Topic: X-ray 'calibre' determination of in situ projectiles, revisited

[Brandon Bertolli]

Dear All

Some discussion was had in the thread on windshield perforation angles:

http://www.afte.org/forum/smf1/index.php?topic=3720.15

I have had very little time to explore my various interests in gunshot wounds and associated forensic investigation in recent years because of a new job that took all my time (yes even after hours).

I have resigned from that job and now I have time to get into this again.

I purchased Brian J. Heard's book yesterday from Waterstones.

http://www.waterstones.com/waterstonesweb/displayProductDetails.do?sku=6335240

There is a section on the radiographic determination of a bullet's calibre, with methodology and photographs previously described to me by Evan Thompson therein.

I regard Evan as a friend, and we have discussed this method before. I have some concerns about it and I would like to investigate this further from the perspective of a radiographer.

What I would like to know first, though, is whether this method has been used to produce evidence that has been pivotal in the outcome of a case. It is all very well for me to be critical of a thing, but my observations might not be relevant if these measurements alone do not sway a case.

Coming back to this matter after a break of a few years has been helpful, as I can see that there are ways in which I can better express my concerns than I have previously (particularly with ray-tracing software which is ideal in some cases to illustrate X-ray imaging pitfalls).

More later, once I hear from you folk whether any cases have been made or broken by X-ray calibre determination as described to me by Evan Thompson and related again in Brian Heard's book.

As always, many thanks for your time.

[Bill Wheatley]

Brandon,

Reach out to Ann Davis (804-786-4707 ext. 26981). She did a presentation at the AFTE Eastern Regional Conference two years ago on X-ray caliber determination used in a case she did, which if memory serves me right was on a horse that was shot by a .303 British. Hope this helps.

Bill

[Brandon Bertolli]

Thanks Bill

I will do that.
I must add that I am not in the employ of, or offering advice to, any legal agency. This is a personal interest of mine.

[Evan Thompson]

Attached photograph depicts known caliber bullets (22 RNL far left to 45 ACP far right) to the question bullet next to the bone.  The standards were held in the same plane (emphasis added) next to the questioned bullet. (Additional positional X-Rays may be needed.) The final report would say something to the effect “that the bullet in question is consistent to a 38/9mm class bullet.”

The second composite photo is of the known standards, as well as a measurement of one of those standards on an X-Ray, which not surprisingly measures smaller than the known standard.  However, if done correctly, the questioned bullet in the same X-Ray would measure smaller as well.  

Not very high tech, but has assisted over the years to answer if the questioned bullet is caliber X or Y.    

[Bob Kennington]

"...What I would like to know first, though, is whether this method has been used to produce evidence that has been pivotal in the outcome of a case. It is all very well for me to be critical of a thing, but my observations might not be relevant if these measurements alone do not sway a case..."

Please give some consideration to the Peter's Methodology, which Al Boehm and I used with success. (And even works for those who are mathematically-impaired—like myself). 

A tip-o-the-hat to Richard Hitchcox for the pdf conversion of the file.

http://www.afte.org/forum/smf1/index.php?topic=4814.30

'Last I saw of my particular case (Victim: Sgt. Fred Pelny of Miami), it was headed for an appeal: Nobody, including myself, was called as a witness in that appeal.

"...More later, once I hear from you folk whether any cases have been made or broken by X-ray calibre determination as described to me by Evan Thompson and related again in Brian Heard's book..."

1) Looking at Evan's x-ray example, I'm really glad my case involved a 44-Special bullet!     

2) From the above link, I'm unclear if Evan wants his dial-caliper identified or not in an x-ray; if so, a series of holes can be drilled for identification purposes.

3) Untested (by me) is a less radio-opaque dial caliper—available from a Swiss manufacturer in a fiber-reinforced plastic.

[Richard Hitchcox]

The files were previously posted in a Member Forum so I am posting them again here since Brandon does not have access to the Member Forum.

Richard

 x-ray.pdf (1703.76 kB - downloaded 1081 times.)

[Brandon Bertolli]

The second composite photo is of the known standards, as well as a measurement of one of those standards on an X-Ray, which not surprisingly measures smaller than the known standard.  However, if done correctly, the questioned bullet in the same X-Ray would measure smaller as well.

This must be a typo: in some circumstances where a radiograph has been printed on hardcopy from a digital system, the reproduced image may be smaller than the actual item that was X-rayed. This is because of digital manipulation from the time the latent image was captured on the recording medium, to the time that the image was output to a printer.
In all other cases (I mean wet film), the X-ray image will be larger than the standard because the latent image is processed to visibility without any manipulation in between and therefore the inherent magnification effect of the X-ray imaging process is unaltered.

The standards were held in the same plane (emphasis added) next to the questioned bullet. (Additional positional X-Rays may be needed.)

