Equipment should be tailored to the needs of patients imaged in the angiography suite. A review of the major imaging components is presented in the following text. The basic management of these components, for example, optimization of radiographic techniques used during the acquisition of images from the production of x-rays, is briefly discussed.

1. Purpose

The generator provides electrical energy from which x-rays are generated. It also controls the production of x-rays.

2. Image acquisition controls

Good image quality requires precise control of the production of x-rays. Lower dose fluoroscopy should be used during catheter placement, whereas a higher dose fluoroscopic mode may be needed for critical positioning of wires and catheters. A radiographic mode is typically provided to create higher quality images for interpretive and archival quality.

a. Tube voltage (measured in units of kVp, kilovoltage peak) determines the kinetic energy of the electrons reaching the anode of the x-ray tube, the energy level of each x-ray in the beam, and the penetrating capability of the x-ray beam. The relative frequency of photoelectric effect and Compton-scattering photon interactions in tissue, determined by x-ray energy, affects both patient radiation dose and subject contrast in the image. The x-ray tube potential should be in the 60 to 80 kVp range to match the effective energy of the x-rays to the k-edge of iodine which improves subject contrast. Tube voltages less than 60 kVp lead to excessive patient radiation dose and should be avoided (1).

b. The tube current (measured in units of milliamperes, mA) determines the flow of electrons from the cathode to the anode of the x-ray tube, which determines the quantity of photons in the beam. The total energy in the beam depends on the number of photons (mA) and the energy carried by each photon (kVp). Tube currents range from 10 to 1,000 mA depending on the size of the selected focal spot size (2).

c. The pulse width (measured in units of milliseconds) is the duration of the exposure. The pulse width should range between 3 msec and 10 msec to adequately freeze motion during imaging. The maximum pulse width for children should not exceed 6 msec (1). The maximum tube current is used to minimize the pulse width when imaging large patients. Small tube currents are used with small body parts to maintain kVp values greater than 60 to avoid excessive patient radiation dose.

d. The pulse rate (pulses per second) is the rate at which images are created. This rate should be proportional to the rate of motion of imaged anatomic structures. Capturing the sequence of motion of rapidly moving objects is called temporal resolution. Fluoroscopic pulse rates range from 30 images per second (pediatric interventional imaging) to 1 to 4 images per second (nonvascular studies). Angiographic pulse rates range from 0.5 to 6 images per second (1). Lower frame rates reduce radiation dose to patients and staff.

e. The size of the focal spot (measured in millimeters) determines the geometric unsharpness in the image, which determines resolution in the image. A smaller focal spot size provides sharper vessel borders, but this improvement must be balanced against less sharpness due to motion that results from the longer pulse widths required by the reduced tube current of the smaller focal spot.

f. Beam filtration is the thickness of filter material inserted in the x-ray beam prior to the patient. Added filtration removes low energy photons from the beam, reduces the patient’s radiation dose, and improves image quality if the tube voltage is reduced to generate a more monoenergetic x-ray beam (3,4). Typical filters range from 0.1 to 0.9 mm of copper. Some manufacturers have chosen to use filter materials other than aluminum such as k-edge (higher z) materials.

g. Dose rate (µGy per image) at the image receptor determines the total number of information carriers used to create the image. An increase in kVp, mA, or pulse width increases the number of photons, reduces image noise due to quantum mottle, and increases the radiation dose rate to the patient.

3. Design

The mid- to high-frequency inverter is the most common generator design due to its low manufacturing cost and compact design.

a. Reproducibility and linearity of x-ray production are improved due to closedloop regulation of the tube current and high voltage, with response as rapid as 0.2 millisecond (2).

b. Automatic recalibration of the x-ray tube as it ages maintains accurate values of the radiographic technique factors.

c. Eighty to 100 kW of power is necessary to penetrate large patients in oblique projections with a low tube voltage, high tube current, and short exposure time. These factors are necessary to visualize small vessels with minimal motion artifacts during rapid image acquisition sequences (3).

4. Hierarchy of adjustment of image acquisition controls

a. When more or less radiation is required at the image receptor due to changes in the thickness of the patient, the generator should follow the sequence below.

(1) Adjust tube current. If additional adjustment is required, the generator should next,

(2) Adjust the pulse width. If additional changes are needed, the generator should next,

(3) Adjust the tube voltage. The tube voltage is adjusted last to maintain appropriate contrast levels in the image.

b. These acquisition parameters are unique as a function of

(1) Type of angiographic study

(2) Size of the patient

(3) Fluoroscopic mode versus image archive. These two modes utilize radiation doses at the image receptor that differ by at least a factor of 100 (1).

5. Configuration of acquisition parameters

Currently, no manufacturer’s “state-of-the-art imagers” automatically provide a sufficient range of radiation output to properly image both the largest and smallest patients (1). In some cases, anatomic program capabilities of the generator allow the selection of appropriate combinations of the image acquisition controls to overcome this deficiency.

