The X-ray and light beam alignment in diagnostic radiology

 SRIM - Society of Radiology and Medical Imaging,
National Congress of Radiology and Medical Imaging,
Craiova, Romania 2023

Abstract — The alignment of X-ray, light and detector fields is a crucial acceptability criterion for a radiological system to perform the exposure with all projections made geometrically correct, thus ensuring an adequately limited and predictable irradiation dose for the patient. However, collimation, according to the investigated tissue performed by the medical staff over misaligned fields, can cause overexposure of the patient due to the larger radiation area than the visible area that was taken into account. Furthermore, as a result of this exposure, a clinically relevant image is not obtained, and the radiological investigation procedure needs to be repeated, which attracts another dose of radiation to be sent to the patient. Therefore, for the radiological equipment, a seemingly simple but, in practice, comprehensive check can significantly reduce the delivered dose, including the unwanted effects of ionizing radiation on human tissue.

This paper exhibits and explains the delicate aspects that can negatively affect the work process associated with handling activity and applying the acceptability criteria for using radiological systems in the diagnostic radiology department. There is also mentioned one of the primary causative sources that can lead to this field alignment errors and the overexposure consequence to avoid its occurrence.

The last part of this paper gives a step-by-step example as guidance to check the alignment and centring of the radiological image fields along with the acceptable obtained values.

I. Introduction

For radiological equipment used in diagnostic radiology, one of the most important aspects related to the geometry of the image fields projected on the patient is the alignment between the X-ray field emitted by the tube and the visible light projection in order to know and define by medical staff that clinically exposed surface.

Also, the two overlapped fields of radiation and light need to be centered on the surface of the image detector, inside of so-called field of view (FOV).

Thus, the alignment of the radiation, light and detector fields practically makes the difference between having a radiographic image or not.

II. The X-ray and light beam projection system

The light and radiation collimation system, although it is an old situation and disseminated to the specialized public through various sources of information, on itemized level there are many aspects that possess hidden characteristics, or rather not taken into account.

To overlap the two fields of radiation and light, we generically have a classic arrangement of the X-ray tube that projects the photons through an exit window of the monobloc, respectively an adjacent light projector that through a guide of the wave provides towards the end a perfectly superimposed light field over the radiation one.

According to figure 1, the light projector is a box that is part of the collimator housing of the radiological system. This projector is constructed simply by a housing that contains a bulb and an optional internal projector mirror that aims to reflect focused light onto the projector's output window. The mirror of the projector is a curved one, and the reflection of the light is made precisely towards the output window. This precise guiding of light to the output window of the projector possesses a well-made design in relation to the model of the bulb used. Projector light bulbs are usually connected to molybdenum holders. Frequently the cable connections that power the lamp are positioned in specialized locations that are shielded from the heat emitted by the bulb.

Although the bulbs are part of a precisely aligned optical system, the light projector assembly allows that bulb to be replaced in the event of a burnout. Even if it seems a bit unfair to us as users, it would be much more advisable to change the entire light projector instead of just changing the bulb. However, having this facility to change only the bulb, it is mandatory to replace it with a bulb model exactly like the one provided by the manufacturer.

Sistem proiector al luminii

Figure 1: The protection mechanism of radiation and light fields.
A) X-ray tube filament, B) Anode of the X-ray tube, C) Window of the shielding dome of the X-ray tube, D) The radiation and light collimation housing, E) The light projector box, F) Mirror for light reflection inside the projector box, G) Optical axis of the mirror in the projector box, H) Reflection curve of the mirror in the projector box, I) Parabolic mirror for light reflection inside the collimation housing, J) Parent parabola curve for reflection of the mirror inside the collimation housing, K) Frame / Collimating blades for the light and X-ray fields, L) Radiation field and light field.
Source: Author

An important aspect that imposes an even greater responsibility on the bulb filament through its position and dimensions, is the presence of a main mirror inside the collimator housing of the radiological system. This mirror is also curved under a parent parabola, having the role of positioning the light field coming from the projector perfectly superimposed as direction and surface on the X-ray provided by the monobloc [10, 11, 12]. By the simple fact that this parabola has a large radius, at the level of observation it gives us the impression that it is a straight mirror, a fact that is far from reality.

