Geometric unsharpness and spatial resolution are affected by focal spot size, Object to Image Receptor Distance ($OID$), and $SID$. For maximum resolution, one must decrease focal spot size, decrease $OID$, and increase $SID$. Distortion is the misrepresentation of size (magnification) or shape. Size distortion decreases as $SID$ increases and increases as $OID$ increases (e.g., using $72\,inches$ for a chest to minimize heart magnification). Shape distortion manifests as elongation or foreshortening. To avoid this, the central ray must be perpendicular and centered to the part and receptor, and the tube must be parallel to the part and receptor. Conversely, shape distortion can be used intentionally by angling the x-ray tube or the body part.