I would solve that with a shader and a sprite...
passing the distance to the shader and it sets transparence to 0 or 1...

here's the shader...:

Code:

---

float4x4 matWorldViewProj;
float3 lineColor = {0.0,1.0,0.0};
float2 vecSkill41;
float4 vecColor;


void circleVS(	in float4 pos	:POSITION,
					in float2 tex	:TEXCOORD0,
					out float4 oPos:POSITION,
					out float2 oTex:TEXCOORD0)
{
	oPos = mul(pos,matWorldViewProj);
	oTex = tex;
}

float4 circlePS(in float2 texcrd:TEXCOORD0):COLOR
{
	float pDist = distance(float2(0.5,0.5),texcrd);
	float4 Color;
	Color.rgb = lineColor;
	if(pDist>vecSkill41.x-vecSkill41.y&&pDist<vecSkill41.x)
		Color.w = vecColor.w;
	else
		Color.w = 0;
	return Color;
}

technique circle
{
	pass p0
	{
		vertexshader = compile vs_2_0 circleVS();
		pixelshader = compile ps_2_0 circlePS();
	}
}

---

...and here a little script you can try it with
Code:

---

MATERIAL* circleMtl =
{
	effect="circle.fx";
}

void main()
{
	float circleAnm = 0;
	level_load(NULL);
	you=ent_create("circle.tga",nullvector,NULL);
	your.tilt = 90;
	your.skill42 = floatv(0.02);
	your.material = circleMtl;
	set(your,TRANSLUCENT);
	watched=you;
	while(1)
	{
		circleAnm += 0.7*time_step;
		if(circleAnm>50)
			circleAnm=0;
		your.skill41 = floatv(circleAnm/100);
		your.alpha = 100-circleAnm*2;
		wait(1);
	}
}

---

have fun with it...(ofc you can also use more than one line with it )