III. The danger of fields misalignment

The light field represents, following the performed alignment process, the same surface that should be precisely irradiated by the X-ray beam. Ideally, this light field and the X-ray field should be identical in size and position. In practice, however, the way the light field is projected can be affected by the type of existing bulb, the size of the filament and its position (according to Figure 2). We consider that the mirror inside the manufacturer projection system is stable and cannot be adjusted in terms of orientation, nor obviously of its curvature. In Europe, the accepted limit level is below 2% of SID [2].

Becuri si filamente

Figure 2: Various types of bulbs.
Source: LiteSourceMedical, Barthelme, et al

Thus, by simply replacing the bulb as a result of its wear and burning, if this is not done by the person representing the manufacturer of the imaging system, the result will be fatal at the level of alignment of the image fields. The most common actions of changing the light projector bulb are carried out by responsible persons other than those accredited, and certainly using other bulbs than the exact one supplied by the manufacturer. As a result of this replacement, the light guide no longer follows exactly the way the manufacturer designed it, and the light projection is altered.

As a contribution to the danger of misalignment of X-rays and light fields, if the bulb is a different model than the one strictly assigned by manufacturer according to the calculation and design of the optical system, the result could be according to Figure 3, meaning the lack of a clearly projected light field on the irradiated tissue of the patient.

Defect aliniere 1

Defect aliniere 2

Figure 3: X-ray and light fields misalignment
A0 - Reference bulb in correct position, A1 - Longer bulb with different filament size and position, A2 - Shorter bulb with different filament size and position, B0 - X-ray field, C1 and C2 - The light field in the case of the long and short bulb, respectively.
Source: Author

IV. X-ray and light beam alignment

These guidelines must be followed before launching the acceptance test procedure that has the role of guaranteeing the correct operation of the radiological system.

4.1  Suspension Levels for Alignment

The suspension levels for fields alignment on X-Ray generator systems [2] are provided in Table 1

Table 1: Suspension levels for alignment. Table information source: Radiation Protection 162 (RP 162) – Criteria for acceptability via medical x-ray equipments

Physical Parameter

Suspension Level


General Radiography Systems

X-ray/light beam alignment

Misalignment in any direction >3% of focus image receptor distance


Light beam/bucky centering

Alignment of crosswire with center of Bucky >1% of focus-image receptor distance



X-ray/Image receptor Alignment

X-ray field extending beyond the image receptor >5mm on any side.

Chest wall side: distance between image receptor and edge >5 mm


Stereotactic biopsy tables

Accuracy of localization

Deviation in alignment > 1mm in X and Y or >3mm in Z.


CT Scanners

CT alignment lights

> ±5 mm


Scan Projection Radiography (SPR) accuracy

> ± 2 mm



Couch top alignment and index accuracy

Deviation > 2 mm from specified distance




4.2 Guideline for verification of fields alignment

In order for an X-ray machine to be authorized for use, it is necessary that immediately after monarization and configuration, as well as periodically at least once every year thereafter, a technical check is carried out which will end with the issuance of a official official verification report. This verification report will attest to the fulfillment of the acceptability criteria. And in order to verify the explained part of alignment and centering of the radiation, light beam and detector field of view (FOV), it is necessary to perform a simple radiographic exposure, in the void, without any patient or fantom. Then the measurements are carried out in order to compare with the limits mentioned in the acceptability criteria figured in section 4.1 of this paper. The meticulous process of exposure and measurement can be accomplished by following steps A-F as guidance [1].

A) Trial exposure of the radiological film-cassette detector

The film-cassette detector cassette with an unimpressed X-ray film is placed on the patient's table. Positioning is made centering the powered on light beam of the collimator, leaving about 3-5 cm free (ie. without exposure) to the edge of the cassette.