I think while the intent is sound, the execution is what I have issues with. With the exception of palpable bullets, or bullets that are known to be in situ in the plane of a precisely identifiable external anatomical landmark, the plane of the bullet is being measured by an additional radiograph taken at 90 degrees to the first one.
The problem is that BOTH bullet image shadows are subject to the effects of beam projection. The second view is subject to beam projection effects and it is my assertion that this makes that measurement of the bullet plane (or distance of the bullet from the recording medium) inaccurate.

How inaccurate, and subject to what variables? Well, that's for me to prove. I can demonstrate the effect by means of raytracing software...I am working on it now.

Other concerns I have (which will affect the Peters method also):

1) How do you know whether the projectile is 'out of round' or not? (could be measuring a narrow diameter on a deformed projectile)

2) How do you know that the edge interval as measured on the radiograph is being measured accurately and consistently? Unlike physical standards, there is no physical 'stop' for the calipers and therefore they are being adjusted by eyeball. The problem that creeps in here is radiographic edge differences between the in situ bullet and the standards, which are affected by adjacent anatomy, movement artefact and differences to do with beam projection (whether the long axis of the standard and the long axis of the bullet in situ are parallel to one another AND the image recording medium).

3) Has anyone done any analysis of bullet profiles per calibre, to determine whether those can be used as an adjunct to the caliper measurements to assert or exclude the calibre of the in situ bullet? I am specifically worried about the length of the bullet and profile differences wthin the same calibre. I found these three 9mm projectiles in my drawer, and I have a very small collection, surely dwarfed by what AFTE members have access to:

[Brandon Bertolli]

Many thanks Richard and Bob for the files.
I bought two AFTE journal CDs a while back, and will read those files too.

[Bob Shem]

Brandon,

Thanks for expressing your concerns about the methodology.  The questions you raise would need to be answered before a skeptical juror would (or should) accept an expert opinion.

[Brandon Bertolli]

The questions you raise would need to be answered before a skeptical juror would (or should) accept an expert opinion.

Who asks the questions?
In fact, have the questions been asked and answered at all?

[Bob Shem]

Let me rephrase that.  

The concerns you raise would need to be addressed . . .

(the concerns in your post four posts back)

Who asks the questions?

Anyone who has doubts about an opinion which declares the caliber of the bullet in the radiograph.  This would include attorneys, judges, juries, forensic scientists, etc.  The main question would be: "How can you assure me that the opinion is reliable?"  This can be demonstrated by validating the procedure, conducting proficiencies, etc.  Like any examination it boils down to "prove it to me".

[Brandon Bertolli]

I'll come back to the issue of 'validating procedures and conducting proficiencies' later.

The main concern I have is that there are X-ray beam geometry effects that aren't being taken into account before the various measurements are done, to arrive at a calibre determination by means of radiography. I have set up a scene in a raytracing program to produce images that may make it easier to understand this phenomenon. The divergent nature of the X-ray beam can be modelled by light rays and I am confident that the images below can be duplicated on an X-ray imaging system. These images demonstrate magnification and beam projection effects when the variables being altered are:

1) Subject to film (or digital recording medium) distance
2) X-ray source to film (or digital recording medium) source
3) Imaging via central ray vs rays not at 90 degrees to the subject and film (or digital recording medium)

I have made all the elements as accurately as I can in the short space of time I have allocated myself to get this out there. Depending on where this leads, I can spend more time on it. Feel free to invite any radiologist or medical physicist to participate, I welcome any criticism of my work here.

I made a 9 x 9 subdivided cube and set it up on a floor, which would be the position of the recording medium (or film). I set up a spotlight, aimed at the center of the cube, 90 degrees to the cube and film. Then I made nine identical spheres, each one fits exactly within any constituent cube of the model. Here is a the model with the spheres lined up in a row, occupying the lowest central row of the cube:



Here is a front view of the setup, zoomed out to see the X-ray source:



This represents the best possible circumstances under which these spheres could be X-rayed, because the central ray of the beam is aimed at the central sphere, the row of spheres is placed directly on the film and the X-ray source is relatively far away from the film, which means that magnification effects are reduced. Note also that this model being presented here assumes a point X-ray source. In reality there is no such thing as a point X-ray source. The source of X-rays is a small area, usually in the order of 1mm squared or less.

I use that cubed matrix to position the spheres, and then I make it invisible for the render, otherwise the image is too busy. I have duplicated the bottom 9 x 9 matrix and left that visible so that you can see where the resultant shadows of the spheres sit on the film, relative to the position of the spheres in the cube.

Here is the render of the best case scenario, from above (viewed from X-ray source towards the film):



The spheres are rendered with a candy texture to make shadows more apparent. Here you can see small shadows projected away from the spheres at the beginning and end of the row.