6. Control console display

Ideally, the control panel should provide a real-time display of all of the acquisition parameters during exposure of the patient. This feature allows the technologist to monitor the performance of the imager during the progression of the examination with respect to the size of the patient on the table.

1. Basic design

The primary components of the x-ray tube consist of a tungsten filament cathode and a spinning anode disk with a tungsten surface. Electrons are boiled off the filament, accelerated to the anode by the tube potential, and stopped by the tungsten surface of the anode. This process converts approximately 1% of the kinetic energy (energy of motion) of the electrons to x-ray energy. The remaining energy is converted to heat at the point of collision on the tungsten anode.

2. Focal spot sizes

a. Multiple focal spot sizes are provided.

(1) Small spot: 0.4 to 0.6 mm with a kW rating of 30 to 50

(2) Large spot: 0.8 to 1.2 mm with a kW rating of 75 to 100

(3) Third spot: 0.3 mm with a kW rating of 10 to 20

b. The choice of focal spots must balance the need for minimal geometric unsharpness (small spot) against the need for minimal motion unsharpness (large spot). The following nominal focal spot sizes are recommended:

(1) Contact arteriography (magnification factors < 1.4) (1)

0.3 mm: infants and toddlers

0.4 to 0.6 mm: children up to small teenagers

0.7 to 1.0 mm: small to large adults

(2) For magnification arteriography

0.2 mm or 0.3 mm for 2 × magnification

0.1 mm for greater than 2 × magnification

3. Anode

a. The anode of the x-ray tube has a large heat load rating to allow the serial imaging techniques required in angiography. This is achieved by

(1) Reducing the anode angle

(2) Enlarging the length of the focal track traced out by the electron collisions on the spinning anode surface (diameter of anode)

(3) Increasing the size of the focal spot

b. The smallest anode angle that provides full coverage of the image receptor by the x-ray field should be chosen. For example, anode angles of 11, 9, and 7 degrees allow coverage of 15-in., 12-in., or 9-in. image receptors, respectively, with a typical source-to-image receptor distance (SID) of 100 cm (3).

c. Some manufacturers have increased the focal track diameter to 8 in. with advanced bearings at a lower speed of rotation (3,000 rpm). This increases loading and reduces the rotor noise. Rotor noise can be stressful during difficult, lengthy cases.

4. Collimation assembly

A collimator assembly is attached to the x-ray tube port from which the x-rays are emitted. This assembly contains adjustable beam blocking blades, selectable beam filters, and adjustable wedge filters (3).

a. The adjustable beam blocking blades shape and limit the area of the x-ray beam at the entrance plane to the patient. Newer units provide (additional cost option) a graphical display of the position of the collimator blades in the field of view while the operator positions the blades. This allows reduction of the area of the x-ray field without radiation to the patient.

b. Adjustable wedge filters reduce intense radiation areas in the beam to improve image quality and reduce patient dose. A graphical display, as described earlier, may be provided to eliminate additional patient dose during the adjustment of the position of these wedges.

c. Although thicker filters have a greater impact on patient dose reduction, the filter thickness must be reduced as the patient size increases to deliver a sufficient number of photons to the image receptor. Most new equipment automatically selects the largest available filter thickness that allows proper penetration of the patient. This frees the operator from managing this image acquisition parameter.

1. Pedestal base

Patient tables are typically floor mounted on a pedestal base. Manufacturers, in response to the growing girth of the largest patients, continue to increase the weight capacity of the tables.

2. Tabletop composition

Carbon fiber tabletops provide the strength required to support an adult cantilevered from the pedestal support while minimizing the attenuation of the diagnostic x-rays.

3. Tabletop dimensions

The length of the tabletop must accommodate the tallest patient. The width must accommodate the patient but be narrow enough to allow adjacency of the image receptor to the exit plane of the patient during lateral imaging.

4. Tabletop motions

a. Vertical motion: Motorized vertical motion sufficient to position any part of the patient’s body at the vertical isocenter of the imaging plane is necessary.

b. Float: The level tabletop must “float” when electromagnets are released to allow axial and transverse motion of the tabletop.

c. Stepping: The tabletop must shift (step) parallel to the axial axis of the patient with the moving bolus of contrast to allow lower extremity angiography. This feature is typically an additional cost option.

d. Tilt: The tabletop tilt ± 15 degrees with respect to level to properly support some interventional procedures (3). This feature is typically an additional cost option.

e. Cradle rotation: The tabletop may rotate the patient about the patient’s axial axis when supine. This feature is typically an additional cost option. It is typically used when the interventional unit is installed in the operating room.

1. X-ray tube/image receptor alignment

The gantry stand supports both the x-ray tube housing and the image receptor/imaging chain. The alignment of the central ray of the x-ray beam to the center
of the image receptor is maintained while the angle of the central ray changes within either the coronal or transverse plane of the patient’s body.

