X-ray Question from Berkley (MCAT)

[Raindrop3]

Could someone please explain this? I've been stuck on trying to make sense of it! Thank you for any thoughts on this.

1) Why do bones show up on medical images more clearly than surrounding tissue?

A) Calcium has a larger atomic radius than carbon, nitrogen and oxygen
B) Calcium has a smaller atomic radius than carbon, nitrogen and oxygen
C) Calcium reflects X-rays more so than carbon, nitrogen and oxygen do
D) Calcium refracts X-rays more so than carbon, nitrogen and oxygen do

Answer is A: I thought that if the atoms were smaller, then the aperture/spacing for the X-rays to diffract would be greater?


2) To analyze blood flow using X rays, a dye is added to the blood. What is true in this scenario?

A) The dye should have an atom with a larger atomic radius than calcium; a longer X-ray can be used than normal
B) The dye should have an atom with a smaller atomic radius than calcium; a longer X-ray can be used than normal
C) The dye should have an atom with a larger atomic radius than calcium; a shorter X-ray can be used than normal
D) The dye should have an atom with smaller atomic radius than calcium; a shorter X-ray can be used than normal

The answer is A : Because the atomic radius is larger than calcium, it can form bonds that diffract wavelengths than calcium bonds can diffract. As a result, a longer X-ray can be used than what is normally used for Calcium. This allows the dye to show up without bones appearing to a notable extent.

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[Cornfed101]

The idea behind X-rays is the larger atoms absorb more of the X-rays. If the atoms are bigger than calcium you can use a longer x-ray wave and it basically is too long for the bones to absorb.

 

[aldol16]

1) Why do bones show up on medical images more clearly than surrounding tissue?

A) Calcium has a larger atomic radius than carbon, nitrogen and oxygen
B) Calcium has a smaller atomic radius than carbon, nitrogen and oxygen
C) Calcium reflects X-rays more so than carbon, nitrogen and oxygen do
D) Calcium refracts X-rays more so than carbon, nitrogen and oxygen do

Answer is A: I thought that if the atoms were smaller, then the aperture/spacing for the X-rays to diffract would be greater?

Generally speaking, medical X-rays don't work by x-ray diffraction (like what you do in crystallography for instance). X-ray imaging is quite straight forward - you shoot X-rays at objects with a detector behind the object. If the object is thick and dense, it doesn't let the X-rays get through it and so you see an opacity on the x-ray. If the object is thin or dispersed, x-rays can get around it to the detector behind. That's why bone shows up as opacities on x-ray film.