We set the distance between the focal point marked on the X-ray tube and film-cassette detector as 100 cm (SID = 100 cm).

Perfectly round metal markers are placed, one at each corner of the rectangular light footprint, in the area perfectly tangent to each pair of perpendicular sides. The perfectly round marker can be a large diameter coin.

Optionally, another round marker or any other object (eg: a set of keys or even a mobile phone) is placed in any lighted area of the box that is considered to be the clinically useful surface. It is not even remotely necessary to position a fixed marker in the center of the box (eg: the only motivation for doing this is that in that area of the center marker the film will be exposed less or not at all, and by default the negative developed film will be transparent in that FOV), thus creating the comfort and visibility of the text written on that film.

B) Delineation of the radiation field

Only the area exposed to X-rays is demarcated with the carioca. This area is the completely black one.

The black exposed area, in most cases, does not have the metal coin marks perfectly positioned on the corners of the rectangle exactly as they were located on the lightprint prior to exposure. Fact for which the two fields are denoted as follows:

  • the radiation field (the black one)
  • the light field (the one over which the coins were placed fixed in the corners of the illuminated cassette)

Consider the radiation field for the first frame on the film, as in the figure 4 below:

Trasare câmp radiații

Figure 4: Delineation of the radiation field
Source: Author

C) Delineation of the light field

The light field is framed, as the imaginary rectangle that would form the light imprint tangent to the round markers visible on the negative developed film. These transparent traces of the coins on the developed film will be in the corners of the rectangle in the exact version with which the cassette was placed on the patient's table before exposure.

The light field marking will look like the example in the image below.

Marginile câmpului luminos

Figure 5: Delineation of the light field
Source: Author

As indicated by the top of the carioca, the sides of the light field are tangential to the tracks of the metal coins.

The values of the distances between the approximately parallel sides that form the radiation field and the light field are noted with the carioca directly on the film.

Fill in the "Collimation and alignment" section of the technical verification bulletin, at the indicator where "light field - radiation field coincidence" is mentioned:

  • next to the imposed value <3% from the table (ie. 3 cm for SID = 100 cm) the larger value of the two obtained as a summation on the vertical axis and the horizontal axis is added (ie on the vertical we have 0.6 + 0.6 = 1.2 cm, and on the horizontal axis we have 0.8 + 1.1 = 1.9 cm. Thus, the larger value will be will be taken into account and written on the film as the appropriate measurement result (ie. 1.9 cm related to the horizontal axis);
  • next to the recommended maximum total value <4% (ie. 4 cm for SID = 100 cm) the result summing all the 4 existing values on each side is completed (ie 0.6 + 1.1 + 0.6 + 0.8 = 3.1 cm).

Valori câmp radiații și lumină

Figure 6: Denoting the difference between the radiation field and the light field
Source: Author

D) Marking of the "Film Center"

The center of the film, sometimes referred to as the receiving center is found at the intersection of the diagonals of the physically developed film. Thus, those diagonals of the film will be drawn with the carioca on the film, respectively, this intersection of the lines will be marked and named with the indication CF (Center of the Film).

Centru film

Figure 7: Marking the center of the film named CF
Source: Author

E) Marking the center of the light field

It is marked with the carioca on the film, the point CL (Center of Light field) as a result of the intersection of the diagonals of the rectangle called the Light Field and defined at section C of this paper.

Centru luminos

Figure 8: Marking the center of the light field referred as CL
Source: Author

Colț centru luminos

Figure 9: Indication of one of the two diagonals of the light field that determines the location of the CL point
Source: Author

F) Marking the center of X-ray field

The intersection of the diagonals of the radiation field defined at section B of this paper is marked with the name CR.