Next, the spheres were moved away from the film, to occupy the uppermost central row in the cube:



The X-ray source hasn't moved, only the spheres have:



Here is the resultant render:



Note how the shadow is now visible in association with all the spheres. Note also that on the final radiograph, you would not see the candy textured spheres, all you would see would be the shadows (usually in white, not black). See my avatar for an example of a bullet shadow near a person's hip.

There are two critical observations:

1) The X-ray shadows are larger than the spheres. This happens with all X-ray imaging that has not been digitally manipulated.
2) The shadows are not central to each sphere (except for the one that was in the path of the central ray). The shadows are projected away from the spheres in the direction of (and subject to) the oblique rays of the X-ray beam.

The next variable we can change is the X-ray source to film distance (in the US it is commonly called SID, and in the UK and SA it is commonly called FFD). If we reduce the SID as follows:



We get this render:



The peripheral spheres have now been X-rayed by more oblique rays than they were previously. Their shadows are subject to increased beam projection effects. They are more magnified, the shadows are more offset with respect to the actual position of the sphere relative to the grid and even more worrying: the profile of the shadow has changed!
To make it more apparent we can bring the X-ray source in even closer as follows:



And then the render is this:



The beam projection and magnification effects are so gross now, that only 4 of the shadows can be seen in the render view and depending on the size of the film or recording medium, shadows could be lost off the film. In trauma radiography this can result in failure to visualise projectiles that are in situ, in obese patients when the projectiles are anterior and the patient is X-rayed supine (with limited operation SID). Further views would have to be done to ensure all skin margins were included to make sure projectiles weren't being missed.

The effect I want you to take note of is the elongation effect of the sphere shadows, where the spheres were not in the path of the central ray.

Remember this is still a very easy setup. The row of spheres is central to the cube and to the X-ray beam. What about a single sphere, placed somewhere in the cube?



If we go back to maximum SID, here is the render:



This equates to real life circumstances where the position of the sphere is not known. Remember you would not see the sphere, you would only see the shadow.

The next two images show the position of the shadow if the SID is reduced by half, as was done with the row of spheres above.





Do you see what has happened? The shadow of the sphere is projected left AND down, not just in a horizontal direction as before. This is because the sphere is not located centrally in the cube (neither in the X nor z plane). Note also the distortion of the shadow because of elongation effects of the X-ray beam.

Continued....

[Brandon Bertolli]

If we switch to a generic bullet shape, then the sequence is more interesting. First, the best case scenario with a row of identical bullets placed directly on the film, with the middle bullet being imaged by the central ray:



Here is the render with hardly any shadows to be seen:



Bullets raised up to occupy the uppermost row in the cube:



The render:



And then a sequence of renders with the SID reduced by half each time:





Note in the last render, the marked profile change of the bullet shadows even though these are identical bullets in the same plane relative to the film! All but the central bullet shadow are affected. Again we lose the peripheral bullet shadows and if we want to see these we have to zoom the camera out artificially:



There is marked change in the profile, position and magnification of the bullet shadows. This effect on a lesser scale is applicable to a row of standards and in my opinion the method with the row of standards should be scrutinized because of the effects of beam geometry as described here.

Here is a sequence with a single bullet, placed in the same position as the single sphere in my previous post. First the X-ray source is moved back to maximum SID:



Here is the render at maximum SID:



Here is a sequence of two images where the SID is reduced by half each time:





Note the position of the bullet shadow relative to the actual position of the bullet in the grid. The shadow is registered on the film AFTER beam projection affects as a result of the bullet not being in the path of the central ray.

This leads to a very pertinent observation. Whether you have two X-ray views at 90 degrees to one another or not, both views of the bullet (both shadows) are subject to beam projection effects. Using the second radiograph to validate the position of the bullet relative to the film in the first radiograph is inherently flawed!

Compare the position of the bullet to its shadow (relative to the bottom of the grid if you imagine this was a side view, not a top view) and tell me whether you are still confident that the measurement of the shadow to the edge of the grid is NOT going to give you a false number with which the final magnification factors are rendered inaccurate? (In other words the position of the standards has not necessarily been in the same plane as the projectile in situ because of measurement errors).

Then you have to consider the profiles. They change with beam projection and the position of the standard in the row.  They cannot all be X-rayed by the central ray, so somewhere there is going to be an error.

When considering the range of profiles that can be produced by beam geometry, AND the range of profiles that exist within the class of bullet being identified, how confident are you that the correct calibre determinations are being made based on the method that I have seen described in Brian Heard's book?

With this in mind I would be very interested to know what validation and proficiencies have been done to render these concerns null?

Of course I will also be interested if someone reckons I am just sprouting nonsense