2. Linear movement of image receptor

Movement of the image receptor parallel to the central ray is accomplished by providing a variable SID of at least 90 to 120 cm (3). This allows the positioning of the input plane of the image receptor close to the exit plane of the patient to minimize geometric unsharpness in the image and to minimize the patient radiation dose.

3. Basic rotational design

While a number of different designs of the gantry are still present in the field (4), the majority of new equipment uses a C-arm geometry to achieve angulation in one dimension. When the x-ray tube and image receptor are rotated on a “C” within a C, both components are rotated about a true pivot point called the isocenter, with a fixed SID. These two design criteria are required to allow accurate rotational angiography and the production of cone beam computed tomography (CT) images from the angiographic device.

4. Single plane configuration

Single plane gantry stands are typically mounted from ceiling supported rails. This maximizes the travel of the gantry to/from the pedestal mounted patient table. This allows the gantry to be parked well away from the patient table in the case of emergency. Most manufacturers also offer their single plane gantry configuration mounted to a fixed location on the floor.

5. Biplane configuration

Biplane configurations, with the lateral plane assembly mounted on ceiling rails, are necessary for neuroangiography of adults and most angiography studies performed on children due to the child’s limited tolerance of iodine contrast media (4). A biplane configuration forces the frontal plane assembly to be mounted on the floor.

6. Robot configuration

At least one manufacturer (5) offers the x-ray tube and image receptor C configuration mounted on a programmable “robot” gantry. The robot is modified from automotive manufacturing. Robots allow more flexible, accurate, reproducible, and rapid motions of the C-arm support. Although this technology should increase the flexibility and applications of the imager, it also significantly increases the cost of the imaging system.

7. Rotational motions

In addition to imaging with the x-ray tube and image receptor stationary, C-arm gantries with a true isocenter allows two types of rotational angiography:

a. Rotational angiography is created by rotating the x-ray tube and image receptor about the isocenter, pulsing the x-ray beam, and collecting a series of two-dimensional (2D) images. The projected view on playback rotates about the patient anatomy at the isocenter.

b. Cone-beam CT uses the same acquisition protocol described earlier. The projection images use “cone beam” reconstruction algorithms to create tomographic slices through the volume. The quality of the tomographic images is not on par with those acquired with conventional CT scanners but can be obtained without transporting the patient to a CT scanner. A gantry rotation of ˜200 degrees in 3 to 5 seconds is required.

1. Number of monitors

a. Single plane configuration: Multiple individual monitors are necessary to adequately view the progression of the examination.

(1) One monitor provides live fluoroscopy.

(2) One monitor provides road maps, which are fluoroscopic images created with a limited amount of contrast media to illustrate the vasculature tree.

(3) One monitor displays patient images from other modalities, for example, three-dimensional, magnetic resonance imaging (MRI), CT, ultrasound (US), or plain radiographs.

(4) One monitor displays the patient’s real-time physiologic monitoring data.

b. Biplane configuration: When two planes of imaging are produced simultaneously, one needs a minimum of two, more typically four additional monitors.

(1) Six total monitors: The two additional monitors relative to the description earlier are used for live fluoroscopy and roadmaps of the lateral plane.

(2) Eight total monitors: The additional monitors are available to allow some combination of 3D, MRI, CT, US images, or plain radiographs to be displayed simultaneously.

c. An alternative is one large (˜60 in.) diagonal monitor. This single monitor is driven by multiple computer-driven inputs to allow the simultaneous presentation of the previously described images. The display system is calibrated with multiple hanging protocols, each one specifically designed to a different operator’s preferences for different types of clinical examinations.

2. Types of monitors

Liquid crystal display (LCD) flat-panel monitors are preferred due to their superior image quality and lack of bulk. The matrix size of individual monitors must be sufficient to display interventional images in full resolution (1,000 × 1,000 matrix). This matrix size allows display of all but plain radiographs at full resolution.

3. Support carriage

a. The support carriage is ceiling mounted on a long set of rails. A transverse set of rails or a long arm mounted on pivot allows movement of the carriage parallel to or transverse to the axis of the patient.

b. Transport of the carriage is designed so that it may be placed on the left side, right side, or the foot end of the patient. The access point of the catheter into the patient’s vasculature determines the correct location of the carriage for a given examination.

4. Patient dose display

Newer equipment displays two types of patient dose information on the monitor carriage (6). See Chapter e-99.

a. Air kerma is a quantitative indication of the ability of the x-ray machine to produce radiation at a specified location from the focal spot. Air kerma, measured in units of mGy, is the cumulative radiation delivered to the entrance plane of the patient’s skin (minus backscatter) during the examination. Medical physicists use this dose metric to estimate the risk of a deterministic radiation injury, for example, skin burn, to the patient.

b. The interventional reference point (IRP) (7) is the assumed entrance plane location for a typical adult relative to the focal spot of the imager. This point is 15 cm toward the focal spot from the isocenter along the central ray of the x-ray beam.