Centru radiații

Figure 10: Marking the center of the radiation field referred as CR
Source: Author

Centru câmp radiații

Figure 11: Indication of one of the two diagonals of the radiation field that determines the location of the CR point
Source: Author

G) Establishment of distances between the centers marked on the radiographic film

After identifying, marking and naming the 3 centers associated with the 3 fields on the developed film, namely "Radiation Center", "Light Center" and "Film/Cassette Center", the distances between them are measured with a standardized ruler.

These measured values are recorded with the carioca on the film:

  • Distance CR – CL (C. Radiation – C. Light) = 0.1 cm
  • Distance CF – CL (C. Film – C. Light) = 0.75 cm
  • Distance CF – CR (C. Film – C. Radiation) = 0.7 cm

These values are completed in the technical verification bulletin in the "Collimation and alignment" section with the secondary indicator entitled "Alignment of X-ray fields, light beam and image detector".

Alinierea și centarea masurate complet

Figure 12: Marking the distances between the centers marked on the film detector, that is, between the light center – CL, the radiation center – CR and the film cassette center – CF
Source: Author

V. Conclusion

  1. Changing the bulb in the light projection system must be done only by the person authorized for this action, and necessarily only with the same bulb model provided by the manufacturer. Any other bulb model of identical wattage placed in the connection slot, even if it fits, will alter the light guide and cause misalignment of the image fields.

  2. The step-by-step guide to the process of verifying the alignment and centering of the radiological image fields must be followed before the acceptance procedure begins; this representing an essential acceptability criterion, but still difficult to apply in the maintenance activity of the radiological system.

References and selected bibliography

  1. Radu Băzăvan, MED-INNO, Operational procedure PO 1-01-04 for carrying out manipulation activities, Revision 4, January 2023

  2. RP 162 (2012) Criteria for Acceptability of Medical Radiological Equipment used in Diagnostic Radiology, Nuclear Medicine and Radiotherapy, Radiation Protection no. 162 (RP 162), Directorate - General for Energy, Directorate D — Nuclear Safety & Fuel Cycle, Unit D4 — Radiation Protection, ISBN 978-92-79-27747-4, Luxembourg: Publications Office of the European Union, 2012

  3. IPEM (2005a) Institute of Physicists and Engineers in Medicine. Recommended Standards for the Routine Performance Testing of Diagnostic X-Ray Imaging Systems, Report 91. York: Institute of Physicists and Engineers in Medicine.

  4. IPEM (2005b) Institute of Physicists and Engineers in Medicine. Commissioning and Routing Testing Of Mammographic X-Ray Systems, Report 89. York: Institute of Physicists and Engineers in Medicine.

  5. EC (1997) - European Commission (1997) Radiation Protection 91: Criteria of acceptability of Radiological (including Radiotherapy) and Nuclear Medicine installations. Luxembourg: Office for Official Publications of the European Communities.

  6. EC (1998) - European Commission (1998) European Guidelines on quality criteria for computed tomography. Luxembourg: Office for Official Publications of the European Communities.

  7. EC (2006) European Commission (2006) European guidelines for quality assurance in mammography screening and diagnosis. 4th Edn. Luxembourg: Office for Official Publications of the European Communities.

  8. IAEA (2011), Quality Assurance Programme for Computed Tomography: Diagnostic and Therapy Applications. Vienna, IAEA (in press).

  9. IEC (2004a) International Electrotechnical Commission. IEC 61223-3-5 Ed 1.0: Evaluation and Routine Testing in Medical Imaging Departments - Part 3-5: Acceptance Tests – Imaging Performance of Computed Tomography X-ray Equipment. Geneva: IEC.

  10. Thorlabs Imaging Systems, USA, Specs and Focal Length of Reflective Collimators & Enhanced Aluminum Coating.

  11. Carl Zeiss, Education in Microscopy and Digital Imaging, Contributing Author: Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida 32310.

  12. Möller-Wedel Optical GmbH, Collimators with fixed focus setting, collimators with adjustable focus setting, reticle turret and double micrometer.